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This report documents a unique multicystic neoplasm of the liver in an 8-month-old boy with a heterozygous germline pathogenic DICER1 variant. This neoplasm, initially considered most likely a mesenchymal hamartoma based on imaging, demonstrated the characteristic histologic pattern of embryonal rhabdomyosarcoma residing in the subepithelial or cambium layer-like zone of the epithelial-lined cysts. Thus, although the differential diagnosis includes mesenchymal hamartoma, a young child with a multicystic mass lesion in the liver, lung, or kidney should both raise the possibility of a germline pathogenic DICER1 variant and also not be mistaken for one of the other hepatic neoplasms of childhood., (© 2021. The Author(s), under exclusive licence to United States & Canadian Academy of Pathology.)

DICER1 syndrome (OMIM 606241, 601200) is a rare autosomal dominant familial tumor predisposition disorder with a heterozygous DICER1 germline mutation. The most common tumor seen clinically is the pleuropulmonary blastoma (PPB), a lung neoplasm of early childhood which is classified on its morphologic features into four types (IR, I, II and III) with tumor progression over time within the first 4-5 years of life from the prognostically favorable cystic type I to the unfavorable solid type III. Following the initial report of PPB, its association with other cystic neoplasms was demonstrated in family studies. The detection of the germline mutation in DICER1 provided the opportunity to identify and continue to recognize a number seemingly unrelated extrapulmonary neoplasms: Sertoli-Leydig cell tumor, gynandroblastoma, embryonal rhabdomyosarcomas of the cervix and other sites, multinodular goiter, differentiated and poorly differentiated thyroid carcinoma, cervical-thyroid teratoma, cystic nephroma-anaplastic sarcoma of kidney, nasal chondromesenchymal hamartoma, intestinal juvenile-like hamartomatous polyp, ciliary body medulloepithelioma, pituitary blastoma, pineoblastoma, primary central nervous system sarcoma, embryonal tumor with multilayered rosettes-like cerebellar tumor, PPB-like peritoneal sarcoma, DICER1-associated presacral malignant teratoid neoplasm and other non-neoplastic associations. Each of these neoplasms is characterized by a second somatic mutation in DICER1. In this review, we have summarized the salient clinicopathologic aspects of these tumors whose histopathologic features have several overlapping morphologic attributes particularly the primitive mesenchyme often with rhabdomyoblastic and chondroid differentiation and an uncommitted spindle cell pattern. Several of these tumors have an initial cystic stage from which there is progression to a high grade, complex patterned neoplasm. These pathologic findings in the appropriate clinical setting should serve to alert the pathologist to the possibility of a DICER1-associated neoplasm and initiate appropriate testing on the neoplasm and to alert the clinician about the concern for a DICER1 mutation., (© 2021. The Author(s).)

Pleuropulmonary blastoma (PPB) is a primary embryonal malignancy of childhood that is characterized by distinct morphologic types: type Ir (regressed), type I (cystic), type II (cystic and solid), and type III (solid). Prognosis varies by PPB type. Most cases are associated with a germline pathogenic mutation in DICER1; however, there is limited data on the factor(s) at a cellular level that drive progression from type I to type III. In this study, we evaluated the expression of p53 and its prognostic implications. A total of 143 PPB cases were included in the study with the following distribution in PPB types: Ir (14%), I (23%), II (32%), and III (31%). P53 expression by immunohistochemistry (IHC) was recorded as four groups: 0%, 1-25%, 26-75%, and 76-100%. All type I PPBs showed 0-25% p53 expression compared to the higher p53 expression (>25%) in type III PPB (p < 0.0001), to support the argument that p53 has a role in tumor progression. In addition, type Ir with the architectural hallmarks of type I PPB, but lacking the primitive cell population, has negligible p53 expression. High p53 expression (staining observed in >25% of the tumor cells) was significantly associated with age over 1 year (p = 0.0033), neoadjuvant therapy (p = 0.0009), positive resection margin (p = 0.0008) and anaplasia (p < 0.0001). P53 expression was significantly associated with recurrence-free survival (p < 0.0001) and overall survival (p = 0.0350), with higher p53 expression associated with worse prognosis. Comparisons of concordance statistics showed no significant difference in prognostication when using morphologic types compared to p53 expression groups (p = 0.647). TP53 sequence was performed in 16 cases; the most common variant identified was a missense variant (12 cases), and in one case a frameshift truncating variant was noted. Based on these findings, we recommend performing p53 IHC in all newly diagnosed cases of types II and III PPB to further aid in risk stratification.

Poorly differentiated thyroid carcinomas (PDTC) in young individuals are rare and their clinical and histopathologic features, genetic mechanisms, and outcomes remain largely unknown. Here, we report a detailed characterization of a series of six PDTC in patients ≤21 years old defined by Turin diagnostic criteria studied for mutations and gene fusions characteristic of thyroid cancer using targeted next-generation sequencing (NGS) and whole-exome sequencing (WES). All tumors had solid, insular, or trabecular growth pattern and high mitotic rate, and five out of six tumors showed tumor necrosis. Targeted NGS assay identified somatic mutations in the DICER1 gene in five of six (83%) tumors, all of which were "hotspot" mutations encoding the metal-ion binding sites of the RNase IIIb domain of DICER1. WES was performed in five cases which confirmed all hotspot mutations and detected two tumors with additional inactivating DICER1 alterations. Of these two, one was a germline pathogenic DICER1 variant and the other had loss of heterozygosity for DICER1. No other mutations or gene fusions characteristic of adult well-differentiated thyroid cancer and PDTC (BRAF, RAS, TERT, RET/PTC, and other) were detected. On follow-up, available for five patients, three patients died of disease 8-24 months after diagnosis, whereas two were alive with no disease. The results of our study demonstrate that childhood- and adolescent-onset PDTC are genetically distinct from adult-onset PDTC in that they are strongly associated with DICER1 mutations and may herald DICER1 syndrome in a minority. As such, all young persons with PDTC may benefit from genetic counseling. Furthermore, their clinically aggressive behavior contrasts sharply with the indolent nature of the great majority of thyroid tumors with DICER1 mutations reported to date.

We report two malignant sacrococcygeal tumors in infants that were associated with pathogenic DICER1 variation. These tumors were composed of primitive neuroepithelium, embryonal rhabdomyosarcoma, and cartilage and initially diagnosed as immature teratomas. One child developed intracranial metastasis and died. The second child underwent surgery and chemotherapy and achieved complete remission. This child subsequently developed five additional DICER1-associated neoplasms by age nine. Genetic analysis revealed that both tumors harbored biallelic pathogenic DICER1 variation. We believe these cases represent another novel subtype of DICER1-associated tumor. This new entity, which we propose to call DICER1-associated presacral malignant teratoid neoplasm, may be difficult initially to distinguish from immature teratoma, but recognizing it as an entity can prompt appropriate classification as an aggressive malignancy and facilitate appropriate genetic counseling, DICER1 germline variant testing, screening, and education.

The pathogenesis of cystic nephroma of the kidney has interested pathologists for over 50 years. Emerging from its initial designation as a type of unilateral multilocular cyst, cystic nephroma has been considered as either a developmental abnormality or a neoplasm or both. Many have viewed