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Lipoblastoma-like tumor (LLT) is a benign soft tissue tumor demonstrating mixed morphologic features of lipoblastoma, myxoid liposarcoma, and spindle cell lipoma but lacking genetic alterations associated with those tumors. LLT was originally thought to be specific to the vulva but has since been reported in the paratesticular region. The morphologic features of LLT overlap with those of "fibrosarcoma-like lipomatous neoplasm" (FLLN), a rare, indolent adipocytic neoplasm considered by some to form part of the spectrum of atypical spindle cell and pleomorphic lipomatous tumor. We compared the morphologic, immunohistochemical, and genetic features of 23 tumors previously classified as LLT (n = 17) and FLLN (n = 6). The 23 tumors occurred in 13 women and 10 men (mean age, 42 years; range, 17 to 80 years). Eighteen (78%) cases arose in the inguinogenital region, whereas 5 tumors (22%) involved noninguinogenital soft tissue, including the flank (n = 1), shoulder (n = 1), foot (n = 1), forearm (n = 1), and chest wall (n = 1). Microscopically, the tumors were lobulated and septated, with variably collagenized fibromyxoid stroma, prominent thin-walled vessels, scattered univacuolated or bivacuolated lipoblasts, and a minor component of mature adipose tissue. Using immunohistochemistry, 5 tumors (42%) showed complete RB1 loss, with partial loss in 7 cases (58%). RNA sequencing, chromosomal microarray, and DNA next-generation sequencing study results were negative for significant alterations. There were no clinical, morphologic, immunohistochemical, or molecular genetic differences between cases previously classified as LLT or FLLN. Clinical follow-up (11 patients [48%]; range, 2-276 months; mean, 48.2 months) showed all patients were alive without disease, and only one patient had experienced a single local recurrence. We conclude that LLT and FLLN represent the same entity, for which "LLT" seems most appropriate. LLT may occur in either sex and any superficial soft tissue location. Careful morphologic study and appropriate ancillary testing should allow for the distinction of LLT from its potential mimics., (Copyright © 2023 United States & Canadian Academy of Pathology. Published by Elsevier Inc. All rights reserved.)

Background: The QT interval in the electrocardiogram (ECG) is a fundamental risk measure for arrhythmic adverse cardiac events. However, the QT interval depends on the heart rate and must be corrected accordingly. The present QT correction (QTc) methods are either simple models leading to under- or overcorrection, or impractical in requiring long-term empirical data. In general, there is no consensus on the best QTc method., Objective: We introduce a model-free QTc method-AccuQT-that computes QTc by minimizing the information transfer from R-R to QT intervals. The objective is to establish and validate a QTc method that provides superior stability and reliability without models or empirical data., Methods: We tested AccuQT against the most commonly used QT correction methods by using long-term ECG recordings of more than 200 healthy subjects from PhysioNet and THEW databases., Results: AccuQT overperforms the previously reported correction methods: the proportion of false-positives is reduced from 16% (Bazett) to 3% (AccuQT) for the PhysioNet data. In particular, the QTc variance is significantly reduced and thus the RR-QT stability is increased., Conclusion: AccuQT has significant potential to become the QTc method of choice in clinical studies and drug development. The method can be implemented in any device recording R-R and QT intervals., (© 2022 Published by Elsevier Inc. on behalf of Heart Rhythm Society.)

Sinonasal mucosal melanoma is a rare tumor arising within the nasal cavity, paranasal sinuses, or nasopharynx (sinonasal tract). This study evaluated 90 cases diagnosed in 29 males and 61 females with median age 68 years. Most tumors involved the nasal cavity and had an epithelioid morphology. Spectrum of research techniques used in this analysis includes targeted-DNA and -RNA next-generation sequencing, Sanger sequencing, fluorescence in situ hybridization and immunohistochemistry. Sinonasal melanomas were commonly driven by RAS (38/90, 42%), especially NRAS (n = 36) mutations and rarely (4/90, 4%) displayed BRAF pathogenic variants. BRAF/RAS mutants were more frequent among paranasal sinuses (10/14, 71%) than nasal (26/64, 41%) tumors. BRAF/RAS-wild type tumors occasionally harbored alterations of the key components and regulators of Ras-MAPK signaling pathway: NF1 mutations (1/17, 6%) or NF1 locus deletions (1/25, 4%), SPRED1 (3/25, 12%), PIK3CA (3/50, 6%), PTEN (4/50, 8%) and mTOR (1/50, 2%) mutations. These mutations often occurred in a mutually exclusive manner. In several tumors some of which were NRAS mutants, TP53 was deleted (6/48, 13%) and/or mutated (5/90, 6%). Variable nuclear accumulation of TP53, mirrored by elevated nuclear MDM2 expression was seen in >50% of cases. Furthermore, sinonasal melanomas (n = 7) including RAS/BRAF-wild type tumors (n = 5) harbored alterations of the key components and regulators of canonical WNT-pathway: APC (4/90, 4%), CTNNB1 (3/90, 3%) and AMER1 (1/90, 1%). Both, TERT promoter mutations (5/53, 9%) and fusions (2/40, 5%) were identified. The latter occurred in BRAF/RAS-wild type tumors. No oncogenic fusion gene transcripts previously reported in cutaneous melanomas were detected. Eight tumors including 7 BRAF/RAS-wild type cases expressed ADCK4::NUMBL cis-fusion transcripts. In summary, this study documented mutational activation of NRAS and other key components and regulators of Ras-MAPK signaling pathway such as SPRED1 in a majority of sinonasal melanomas., (© 2022. This is a U.S. Government work and not under copyright protection in the US; foreign copyright protection may apply.)

Background: Package leaflet provides information about medicinal product to the user. Printed leaflet is familiar and available, however poorly legible, especially when containing multiple languages. It is resourceful to update, has potential to go missing or get damaged, and is environmentally burdensome. The pharmaceutical manufacturers in the Baltic countries have been granted permission to market selected hospital medicinal products without printed package leaflet. The industrial pilot project is expected to promote availability of medicinal products and patient safety via increased access to medicinal information., Objective: Only few countries in Europe have derogated from Article 58 of Directive 2001/83/EC. Knowledge about the effects of removal of paper package leaflet from the medicinal product is limited, and related publications are scarce. Current interview study is identifying the obstacles during the implementation of the industrial project, investigating the potential environmental impact, and searching for further opportunities for the package leaflet in development of medicinal products., Methods: Real-time person-to-person semi-structured interviews with relevant stakeholders were conducted, and transcripts were analysed by content analysis to identify themes., Results: Results demonstrated general support for removing package leaflet from selected hospital products. Main difficulties of the industrial project regarded the need for clear communication and practical disadvantages of project setup. Main benefits included educational aspect of increasing awareness about product information and strengthened collaboration. Majority of participants felt doubtful about the impact of the industrial project on people's awareness of ecological issues and they admittedly lacked sufficient information on the environmental impact of pharmaceutical packaging., Conclusion: The removal of paper leaflet could be extended to more products based on the positive feedback for the industrial pilot project. However, it is paramount that the format of electronic product information would need to be enhanced first to improve readability., Competing Interests: None., (© 2022 The Authors.)

Primary malignant melanoma of esophagus is very rare, and its clinicopathologic and genetic features have not been extensively investigated. In this study, 20 tumors from 14 male and 6 female patients (40-79 years old) were evaluated. Dysphagia, chest pain, and weight loss were frequent symptoms. Thirteen melanomas, including two with multiple lesions, involved the distal third of esophagus. The median tumor diameter was 6 cm. Epithelioid morphology, moderate atypia, and pigmentation were typical findings. None of the patients had melanoma elsewhere, and all tumors exhibited a junctional peri-epithelial component consistent with a primary lesion. The median mitotic activity was 11 per 10 high-power fields (range, 0-31). Nine patients died of tumor within 4-22 months, however, two showed long-term (96 and 104 months) survival. In 15 cases, tissue for further immunohistochemical and molecular studies were available. BRAF, KIT, and NRAS mutation status was assessed by Sanger sequencing in all 15 tumors. The next-generation sequencing of 50 or 409 genes was performed in five and three cases, respectively. IGF1R expression indicating activation of the IGF axis was seen in 82% (9/11) of tumors. However, no BRAF mutations were identified. In 33% (5/15) of tumors, NRAS mutations were detected. KIT expression was seen in 50% (7/14) of melanomas including single KIT mutant. Two of three tumors evaluated with 409 genes panel revealed multiple driver mutations indicating sub-clonal expansion, whereas a single mutation (TSC1 p.H371Q) was the sole change in the third case. SF3B1 p.K666T and p.R625C mutations were detected in two cases. However, no co-occurrence of SF3B1 and GNAQ or GNA11 mutations, seen in uveal melanoma, was detected. FBXW7 p.R465C and p.R479G mutations, linked to cancer progression, were found in two of eight tumors. In summary, esophageal melanoma mutation profile indicates complexity of molecular mechanisms underlying its pathogenesis.

The CD274 (PD-L1)/PDCD1 (PD-1) pathway is crucial for the modulation of immune responses and self-tolerance. Aberrantly expressed CD274 allows tumor cells to evade host immune system and is considered to be a mechanism of adaptive immune resistance. Inhibition of the CD274/PDCD1 immune checkpoint offers a promising new therapeutic strategy. Although CD274-expressing tumor cells have been identified in different types of tumors including colorectal cancer, clinicopathologic profile of these CD274-positive tumors has not been extensively studied. In this study, 454 primary colorectal carcinomas were analyzed histologically and immunohistochemically for CD274, mismatch repair (MMR) proteins, intestinal differentiation marker (CDX2), and stem cell markers (ALCAM, ALDH1A1, and SALL4). CD274-positive colorectal carcinomas (54/454 (12%)) usually (83%) involved the right or transverse colon with poorly differentiated and solid/medullary histology. On the basis of multivariate logistic regression analysis, CD274 positivity was significantly associated with poorly differentiated histotype (OR: 3.32; 95% CI: 1.46-7.51; P=0.004), MMR deficiency (OR: 10.0; 95% CI: 4.66-21.5; P<0.001), and 'stem-like' immunophenotype defined by the loss or weak expression of CDX2 and ALCAM-positivity (OR: 5.51; 95% CI: 1.66-18.3; P=0.005). Mutation analysis of 66 arbitrary selected colorectal carcinomas revealed that CD274-positive tumors usually (88%) carried the BRAF V600E mutation. Thus, colorectal carcinomas defined by CD274 positivity displayed features associated with tumors arising via the serrated neoplasia pathway. Moreover, colorectal carcinomas characterized by lack of CDX2 and prominent expression of ALCAM frequently (71%) showed CD274 positivity. This might suggest association of CD274 expression with 'stem-like' phenotype. Further evaluation of a larger cohort or experimental analyses would be needed to confirm this notion.

Gastrointestinal stromal tumors (GISTs) are mesenchymal tumors usually driven by the mutational activation of receptor tyrosine kinases, KIT, or PDGFRA. Oncogenic activation of phosphatidylinositide-3-kinase (PI3K), a downstream effector in the KIT signaling pathway, has been identified in different types of cancer, with the PI3K 110α subunit encoded by PIK3CA being a common mutational target. In this study, the mutational hotspot in the PIK3CA kinase domain encoded by exon 20 was evaluated in 529 imatinib-naive GISTs using PCR amplification and Sanger sequencing. Eight mutations (two co-existing in one tumor) were identified. Subsequently, The cobas PIK3CA Mutation Test was employed to evaluate mutational hotspots in exons 1, 4, 7, and 9 in 119 PIK3CA exon 20-wild type tumors. In two cases, mutations in exons 1 and 9 were identified. In one GIST, previously undetected by Sanger sequencing, the exon 20 mutation was discovered. Altogether, eight primary and two metastatic GISTs carried PIK3CA mutations. The size of primary PIK3CA-mutant GISTs was ≥14 cm (mean size 17 cm), and mitotic activity varied from 0 to 72 per 50HPF (mean 5/50HPF). Follow-up data showed short survival in 6 of 7 studied cases. Detection of PIK3CA mutations in large or metastatic KIT-mutant GISTs may suggest that PIK3CA-mutant clones have a proliferative advantage during disease progression. Tyrosine kinase inhibitors have been successfully used in GIST treatment. However, resistance frequently develops due to secondary KIT mutations or activation of downstream to KIT signaling pathways, such as the PI3K/AKT/mTOR pathway. PIK3CA mutations similar to the ones detected in GISTs have been shown to cause such activation. Therefore, genotyping of PIK3CA in GISTs might help to pinpoint primary and metastatic tumors with the potential to develop resistance to tyrosine kinase inhibitors and guide therapy with PI3K inhibitors.

Glomangiopericytoma (sinonasal-type hemangiopericytoma) is a rare mesenchymal neoplasm with myoid phenotype (smooth muscle actin-positive), which distinguishes this tumor from soft tissue hemangiopericytoma/solitary fibrous tumor. Molecular genetic changes underlying the pathogenesis of glomangiopericytoma are not known. In this study, 13 well-characterized glomangiopericytomas were immunohistochemically evaluated for β-catenin expression. All analyzed tumors showed strong expression and nuclear accumulation of β-catenin. Following this observation, β-catenin glycogen serine kinase-3 beta phosphorylation region, encoded by exon 3, was PCR amplified in all cases and evaluated for mutations using Sanger sequencing. Heterozygous mutations were identified in 12 of 13 tumors. All mutations consisted of single-nucleotide substitutions: three in codon 32 (c.94G>C (n=2) and c.95A>T), four in codon 33 (two each c.98C>G and c.98C>T), two in codon 37 (c.109T>G), one in codon 41 (c.121A>G), and two in codon 45 (c.133T>C). At the protein level, these substitutions would lead to p.D32H, p.D32V, p.S33C, p.S33F, p.S37A, p.T41A, and p.S45L mutations, respectively. Previously, similar mutations have been reported in different types of cancers and shown to trigger activation of β-catenin signaling. All analyzed glomangiopericytomas showed prominent nuclear expression of cyclin D1, as previously shown for tumors with nuclear expression of β-catenin as a sign of oncogenic activation. These results demonstrate that mutational activation of β-catenin and associated cyclin D1 overexpression may be central events in the pathogenesis of glomangiopericytoma. In additon, nuclear accumulation of β-catenin is a diagnostic marker for glomangiopericytoma.

Smooth muscle tumors are here considered an essentially dichotomous group composed of benign leiomyomas and malignant leiomyosarcomas. Soft tissue smooth muscle tumors with both atypia and mitotic activity are generally diagnosed leiomyosarcomas acknowledging potential for metastasis. However, lesions exist that cannot be comfortably placed in either category, and in such cases the designation 'smooth muscle tumor of uncertain biologic potential' is appropriate. The use of this category is often necessary with limited sampling, such as needle core biopsies. Benign smooth muscle tumors include smooth muscle hamartoma and angioleiomyoma. A specific category of leiomyomas are estrogen-receptor positive ones in women. These are similar to uterine leiomyomas and can occur anywhere in the abdomen and abdominal wall. Leiomyosarcomas can occur at any site, although are more frequent in the retroperitoneum and proximal extremities. They are recognized by likeness to smooth muscle cells but can undergo pleomorphic evolution ('dedifferentiation'). Presence of smooth muscle actin is nearly uniform and desmin-positivity usual. This and the lack of KIT expression separate leiomyosarcoma from GIST, an important problem in abdominal soft tissues. EBV-associated smooth muscle tumors are a specific subcategory occurring in AIDS or post-transplant patients. These tumors can have incomplete smooth muscle differentiation but show nuclear EBER as a diagnostic feature. In contrast to many other soft tissue tumors, genetics of smooth muscle tumors are poorly understood and such diagnostic testing is not yet generally applicable in this histogenetic group. Leiomyosarcomas are known to be genetically complex, often showing 'chaotic' karyotypes including aneuploidy or polyploidy, and no recurrent tumor-specific translocations have been detected.

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors of the gastrointestinal tract. A great majority of GISTs is driven by pathological activation of KIT or platelet-derived growth factor receptor-α (PDGFRA), two closely related receptor tyrosine kinases. However, other genetic changes including gain-of-function BRAF mutations and loss of succinate dehydrogenase (SDH) complex activity have been identified in the subsets of KIT-, PDGFRA-wild type tumors. Genetic mutations affecting KIT, PDGFRA, BRAF and SDH complex functions are believed to be mutually exclusive events. Recently, KRAS codon 12 and 13 mutations were reported in a small subset of KIT or PDGFRA mutant GISTs. Moreover, in in vitro experiments, KIT mutants with concurrent KRAS mutation showed resistance to imatinib, a receptor tyrosine kinase inhibitor used in GIST treatment. The aim of this study was to evaluate a large cohort of GISTs to define frequency and clinical significance of KRAS mutations in this type of cancer. A well-characterized cohort of 514 GISTs was screened for KRAS mutations using Sanger sequencing (n=450) and pyrosequencing (n=64). In all, 350 gastric, 100 intestinal and 64 primary disseminated GISTs were analyzed. No KRAS mutations were found. In GIST, KRAS mutations are extremely rare if they exist (<0.2%). Thus, mutational activation of KRAS does not seem to play any significant role in the development and progression of this type of cancer.

Gastrointestinal stromal tumors (GISTs) are clinically distinct mesenchymal tumors, which generally result from expression of mutant KIT or PDGFRA receptor tyrosine kinase oncogenes. Most GISTs feature strong expression of KIT that serves as a crucial diagnostic adjunct. However, a subset of tumors lacks KIT expression and otherwise may also be difficult to distinguish from other sarcomas, including leiomyosarcoma. Because various carbonic anhydrase (CA) isozymes have been identified as potential treatment targets against different cancers, we evaluated CA II expression in 175 GISTs. Western blotting experiments indicated that CA II is highly expressed in GIST cell lines. Immunohistochemically, 95% of GISTs showed positive signal. The CA II expression in GISTs did not correlate with particular KIT or PDGFRA mutation types. CA II immunoreactivity was absent or low in other mesenchymal tumor categories analyzed. High CA II expression was associated with a better disease-specific survival rate than low or no expression (Mantel-Cox test, P<0.0001). The present results indicate that CA II is overexpressed in most GISTs, is quite selective to this tumor type among mesenchymal tumors, and therefore might be a useful biomarker in diagnostics.

The inflammatory fibroid polyp is a rare benign lesion occurring throughout the digestive tract. It usually forms a solitary mass, characterized by a proliferation of fibrovascular tissue infiltrated by a variable number of inflammatory cells. The etiology of this lesion is unknown and conflicting histogenetic theories have been proposed. Recently, mutations in platelet-derived growth factor receptor (PDGFRA) and PDGFRA expression were reported in gastric inflammatory fibroid polyps. In this study, PDGFRA exons 12, 14, and 18 were screened for activating mutations in 60 small intestinal inflammatory fibroid polyps. In addition, the PDGFRA expression was evaluated immunohistochemically. Mutations in PDGFRA were identified in 33 of 60 (55%) cases, whereas 95% expressed PDGFRA. There were 26 deletions, three deletion-insertions, duplication, and single nucleotide substitution in exon 12, and a single nucleotide substitution and deletion in exon 18. The majority (n=23) of exon 12 deletions were 1837&#95;1851del leading to S566&#95;E571delinsR. However, 1835&#95;1852delinsCGC leading to the same S566&#95;E571delinsR, were found in two tumors. Three inflammatory fibroid polyps had 1836&#95;1850del leading to S566&#95;E571delinsK. A complex deletion-insertion affecting a similar region (1837&#95;1856delinsGATTGATGATC) and leading to S566&#95;I573delinsRIDDL was identified once. In addition, duplication and single nucleotide substitution were found 5' to the common inflammatory fibroid polyp mutational 'hot spot'. These mutations consist of 1808&#95;1828dup leading to I557&#95;E563dup, and 1821T>A resulting in 561V>D substitution. A 2664A>T and 2663&#95;2674del leading to 842D>V and D842&#95;H845del, respectively, were identified in exon 18. Similar gain-of-function PDGFRA mutations reported in gastrointestinal stromal tumors have been considered to be a driving pathogenetic force. This study showed consistent expression and common mutational activation of PDGFRA in small intestinal inflammatory fibroid polyps as in their gastric counterparts, and these lesions should be considered PDGFRA-driven benign neoplasms. We also suggest that these polyps may develop from earlier described PDGFRA-positive mesenchymal cells distributed along the villus membrane after oncogenic PDGFRA activation.

Gastrointestinal stromal tumors (GISTs) are mesenchymal neoplasms driven by oncogenic, mutational activation of KIT or platelet-derived growth factor receptor alpha (PDGFRA). GIST-specific KIT or PDGFRA mutations have been linked to tumor location, tumor cell morphology and clinical behavior. The purpose of this study was to evaluate the clinicopathologic profile of GISTs that have KIT exon 13 or exon 17 mutations. Through the collaboration of several GIST research groups, we gathered 54 cases from the pre-imatinib era that had such primary mutations. From our observations and those in the literature, we estimate that the frequency of these mutations is no higher than 1-2%. Almost all (32 of 33, 97%) of the KIT exon 13 mutations were the 1945A>G substitution leading to Lys642Glu. A majority (15 of 21, 71.4%) of the KIT exon 17 mutations were the 2487T>A substitution leading to Asn822Lys. Demographic and clinicopathologic data were available for 26 and 14 KIT exon 13 and exon 17 mutant GISTs, respectively. Median age and male to female ratio were similar to ones reported in other GIST studies. Small intestinal tumors were two times more frequent than gastric ones among KIT exon 17 mutants. Also, intestinal tumors were slightly overrepresented among KIT exon 13 mutants when compared with population-based studies. The majority of KIT exon 13 or exon 17 mutants had a spindle-cell morphology and only a few had epithelioid features. Tumor size varied from 1.2 to 25 cm and average mitotic rates were 9.5 and 4.2 for KIT exon 13 and exon 17 mutants, respectively. Gastric KIT exon 13 mutant GISTs tend to be slightly larger and more aggressive than gastric GISTs in average, whereas the behavior of small intestinal GISTs with KIT exon 13 mutations does not differ from other small intestinal GISTs. The latter is also true for all KIT exon 17 mutant GISTs.

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors of gastrointestinal tract. GISTs range from benign indolent neoplasms to highly malignant sarcomas. Gain-of-function mutations of tyrosine kinase receptors, KIT or PDGFRA, have been identified in most GISTs. In this study, we report 36 GIST patients whose tumors had homozygous KIT exon 11 mutations detected by direct sequencing of PCR products. Loss of heterozygosity in KIT locus and other chromosome 4 loci were documented in majority of these tumors. However, fluorescence in situ hybridization with KIT locus-specific probe and chromosome 4 centromeric enumeration probe showed no evidence of KIT hemizygosity in a majority of analyzed cases. These findings are consistent with duplication of chromosome 4 with KIT mutant allele. Homozygous KIT exon 11 mutations were found in 33 primary tumors and 7 metastatic lesions. In two cases, shift from heterozygosity to homozygosity was documented during tumor progression being present in metastases, but not in primary tumors. Among primary GISTs, there were 16 gastric, 18 intestinal and 2 from unknown locations. An average primary tumor size was 12 cm and average mitotic activity 32/50 HPFs. Out of 32 tumors 29 (90.6%) with complete clinicopathologic data were diagnosed as sarcomas with more than 50% risk of metastatic disease, and 26 of 29 patients with follow-up had metastases or died of disease. An average survival time among pre-imatinib patients, who died of the disease was 33.4 months. Based on these findings, we conclude that presence of homozygous KIT exon 11 mutations is associated with malignant course of disease and should be considered an adverse prognostic marker in GISTs.

Gastrointestinal stromal tumors (GISTs) are common mesenchymal tumors of the gastrointestinal tract. Activating KIT or PDGFRA (platelet-derived growth factor receptor alpha) mutations have been shown to be a major force in GIST pathogenesis. Recently, a previously undescribed N659K PDGFRA exon 14 mutation has been reported in GISTs. The purpose of this study was to evaluate the frequency of GISTs with PDGFRA exon 14 mutations and define the clinicopathologic profile of such tumors. In all, 200 GISTs negative for mutations in KIT exons 9, 11, 13 and 17 and PDGFRA exons 12 and 18 were evaluated for PDGFRA exon 14 mutations by PCR amplification and direct sequencing. Mutations were found in 11 of 119 (9%) gastric GISTs. None of the 81 GISTs from other than gastric location had such a PDGFRA mutation. A majority of these mutations (eight cases) represented simple 2125C>A or C>G missense mutations, leading to substitution of the lysine for asparagine (N659K). However, in two cases, 2123A>T missense mutations leading to substitution of the tyrosine for asparagine (N659Y) was found instead. Of 11 PDGFRA N659-mutant GISTs, 10 had pure epithelioid morphology. One tumor had mixed, predominantly spindle and focally epithelioid cell morphology. Frequency of PDGFRA N659-mutant GISTs among pure epithelioid GISTs was almost 19%. Immunohistochemically, the majority (64%) of these tumors lacked KIT expression or showed only focal scattered KIT positivity. Tumor size ranged from 2.5 to 16 cm (average 7.1 cm). Low mitotic activity, 5 cm tumors. Based on mitotic activity and tumor size, six tumors were classified as probably benign with very low malignant potential. Low to moderate malignant potential and high malignant potential was suggested in three and two tumors, respectively. In four cases with moderate or high malignant potential GISTs, a long-term follow-up (average 235.5 months) showed favorable course of disease.

Schwannomas have been variably observed to be glial fibrillary acid protein (GFAP) and occasionally keratin positive, with antibodies reacting with multiple keratins (pankeratins, keratin cocktail (CK), but specific keratin polypeptides (K) have not been examined for in schwannoma. Since we observed CK positivity in retroperitoneal schwannomas, we wanted to study a large group of retroperitoneal and peripheral schwannomas with GFAP, CK and Ks to explore the frequency and biologic background of this finding. We immunohistochemically evaluated a large number of retroperitoneal (n=115) and peripheral schwannomas (n=22) for GFAP, 16 individual K and AE1/AE3 keratin cocktail. The great majority (104/115, 90%) of retroperitoneal schwannomas were positive for GFAP, and 72/104 (69%) cases were positive for AE1/AE3, often extensively. Both markers highlighted the cellular Antoni A areas, particularly adjacent to the capsule, myxoid or degenerative areas, and perivascularly. Most cases 87/104 (84%) stained for both AE1/AE3 and GFAP at least focally. No tumors stained for keratins that were GFAP negative. None of the immunostains for individual K showed positivity comparable to that obtained with AE1/AE3 CK. However, 62% were focally positive for high molecular weight K1 and 8/61 (13%) for K7. None of the retroperitoneal schwannomas were positive for other keratins including K2, 4, 5, 8, 9, 10 and K14-20. Peripheral schwannomas showed GFAP-positivity in only three of 22 cases (14%), and all were negative for keratins, both cocktail and individual K. We conclude that crossreactivity of AE1/AE3 with other intermediate filament proteins, such as GFAP, as previously observed in brain and glioma tissue, probably accounts for the extensive keratin-positivity seen in some retroperitoneal schwannomas. However, focal expression of K1 and K7 cannot be ruled out. Keratin-positive schwannomas should not be confused with other keratin-positive tumors, such as sarcomatoid carcinoma, mesothelioma, and synovial sarcoma.

Histiocytic sarcoma (HS) is a rare but controversial hematopoietic neoplasm. In the past, malignancies have been misclassified as histiocytic tumors due to overlapping histologic features and inadequate phenotypic data. CD163, a recently characterized hemoglobin scavenger receptor, appears to be a 'specific' marker of histiocytic lineage and a promising diagnostic tool for evaluating histiocytic neoplasms. Five cases of HS were studied to further elucidate the clinicopathologic features of these rare tumors and to demonstrate the diagnostic utility of CD163. Criteria for diagnosis included histologic and immunohistochemical evidence of histiocytic differentiation, CD45 positivity, and exclusion of lymphoid, epithelial, melanocytic and dendritic cell phenotype. Sites of disease included the colon (two cases), palate, inguinal lymph node, and testis. The clinical course was aggressive in 4/5 patients (survival=2-15 months). One patient with localized disease of the palate, survived 17 years after diagnosis. All patients with poor survival had tumors > or =3.5 cm. Histologically, all cases showed diffuse architecture with large, discohesive polygonal cells. Spindling of cells was focally noted. Hemophagocytosis was identified in 3/5 cases. A prominent inflammatory background was present in 4/5 tumors. All cases were immunoreactive for CD45, CD163, CD68, and lysozyme. S-100 was focally positive in 4/5 cases. Antibodies for melanocytic, epithelial, lymphoid, and dendritic cell markers were negative. Molecular studies showed monoclonal IgH gene rearrangements in three cases. Our findings suggest that HS is an uncommon neoplasm frequently extranodal in presentation and aggressive in behavior, with rare exceptions. Stage of disease and possibly tumor size are significant prognostic indicators. Molecular studies remain controversial in the diagnosis. The morphologic and phenotypic features are relatively uniform; however, the diagnosis requires exclusion of more common neoplasms by extensive immunophenotypic studies. CD163 appears to be a specific histiocytic marker and is important in establishing the diagnosis of HS.

Mutational activation of KIT or PDGFRA is considered an early step in pathogenesis of gastrointestinal stromal tumors (GISTs); however, other nonrandom genetic changes have also been identified. At least three common regions of deletions on chromosome 22q, which may harbor putative tumor suppressor genes, have been defined. However, mapping of these regions has been inconsistent. It has also been speculated that GI autonomous nerve tumors (GANTs), GISTs with ultrastructural features suggestive of autonomic nerve differentiation, are characterized by a specific deletion involving 22q13 cytogenetic region. This study was undertaken to evaluate loss of heterozygosity (LOH) on chromosome 22q in 50 GISTs, including 10 GANTs. Four tumors were incidental minimal lesions

Gastrointestinal stromal tumors (GISTs) are KIT expressing spindle cell, epithelioid and rarely pleomorphic mesenchymal tumors. The majority of GISTs show gain-of-function KIT mutations. However, GISTs without KIT mutations and GISTs with weak or lack of immunohistochemical KIT expression have also been reported. Recently, gain-of-function mutations in exon 18 (activation loop) and exon 12 (juxtamembrane domain) of the PDGFRA were identified in such tumors. The purpose of this study was to test the hypothesis that PDGFRA mutation may define a specific clinicopathologic subgroup of GISTs. A total of 447 KIT exon 11 (juxtamembrane domain) mutation-negative GISTs were studied. DNA samples were obtained from formaldehyde-fixed paraffin-embedded tissues. Genomic sequences of PDGFRA exons 18 and 12 were evaluated for the mutations by PCR amplification and direct sequencing. PDGFRA exon 18 mutations were identified in 122 of 346 (35.3%) gastric GISTs and two of 75 (2.7%) intestinal GISTs. A great majority of these mutations represented simple T to A missense mutation at the codon 842 leading to substitution of the valine for aspartic acid (D842 V). However, in-frame deletions and deletions with point mutations clustering between codons 841-847 were found in approximately 23% of all exon 18 mutations. Mutations in PDGFRA exon 12 were found only in 10 of 170 (5.8%) gastric and one of 54 (1.9%) intestinal GISTs negative for KIT exon 11 and PDGFRA exon 18 mutations. There were seven substitutions of aspartic acid for valine at codon 561 (V561D) and four in-frame deletions with point mutations clustering between codons 566 and 571. The majority of GISTs with PDGFRA mutations had pure or predominant epithelioid morphology. Low mitotic activity, < or =5 mitoses/50HPF was detected in 81% of analyzed GISTs including larger, >5 cm tumors. Based on long-term follow-up (average 135 months), a majority (83.5%) of GISTs with PDGFRA mutations followed a benign course.

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors of the gastrointestinal tract. GISTs express KIT and have KIT mutations. Majority of these mutations cluster in the 5' end of the KIT juxtamembrane domain. Little is known about the clinicopathological profile of GIST carrying internal tandem duplications in the 3' end of KIT juxtamembrane domain (ITDs in the 3' KIT-JM). In this study, 500 immunohistochemically KIT-positive GISTs were screened for this type of mutation, and 18 cases were identified (3.6%). The majority of the ITDs consisted of 1 to 18 codon duplications, with Tyr(578), Asp(579), and Leu(576) being the most commonly duplicated codons. There were 14 gastric (78%), 2 small intestinal (11%), and 2 anal (11%) primary tumors diagnosed in 12 females and 6 males with median age of 71 years. The frequency of IDTs in gastric GISTs was 6.5% and was only 0.5% in intestinal GISTs. There was a strong female predominance (79%) among the patients with gastric tumors. Histologically, 16 GISTs were spindle cell, and 2 had epithelioid morphology. The sizes of primary tumors varied from 1 to >20 cm. Based on the combination of tumor size and mitotic activity, six tumors were classified as benign or probably benign, eight as having uncertain malignant potential, and only four as malignant. Follow-up data available in 17 patients confirmed the malignant course of disease in 3 cases. Only one of the tumors classified as potentially malignant metastasized, although the follow-up was limited in some cases. In summary, the great majority of GISTs with ITDs in the 3' KIT-JM were mitotically inactive tumors occurring predominantly in the stomach and that seemed to have a favorable course. This suggests that presence of these IDTs may define a clinicopathologically favorable subset of GISTs. The consequence of these mutations to KIT signaling should be investigated.

A significant percentage of conventional schwannomas, whether sporadic or associated with neurofibromatosis 2 (NF2), show loss of heterozygosity (LOH) at NF2 and/or NF2 inactivating mutations. Similarly, a significant percentage of neurofibromas show LOH at NF1 and/or NF1 inactivating mutations. There are no molecular genetic data on gastrointestinal (GI) nerve sheath tumors traditionally diagnosed as benign schwannomas, rare neoplasms possibly derived from the schwannian elements dispersed between the smooth muscle fibers. In this study, we analyzed 1 esophageal, 16 gastric, 1 small intestinal, and 2 colonic tumors of such type. Histologically, all were spindle cell neoplasms positive for S-100 protein, vimentin, and glial fibrillary acidic protein, and negative for smooth muscle markers, KIT, CD34, neurofilament proteins, and HMB45. Focal or extensive lymphoid cuffs, often containing germinal centers, were present in most cases. None of the patients had NF2 or NF1. Chromosomes 22 and 17, particularly NF2 and NF1 loci, were analyzed for LOH in all GI tumors and for comparative purposes in 10 conventional schwannomas. LOH on 22q was seen in 40% of conventional schwannomas but in only 5% (1 of 20) of GI schwannomas. PCR amplification followed by direct sequencing of PCR products failed to identify mutations in NF2 coding sequences (exons 1-15) in 13 cases, including a case with LOH on 22q. Losses on 17q involving NF1 were seen in both GI and conventional schwannomas in 50% and 33% of analyzed tumors, respectively. LOH at NF1 might be one of the genetic features seen in peripheral nerve sheath tumors from different locations and should be interpreted with caution. However, lack of NF2 alterations strongly supports the hypothesis that GI schwannomas represent a morphologically and genetically distinct group of peripheral nerve sheath tumors that are different from conventional schwannomas.

We encountered a child with an intraosseous small round cell tumor that was negative for LCA, CD20 (L26), and CD3 and positive for vimentin, CD99 (MIC-2), and periodic acid-Schiff. The tumor exhibited rosette-like formations. This case was initially interpreted as Ewing's sarcoma (ES); however, additional studies revealed positivity for CD79a, CD43, and TdT expression, and an immunoglobulin heavy chain gene rearrangement (IgH-R) by polymerase chain reaction (PCR) established this to be a precursor B-lymphoblastic lymphoma. Because the differential diagnosis of ES and lymphoblastic lymphoma can be difficult and the differential diagnostic value of leukocyte antigens and immunoglobulin heavy chain gene rearrangement studies have not been fully evaluated, we conducted a more extensive investigation on 33 (21 soft tissue and 12 intraosseous) ES cases. Cases were retrieved from the files of the Department of Pathology at Georgetown University and from the Soft Tissue Registry of the Armed Forces Institute of Pathology. The cases were studied by light microscopy, immunohistochemistry, and PCR for IgH-R and T cell receptor gamma chain gene rearrangement (Tgamma-R). There were 17 females and 16 males; the mean age was 29.3 years. Locations included the extremities (n = 17) and trunk (n = 16). All cases fit the ES spectrum by light microscopy and immunohistochemistry, as previously determined, and were negative for lymphoid markers (LCA, CD3, CD20, CD43, CD79a, and TdT), CD10 and CD34. CD99 was positive in 31/33 and bcl-2 was weakly positive in 13/33 cases. All 21 cases studied for gene rearrangements by PCR were negative for IgH-R and Tgamma-R. Distinction of intraosseous lymphoblastic lymphoma from ES may be difficult because lymphomas may occasionally exhibit unexpected morphologic and immunophenotypic properties including LCA, CD3 and CD20 negativity and cytokeratin positivity. Additional analysis using CD79a, CD43, TdT, and PCR should be performed to avoid misdiagnosis. True ES is negative for lymphoid markers including CD79a, CD43, and TdT, as well as for IgH-R and Tgamma-R.

Most mesenchymal tumors of the gastrointestinal tract are currently classified as specific gastrointestinal stromal tumors. However, true leiomyomas are more common in the esophagus, and they have been occasionally noted in the colon and rectum, but the small number of reported cases does not allow for clinicopathologic profiling. This study was undertaken to characterize 88 tumors of the muscularis mucosae of the colon and rectum. Seventy tumors were obtained form the files of AFIP and 18 cases from the Department of Pathology of the Haartman Institute of the University of Helsinki. The lesions, except one, were removed by snare polypectomy as incidental lesions at cancer or polyp surveillance; one small tumor was an incidental finding in the rectal resection specimen. The tumors had a significant male predominance in both institutions (overall 2.4:1). They occurred in age range of 38 to 85 years (median 62 years). The lesions were typically small (range 1 to 22 mM, median 4 mM) and located predominantly in the rectum and sigmoid (72%). All tumors were composed of well-differentiated, eosinophilic smooth muscle cells that were seen immediately beneath the mucosa obliterating the muscularis mucosae layer and merging with it. Two tumors had significant atypia ("symplastic leiomyoma"); mitotic activity was seen in one of these tumors, but not in others. The lesional cells were uniformly positive for smooth muscle actin and desmin and negative for CD34, CD117 and S100-protein, based on immunohistochemical studies on 20 to 24 cases with each marker. No gastrointestinal stromal tumors were identified among the tumors of muscularis mucosae, and no CD117-positive cells, except mast cells, were seen in the muscularis mucosae layer. None of the patients had morbidity related to the tumor. Based on follow-up data on 29 patients, leiomyomas of muscularis mucosae are benign. They should be separated from gastrointestinal stromal tumors that have a clinicopathologic spectrum including frequent disease-related mortality. Snare polypectomy is an adequate treatment, but ensuring the complete removal and follow-up are necessary precautions for tumors with any atypia or mitotic activity.

Gastrointestinal (GI) stromal tumor (GIST) is the designation for the major subset of GI mesenchymal tumors and encompasses most tumors previously classified as GI smooth muscle tumors. Although GISTs typically express CD117 (KIT), often express CD34, and sometimes express alpha-smooth muscle actin (SMA), the relative frequency of these markers has not been characterized in large series of GISTs of different sites, and the CD117 expression has not been fully characterized in intra-abdominal tumors. In this study, we immunohistochemically analyzed 292 GISTs throughout the GI tract, including omentum and mesentery, and compared the immunoreactivities with 211 other tumors that may enter in the differential diagnosis. GISTs were defined in this study as CD117-positive primary spindied or epithelioid mesenchymal tumors of the GI tract, omentum, or mesentery. The CD34 positivity of GISTs varied from 47% in small bowel to 96 to 100% in rectum and esophagus, whereas SMA expression showed the opposite patterns and was most frequent in the GISTs of small bowel (47%) and rarest in the GISTs of rectum and esophagus (10-13%). Desmin was seen only occasionally. S100 positivity was rare but was seen most frequently in small intestinal GISTs (15%). True leiomyomas from esophagus, muscularis mucosae of colorectum, and pericolic leiomyomas similar to uterine leiomyomas were negative for CD117 and CD34 and positive for SMA and desmin (46 of 46). Inflammatory fibroid polyps of stomach and small intestine were negative for CD117 but were often positive for CD34 (6 of 8) and variable for SMA (3 of 8). Inflammatory myofibroblastic tumors involving gastric or colonic wall were negative for CD117 but some showed CD117-positive endothelia. GI schwannomas were all negative for CD117 and positive for S100 protein (11 of 11). Extremely focal CD117 positivity was seen in the neoplastic cells of some retroperitoneal leiomyosarcomas and liposarcomas. Among other CD117-positive tumors were intestinal metastatic melanomas (8 of 11) and extraskeletal Ewing's sarcomas (5 of 11), two of which were abdominal. In conclusion, strong CD117 expression defines most primary GI mesenchymal tumors as GISTs, which show different patterns for CD34 and SMA in various parts of the GI tract. Some unrelated CD117-positive tumors (melanomas, Ewing's sarcomas) should not be confused with GISTs.

Epithelioid sarcoma is a distinctive, rare soft tissue sarcoma that typically involves the distal extremities in young adults, and shows epithelioid morphology and immunohistochemical markers of epithelial differentiation. The genetic background of epithelioid sarcoma is poorly understood, and knowledge of it could give insights into the pathogenesis of this tumor and its possible relationship with other malignant tumors. In this study, we analyzed DNA copy number changes in 30 epithelioid sarcomas by comparative genomic hybridization. DNA was extracted from microdissected samples of formaldehyde-fixed and paraffin-embedded tumors with a minimum of 60% of tumor cells in each sample. Sixteen tumors (53%) showed DNA copy number changes at one to six different genomic sites. The majority of the changes were gains, seen in 14 tumors, whereas 10 tumors showed losses. The most common recurrent gains were at 11q13 (five cases), 1q21-q23 (four cases), 6p21.3 (three cases), and 9q31-qter (three cases). High-level amplifications were detected once in 6p21.3-p21.1 and once in 9q32-qter. Recurrent losses were seen at 9pter-p23 (three cases), 13q22-q32 (three cases), 1p13-p22 (two cases), 3p12-p14 (two cases), 4q13-q33 (two cases), 9p21 (two cases), and 13q32-qter (two cases). The most common recurrent gain at 11q13 was seen in both classic cases and angiomatoid and rhabdoid variants supporting the relationship of these variants with the classic epithelioid sarcoma. Expression of cyclin D1 gene, located in 11q13, was immunohistochemically detected in nine of 15 cases including three of five cases with gain of 11q13, suggesting its involvement in epithelioid sarcoma. The observed comparative genomic hybridization changes give targets for future genetic studies on epithelioid sarcoma.

Primary angiosarcoma of the spleen is a rare neoplasm that has not been well characterized. We describe the clinical, morphologic, and immunophenotypic findings of 28 cases of primary splenic angiosarcoma, including one case that shares features of lymphangioma/lymphangiosarcoma. The patients included 16 men and 12 women, aged 29 to 85 years, with a mean of 59 years and median of 63 years. The majority of patients (75%) complained of abdominal pain, and 25% presented with splenic rupture. The most common physical finding was splenomegaly (71%). Seventeen of 21 patients were reported to have anemia. Macroscopic examination showed splenomegaly in 85% cases. Sectioning revealed discrete lesions in 88% of cases, ranging from well-circumscribed firm nodules to poorly delineated foci of necrosis and hemorrhage associated with cystic spaces. Microscopically, the tumors were heterogenous; however, all cases demonstrated at least a focal vasoformative component lined by atypical endothelial cells. Solid sarcomatous, papillary, and epithelioid growth patterns were observed. The solid sarcomatous component resembled fibrosarcoma in two cases and malignant fibroushistiocytoma in one case. Hemorrhage, necrosis, hemosiderin, extramedullary hematopoiesis, and intracytoplasmic hyaline globules were frequently identified. A panel of immunohistochemical studies revealed that the majority of tumors were immunoreactive for at least two markers of vascular differentiation (CD34, FVIIIRAg, VEGFR3, and CD31) and at least one marker of histiocytic differentiation (CD68 and/or lysozyme). Metastases developed in 100% of patients during the course of their disease. Twenty-six patients died of disease despite aggressive therapy, whereas only two patients are alive at last follow-up, one with disease at 8 years and the other without disease at 10 years. In conclusion, primary splenic angiosarcoma is an extremely aggressive neoplasm that is almost universally fatal. The majority of splenic angiosarcomas coexpress histiocytic and endothelial markers by immunohistochemical analysis, which suggest that some tumors may originate from splenic lining cells.

C-kit proto-oncogene product (KIT, CD117) is a tyrosine kinase growth factor receptor for stem cell factor. This receptor is important for the development and maintenance of hematopoietic stem cells, mast cells, germ cells, melanocytes, and interstitial cells of Cajal and is constitutively expressed in them. Among mesenchymal tumors, KIT seems to be specific for the gastrointestinal stromal tumors, which consistently express this protein. Activating mutations in the tyrosine kinase or juxtamembrane domains of c-kit gene have been found in mastocytoma, seminoma, and gastrointestinal stromal tumors. Following up our initial observation of KIT expression in one angiosarcoma, we examined 50 angiosarcomas, 13 Kaposi sarcomas, 10 epithelioid hemangioendotheliomas, and 31 hemangiomas of different types for KIT expression using a polyclonal antiserum specific to KIT. Adult and fetal tissues and neovascular endothelia in 20 carcinomas were studied for comparison. More than half (56%) of the angiosarcomas representing different clinicopathologic and histologic subtypes and 2 of 13 Kaposi sarcoma were KIT positive. All epithelioid hemangioendotheliomas and hemangiomas were negative, with the exception of two infantile hemangiomas that showed KIT reactivity. The fetal capillary endothelia of lungs, placenta, and soft tissues were also KIT positive, although in soft tissues and placenta, KIT positivity was more prominent in the first trimester. However, endothelia of adult vessels and neovascular capillaries of carcinomas were negative. None of the four KIT-positive angiosarcomas and one KIT-positive Kaposi sarcomas that were studied showed mutations in the juxtamembrane or tyrosine kinase domains of the c-kit gene. These results indicate that KIT expression occurs in a subset of angiosarcomas, and the expression probably represents oncofetal expression (i.e., reversion of the tumor cell phenotype to that of fetal endothelial cells that may show KIT expression).

Kaposi's sarcoma (KS) is a vascular tumor, the pathogenesis of which has been suggested to include human herpesvirus 8 (HHV-8) as well as various cytokines and growth factors. Very little is known about cytogenetic and molecular genetic changes in KS. We studied DNA copy number changes in KS and found a recurrent gain at 11q13. We then analyzed the amplification and expression status of two known oncogenes, FGF4 and INT2, residing at 11q13. Comparative genomic hybridization, interphase fluorescence in situ hybridization with yeast artificial chromosome probes containing FGF4 and INT2, and immunoperoxidase immunostaining with anti-FGF4 and -INT2 antibodies were used on 12 KS samples. All samples tested were shown by polymerase chain reaction to be HHV-8 positive. A recurrent gain at 11q13 was shown by comparative genomic hybridization in 4 of 10 cases studied. Of six cases studied by interphase fluorescence in situ hybridization, four showed a 3- to 4-fold amplification with the probes containing FGF4 and INT2. Expression of FGF4 and INT2 was found in nine and three cases, respectively, of nine studied. Amplification and expression of these genes is particularly interesting in the context of oncovirus involvement, because INT2 is a homolog of mouse int2 which causes mammary carcinoma in mice when activated by integration of retrovirus mouse mammary tumor virus. This raises the question of whether HHV-8 represents an integrating oncovirus that causes amplification and activation of genomic oncogenes in humans.

Background: The FHIT gene on human chromosome 3p14.2 is deleted in a variety of malignant tumors, including clear cell renal carcinomas (RCCs) resulting in a loss of expression of Fhit protein. The Fhit expression in specific subtypes of renal carcinomas has not been characterized. We have investigated the association of Fhit expression with particular subtypes of renal tumors to determine the role and specificity of this putative tumor suppressor gene in renal neoplasia., Material and Methods: The immunohistochemical expression of Fhit was tested in normal kidneys and in 109 renal neoplasms consisting of 51 clear cell RCCs, 26 papillary RCCs, two chromophobe carcinomas, six oncocytomas, four pelvic transitional cell carcinomas and 20 Wilms' tumors from formalin fixed and routinely processed tissue., Results: Normal renal tubules expressed Fhit strongly and consistently. The majority (78%) of clear cell RCCs showed reduced or absent expression of Fhit, whereas the majority (74%) of papillary carcinomas, all chromophobe renal cell carcinomas, and oncocytomas were strongly positive. Sixty-eight percent of low-grade (G1 plus G2) but only 9% of high-grade (G3 plus G4) clear cell carcinomas were Fhit negative. Wilms' tumors demonstrated focal staining in the epithelial component in 8 of 20 cases (40%)., Conclusions: The loss of Fhit expression in a high percentage of clear cell RCCs with conservation of Fhit in other types of tumors supports the proposed role of FHIT alterations in the genesis of clear cell carcinomas in contrast to other types of renal epithelial tumors. FHIT expression may play a role in epithelial differentiation of nephroblastomas (Wilms' tumors).

Currently, the immunohistochemical evaluation of smooth muscle differentiation is usually based on desmin, which also reacts with skeletal muscle and is not present in all smooth muscle tumors, and alpha-smooth muscle actin, which reacts with myoepithelial cells. Neither marker typically reacts with gastrointestinal stromal tumors (GISTs), previously classified as smooth muscle tumors or presently often classified as smooth muscle/stromal tumors. Two cytoskeleton-associated actin-binding proteins, calponin (CALP) and h-caldesmon (HCD), are putative smooth muscle markers that also react with myoepithelia. These markers are of particular interest in the immunohistochemical analysis of tumors; neither of them has been extensively documented in soft tissue tumors. In this study, we evaluated selected normal and reactive tissues and more than 250 mesenchymal tumors for CALP and HCD. Both markers were expressed in parenchymal and vascular smooth muscle cells in various organs and in myoepithelial cells. CALP also reacted with myofibroblasts of desmoplastic stroma. All of our 25 benign smooth muscle tumors from various locations were positive for CALP and HCD, as were most of the retroperitoneal and uterine leiomyosarcomas. HCD was more specific, because CALP also reacted with myofibroblastic lesions. The common reactivity of malignant fibrous histiocytomas with CALP and HCD suggests a combination of myofibroblastic and smooth muscle differentiation in these tumors. The GISTs (c-kit positive, usually actin negative) showed nearly consistent HCD reactivity, suggesting traits of smooth muscle differentiation. GISTs were usually CALP negative and showed a CALP expression pattern similar to that of alpha-smooth muscle actin. Although nonmuscle, nonmyofibroblastic tumors were negative for CALP and HCD, synovial sarcomas showed streaks of CALP-positive cells of unknown significance. CALP and HCD should be explored as markers to identify myofibroblastic and smooth muscle cell differentiation in mesenchymal tumors.

We describe 12 patients with a distinctive variant of pleomorphic liposarcoma that histologically shows epithelioid features and focally resembles a solid carcinoma. The tumors occurred in nine men and three women (median age, 63 yr; range, 40-78 yr). Five tumors were in the thigh, two in the chest wall, two in the axilla, and one each in the retroperitoneum, groin, and calf. Most were 15 to 20 cm in maximal diameter. They consisted of sheets of epithelioid-appearing cells with ample, variably eosinophilic cytoplasm, often showing a honeycomb-like pattern of cell borders and little if any collagenous extracellular matrix. Their histologic features often resembled those of renal clear cell carcinoma or adrenal cortical carcinoma, but all showed evidence of adipocytic differentiation, and five also showed focal spindle cell components. One patient whose tumor in the thigh had been originally diagnosed as metastatic renal carcinoma had undergone nephrectomy without a finding of a kidney tumor. All of the cases were positive for vimentin; 6 of 11 cases were positive for S-100 protein, usually focally; 5 of 11 were focally positive for keratins; and all were negative for epithelial membrane antigen, muscle actins, desmin, and CD34. High mitotic activity (mean, 42 mitotic figures per 10 high power fields) and high MIB-1-positive proliferative fraction (>30%) were seen in all of the cases, and nuclear p53 immunoreactivity was detected in five of seven cases. Of the eight patients with complete follow-up, five died of disease (median survival, 6 mo), two died of unrelated causes 10 and 18 years later, and 1 was alive and well 24 years later. The epithelioid variant of pleomorphic liposarcoma is a high-grade tumor that must be distinguished from malignant epithelial tumors, especially in view of the keratin immunoreactivity of some of these neoplasms.

The clinicopathologic features of five cases of a fibrocartilaginous mass developing in the nuchal ligament, the nuchal fibrocartilaginous pseudotumor, are described. Only six examples of this lesion were previously reported in the English-language medical literature. The lesions clinically manifested in five adults (3 women, 2 men) ranging in age from 22 to 46 years (mean, 37 yr). The process presented as a nodular mass that was asymptomatic in three patients and accompanied by vague neck pain or stiffness in the remaining two. Three patients related a history of head and neck trauma that preceded the discovery of the tumor. All of the tumors were situated in the deep soft tissue overlying the posterior aspect of the lower cervical vertebrae. The five patients were managed by complete local excision. The tumors measured 1.3 to 3.0 cm. in greatest dimension (mean, 2.5 cm.). Microscopically, the lesion consisted of a poorly delineated, nodular proliferation of moderately cellular fibrocartilaginous tissue arising within the substance of the nuchal ligament and extending into the surrounding soft tissues. No cytologic atypia or mitotic activity was identified. Follow-up data from four of the cases in this study (range, 10-324 mo) and four previously reported examples with follow-up (range, 3-12 mo) show no evidence of recurrent or persistent disease after simple excision. The nuchal fibrocartilaginous pseudotumor is a benign lesion caused by fibrocartilaginous metaplasia of the lower portion of the nuchal ligament, probably as a result of localized trauma or chronic mechanical stress.

Leukocyte common antigen (CD45/LCA) and keratin expression are generally mutually exclusive in diagnostic surgical pathology. CD45 reactivity is a reliable indicator of the hematolymphoid nature of a tumor, whereas keratin reactivity is typical of epithelial differentiation (carcinomas and some sarcomas). Some lymphomas, however, might lack detectable CD45 expression, whereas occasional ones might express keratins. CD45 immunoreactivity has been considered exquisitely specific for hematopoietic cells. We report three undifferentiated or neuroendocrine carcinomas that showed membrane-associated immunoreactivity for CD45 in addition to showing distinctive keratin cocktail (AE1/AE3) and epithelial membrane antigen reactivity (all cases); also, keratin 7 was demonstrated in one case and keratin 19 in another. Two cases were lymph node metastases of undifferentiated carcinomas, one of them from the lungs and the other of an unknown origin; the former case showed neuroendocrine features. The third case represented a pulmonary large-cell undifferentiated carcinoma. These cases were negative for lineage-specific leukocyte antigens and did not show clonal immunoglobulin heavy-chain gene rearrangements. Electron microscopic studies demonstrated desmosomes and keratin-like tonofilaments in all three cases, thus confirming the epithelial nature of these tumors. The exceptional membrane staining for CD45 seen in these undifferentiated carcinomas might be comparable to experimentally detected incorporation of leukocyte antigens into the cell membranes of nonleukocytic cells in a leukocyte-rich environment. This rare diagnostic pitfall should be considered in the diagnostic surgical pathology of undifferentiated tumors. It is best avoided by employing a panel of leukocyte and epithelial antigens and by use of electron microscopy, if possible.

Gastrointestinal stromal tumors (GISTs) represent a distinct and the most important subset of mesenchymal tumors of the GI tract. These tumors are both phenotypically and genotypically different from true leiomyomas and usually express CD34, a hematopoietic progenitor cell antigen. CD34, however, is also present in a wide variety of fibroblastic and endothelial cell tumors. In this immunohistochemical study of CD117, we evaluated 85 cases of GIST and more than 150 other mesenchymal tumors, including leiomyomas and schwannomas. CD117, the c-kit proto-oncogene product, is expressed in subsets of hematopoietic stem cells, mast cells, melanocytes, and interstitial cells of Cajal of the GI tract. CD117 was almost always (85%) expressed in both benign and malignant GISTs. CD117 was observed both in the spindle cell and epithelioid subtypes of GISTs in all locations. In addition to reacting with the CD34-positive GISTs, CD117 was positive in some CD34-negative cases. Approximately one-third of GISTs coexpressed CD117 and smooth muscle actins. In contrast, true leiomyomas (desmin and actin-positive) and schwannomas in both GI and peripheral locations were consistently negative for CD117. Solitary fibrous tumors and Kaposi's sarcomas, which are typically CD34 positive, were consistently CD117 negative. Among the CD34-positive tumors that showed occasional CD117 reactivity were dermatofibrosarcoma protuberans (1 of 7) and hemangiopericytoma (2 of 10). Other mesenchymal tumors that were variably CD 117 positive included clear cell sarcoma (7 of 15), metastatic melanoma (9 of 25), and malignant fibrous histiocytoma (1 of 20). These results indicate that CD117 is a specific marker for GIST among tumors that occur in the GI tract and adjacent regions. CD117 expression also separates GISTs from true leiomyomas and gastric schwannomas.

Identification of the t(X;18)(p11.2;q11.2) translocation and detection of the resulting SYT-SSX1 or SYT-SSX2 fusion transcripts are useful diagnostic markers for synovial sarcoma. In this study, we developed a polymerase chain reaction (PCR) assay to amplify SYT-SSX fusion transcripts. The primer sequences were designed to generate small PCR products and to amplify sequences of all known SSX genes as fusion partners for the SYT gene. RNA was obtained from formaldehyde-fixed and paraffin-embedded tissues of 22 immunohistochemically characterized synovial sarcomas, 6 of them cytogenetically confirmed as t(X;18) positive. The SYT-SSX fusion transcripts were detected in 21 of the 22 analyzed cases. The type of the fusion was identified by the specific restriction enzyme digestion pattern as SYT-SSX1 in 13 cases and SYT-SSX2 in 7 cases; in 1 case, the type could not be assigned. None of the cases showed involvement of the SSX3, SSX4, or SSX5 genes, the other members of the SSX gene family. In seven cases, the SYT-SSX1 or SYT-SSX2 fusion transcripts were demonstrated in frozen tissue using a different PCR assay. The PCR products were confirmed as SYT-SSX sequences by sequencing in five randomly selected cases. Fifteen other sarcomas and related tumors were negative for SYT-SSX fusion transcripts. The PCR assay used in this study performs well in formaldehyde-fixed and paraffin-embedded tissue, and it shows a high specificity. This assay can be used as an adjunct test for diagnostically difficult cases or in retrospective studies to refine the diagnosis of synovial sarcoma in archival material.

Alveolar soft part sarcoma (ASPS) is a rare, histologically distinctive soft tissue sarcoma typically occurring in children and young adults. Although the tumor often shows focal expression of muscle markers, its relationship with rhabdomyosarcoma is not established. The genetic background of ASPS is poorly understood. This study was undertaken to analyze the DNA copy number changes in 13 cases of ASPS using comparative genomic hybridization (CGH) on formaldehyde-fixed, paraffin-embedded tissue sections. Four ASPS cases showed DNA copy number changes. Gains were more common than losses. Gains observed in more than one case included 1q, 8q, 12q and 16p. Although these findings do not show consistent DNA copy number changes in ASPS, they give preliminary clues to genomic areas that might be important in the pathogenesis of ASPS.

Solitary fibrous tumor (SFT) presenting in the pleura and other soft tissue sites and hemangiopericytoma (HPC) presenting at various soft tissue sites are mesenchymal tumors that share many histologic and immunohistochemical features. This raises the questions of whether these tumors are related and whether they belong within the spectrum of a single biologic entity. The behavior of both SFTs and HPCs is difficult to predict histologically. The genetic background of both SFTs and HPCs is poorly known, but it could be helpful in the evaluation of malignancy and could give clues to their possible relationship. In this study, we analyzed 15 SFTs and 11 HPCs by comparative genomic hybridization (CGH), a powerful molecular cytogenetic tool that can be applied to DNA extracted from formaldehyde-fixed and paraffin-embedded tissue. All of these tumors were immunohistochemically similar and showed reactivity for CD34-antigen but not for keratins, desmin, or muscle actins. Only 1 SFT smaller than 10 cm showed DNA copy number changes (a single loss in chromosome 13), but 7 of 8 SFTs larger than 10 cm (including all 4 tumors with more than 4 mitoses per 10 high power fields) showed changes, mostly chromosomal gains in 5q 7, 8, 12, and 18. Four cases showed losses, two of them in chromosome 13 and two others in 20q. These findings suggest that CGH might be useful in the evaluation of malignant transformation in SFT. The most common change, gain of the entire chromosome 8, seen in two cases as the only change, suggests trisomy 8 and parallels a similar finding previously described in other fibrous tumors, such as subsets of desmoid fibromatosis and infantile fibrosarcoma. In contrast, HPCs, including large and mitotically active tumors, showed no DNA copy number changes on CGH. This suggests that HPC is genetically different from SFT.

Low-grade mucosa-associated lymphoid tissue (MALT) lymphomas of the salivary gland are usually indolent diseases with a protracted clinical course. Recurrent multifocal disease has been shown to represent identical clones in some cases, and intraclonal variation resulting from continuing somatic hypermutation has been described, but emergence of novel, major clones upon recurrent disease has not been documented. We analyzed three consecutive biopsy specimens of parotid lymphoid infiltrates of a young woman with Sjögren's disease. The immunoglobulin heavy chain (IgH) gene rearrangements were first amplified using FR2/LJH-VLJH consensus primers. Then, the PCR products were cloned, sequenced, and compared. On the basis of the sequences of the complementarity determining region 3 (CDR3), clone-specific primers were designed and used to evaluate the presence of similar sequences in different biopsy specimens. Recurrent parotid lymphoid infiltrates during a span of 9 years showed histologically similar features consistent with low-grade MALT lymphoma. Polymerase chain reaction amplification showed a clonal pattern of IgH gene rearrangement in all of the lesions with similar product sizes, suggesting the identity of the clones, but two major clones with different CDR3 sequences were found. Intraclonal variation was seen among the sequences seen in the three lesions consistent with the occurrence of somatic hypermutations. Primers specific to the clone seen in the first two lesions failed to amplify products from the third lesion, but primers specific to the third clone showed similar products in the second clone in a small quantity, indicating that this clone persisted and expanded. Our results suggest that different B-cell clones might dominate during the course of low-grade MALT lymphoma of the salivary gland. This implies that in some cases, these processes can represent oligoclonal B-cell proliferations.

Mantle cell lymphoma (formerly known as intermediate lymphocytic lymphoma, diffuse centrocytic lymphoma, or diffuse small cleaved lymphoma) is one of the small cell non-Hodgkin lymphoma entities that is clinically more aggressive than small lymphocytic lymphoma, and needs to be separated from it. Mantle cell lymphoma is strongly associated with the t(11;14) chromosomal translocation that rearranges the bcl-1 oncogene (PRAD-1 gene) and immunoglobulin heavy chain gene. In this study, we developed a nested polymerase chain reaction system to evaluate the t(11;14) translocation. The study material consisted of 10 mantle cell lymphomas fulfilling the criteria suggested by other authors (P. M. Banks et al. Surg Pathol 16:637, 1992). A novel nested polymerase chain reaction system was used to evaluate the bcl-1 breaks in the major translocation cluster using two successive polymerase chain reaction amplifications. This reaction yielded a background-free single product of the size of 200 to 300 base pairs in four of 10 mantle cell lymphomas. The identity of the product from the nested polymerase chain reaction was confirmed by Southern blotting followed by hybridization with a specific probe. The amplification products were also evaluated by Sst-I, Alu-I, Dde-I, and Ita-I restriction enzymes and showed different patterns of digestion reflecting individual differences between the MTC/ IgH junctions. A selection of other low-grade lymphomas, including lymphocytic, follicular, and mucosa-associated lymphoid tissue lymphoma, and hairy cell leukemia and 29 hyperplastic lymph nodes were negative. This nested polymerase chain reaction system for the t(11;14) translocation involving major translocation cluster offers a convenient specific identification for mantle cell lymphoma. However, this test has a limited diagnostic power because only about half of the mantle cell lymphomas show the bcl-1 breaks in the major translocation cluster. The test performs well in formaldehyde-fixed and paraffin-embedded material, allowing the study of large numbers of retrospective cases of mantle cell lymphomas.

Merkel cell carcinoma and malignant lymphoma are important differential diagnoses for undifferentiated cutaneous round cell tumors and immunohistochemistry is instrumental in their evaluation. We describe a case of a 73-year-old man who had cutaneous large cell lymphoma in the right leg (immunophenotype CD45+, CD19+, CD20+ CD22+, lambda clonal, cytokeratin-, NSE-) and lymphoma in left leg simulating Merkel cell carcinoma showing absence of leukocyte antigens (CD45-, CD20-, no light chains) and focal expression of keratin and NSE. However, analysis of polymerase chain reaction amplification products of DNA extracted from both lesions showed two amplifiable sharp bands indicating clonal rearrangements of both alleles of the immunoglobulin heavy chain. Cloning and sequencing of the products from left and right leg lesions showed either 100% homology (one band), or close similarity (the other band), indicating that both tumors were derived from the same B-cell lymphoma clone. This case shows the value of polymerase chain reaction and sequencing in analyzing the ultimate nature of lymphoproliferations and illustrates the potential limitations of immunophenotyping.

Keratin 20 is a recently identified keratin protein distributed particularly in the epithelial cells of the gastrointestinal tract. In this study, keratin 20 was immunohistochemically evaluated in 788 epithelial tumors of different organs. Keratin 20 was consistently present in colonic adenocarcinomas and their metastases in lymph nodes, liver, lung, and ovaries; most primary and metastatic colon carcinomas showed high numbers of positive cells independent of their level of differentiation. Adenocarcinomas of the upper gastrointestinal tract, pancreas, and cholangiocarcinomas showed variable reactivity. Hepatocellular carcinomas and carcinoid tumors often showed focal reactivity limited to scattered tumor cells. In contrast, keratin 20 was virtually absent in primary adenocarcinomas of lung, ovaries, and endometrium. Notable exceptions among ovarian tumors were the mucinous neoplasms that showed variable, sometimes significant keratin 20 reactivity. Transitional cell carcinomas irrespective of grade were usually positive, whereas most prostatic and renal adenocarcinomas were negative or showed only single positive cells. Typically negative were squamous cell carcinomas of all organs and carcinomas of the breast. Merkel cell carcinomas of the skin showed consistent reactivity, whereas small cell carcinomas of the lung were negative. On the basis of these observations, keratin 20 seems to be a suitable adjunct marker to evaluate the primary origin of carcinomas in specific contexts, especially to separate adenocarcinomas of gastrointestinal versus nongastrointestinal origin.

Sixty vascular tumors including 23 angiosarcomas, 300 nonvascular tumors, and selected normal tissues were immunohistochemically evaluated with antibodies to CD31, CD34, and von Willebrand factor (vWF), and monoclonal antibody BNH9, to test the sensitivity and specificity of these markers in the identification of endothelial cells and vascular tumors. Formaldehyde-fixed paraffin-embedded tissues and avidin biotin complex immunostaining were used. All markers labeled normal vascular and lymphatic endothelial cells approximately equally with the exception of CD34 which showed inconsistent expression within the lymphatics. In addition, antibody to CD31 reacted with platelets and megakaryocytes, CD34 with fibroblasts and aortic smooth muscle cells, and BNH9 with many epithelial cells including squamous and gastrointestinal epithelia. Antibody to vWF often showed significant stromal background staining which made the staining occasionally uninterpretable. Benign vascular tumors showed rather uniform staining with all antibodies. However, angiosarcomas were heterogeneous; CD31 was positive in 21/27, CD34 in 25/27 cases, BNH9 in 22/25, and vWF in 18/27 cases. Epithelioid hemangioendotheliomas showed consistent labeling for vWF, but were inconsistently labeled with antibodies to the other markers. Kaposi's sarcoma was positive for both CD31 and CD34. In addition, antibody to CD34 labeled the tumor cells in hemangiopericytoma, cerebellar hemangioblastoma, meningioma, most epithelioid sarcomas, dermatofibrosarcomas, and in a few other sarcomas. CD31, in turn, was not found in sarcomas other than angiosarcomas, but labeled weakly occasional carcinomas and mesotheliomas. Many adenocarcinomas and the glandular component of synovial sarcoma were BNH9 positive.(ABSTRACT TRUNCATED AT 250 WORDS)

Ten adrenocortical carcinomas including two tumors with clinically detectable corticosteroid production, were immunohistochemically analyzed for their intermediate filament proteins, and for neuroendocrine markers. Keratins were present in 6 of 10, vimentin in all 10, and the 68 kilodalton kD neurofilament subunit protein in 6/10 tumors. Keratins numbers 8 and 18 were most prevalent, whereas only traces of keratins 19 and 7 were found. Eight tumors were positive for synaptophysin at least focally, and 3 showed extensive positivity in more than 30% of tumor cells. The tumors showed approximately similar levels of neuron-specific enolase (NSE) expression as judged by immunohistochemistry. Chromogranin was not detected, and there was no immunoreactivity for 3 neuropeptides (calcitonin, gastrin, somatostatin). In normal adrenal cortex, neuron-specific enolase, synaptophysin and neurofilaments were restricted to the nerves seen between the cortical cells. Electron microscopy revealed clusters of dense-core granules in 4 of 5 tumors, consistent with neuroendocrine granules. These results indicate that adrenocortical carcinomas may show signs of neuroendocrine differentiation and share some features with the adrenal medullary tumors.

The presence of intermediate filament proteins of cytokeratin/prekeratin type and vimentin type was evaluated in non-neoplastic thyroid glands and in different types of thyroid neoplasms. Follicular epithelium of both normal and goitrous thyroids showed a strong reaction with anticytokeratin antibodies that widely cross-react with various simple epithelia. On the other hand, in normal thyroid, there were only occasionally (in one of 12 cases) solitary cells reacting with antibodies to epidermal prekeratin. In nodular goiters, such cells were often seen (eight of 18), especially among the lining cells of cysts, and in chronic thyroiditis in all (12 of 12) cases. Only the stromal cells and intraluminal macrophages reacted with antibodies to vimentin. Neoplastic cells of papillary carcinomas showed a positive staining reaction both with antibodies to cytokeratins and to epidermal prekeratin. Follicular carcinoma cells, although positive for cytokeratins, could generally not be stained with antibodies to epidermal prekeratin. Medullary carcinoma cells also showed cytokeratin positivity and, only occasionally, positivity for epidermal prekeratin. Anaplastic carcinomas were also reactive with antibodies to cytokeratin but, for the most part, were negative for epidermal prekeratin. Interestingly, some neoplastic cells of all types of thyroid carcinomas also appeared to contain vimentin, as shown with both polyclonal and monoclonal antivimentin antibodies. In contrast to carcinomas, the intermediate filaments of thyroid sarcomas and lymphomas were only of vimentin type. Furthermore, it was found that the papillary structures in benign goiters were only reactive with cytokeratin antibodies and lacked, in contrast to papillary carcinomas, epidermal prekeratin-like immunoreactivity. Hence, the analysis of intermediate filament proteins of thyroid tumors can be utilized to differentiate between papillary and follicular carcinomas and between benign and malignant papillary lesions as well as between anaplastic thyroid carcinomas and sarcomas or lymphomas.

Twelve cases of gastrointestinal neuroendocrine tumors, including eight carcinoids and four pancreatic islet cell tumors or their metastases, were immunohistochemically analyzed for the expression of different types of intermediate filament proteins. All of the tumors showed cytokeratin positivity in immunostaining, and the Western blotting technique revealed 45- and 52-kilodalton cytokeratins in carcinoid tumors. Three of the islet cell tumors, but none of the carcinoid tumors, showed, in addition, varying numbers of neurofilament-positive tumor cells when evaluated with rabbit and mouse monoclonal antineurofilament antibodies. The presence of only the 70-kilodalton neurofilament and cytokeratin polypeptides in an islet cell tumor was revealed also by using the Western blotting technique. On the other hand, both fetal and adult pancreatic islet cells showed only cytokeratin positivity. Neurofilament-positive epithelial cells were not found in normal small intestines either. The results show epithelial characteristics in normal gastrointestinal neuroendocrine cells and neuroendocrine tumors by their expression of cytokeratin. In addition, some islet cell tumors display the 70-kilodalton neurofilament protein which suggests the acquisition of a new type of intermediate filament during the neoplastic change.

Acetone-fixed frozen sections of 15 malignant melanomas of the skin with metastases were studied immunohistochemically for the presence of different types of intermediate filament proteins, synaptophysin, muscle cell actins, and desmoplakins. One of the melanomas was a primary toe tumor, and the others mainly regional lymph node metastases. The original diagnosis of melanoma was reconfirmed in each case, and the melanoma diagnosis of the metastases was verified by S100 protein immunostaining in all cases and by a monoclonal antibody to melanoma cells (NK1C3) in 7 cases. All melanomas were prominently vimentin-positive. In 10 of 15 cases, immunoreactive keratin could be demonstrated with antibody CAM 5.2. The presence of keratins was confirmed in selected cases with three other monoclonal antibodies including AE1, PKK1, and a monoclonal antibody specific for keratin number 18. Desmoplakin, another marker of epithelial differentiation, was not found in melanoma cells. Two melanomas contained neurofilament-positive tumor cells, which were however negative for synaptophysin. Desmin, muscle actins, and glial fibrillary acidic protein were not found in the neoplastic cells. On the basis of the present results one could conclude that the protein composition of the cytoskeleton of melanomas is more complex than has been previously thought and most importantly that melanomas may contain keratins.

The expression of merosin and laminin was studied in human schwannomas, plexiform neurofibromas, and malignant schwannomas immunohistochemically using monoclonal antibodies. Merosin is a unique novel tissue-specific basement membrane protein found in basement membranes of trophoblast, striated muscle, and Schwann cells. Merosin is related to laminin, another basement membrane protein with which it shows a homologous C terminal domain. In schwannomas, merosin was only found in areas where tumor cells were in contact with stromal or vascular tissue. Laminin, however, was present in all tumor cell basement membranes. In plexiform neurofibromas large amounts of both merosin and laminin were seen in Schwann cell basement membranes. Very little of either protein was found in malignant schwannomas. Thus merosin is present almost exclusively in highly differentiated Schwann cell neoplasms, and its distribution is more restricted than that of laminin. The expression of merosin in plexiform neurofibromas and in the schwannoma cells juxtaposed to the mesenchymal cells suggests that this protein is induced epigenetically in well-differentiated cells in contact with connective tissue or vascular components.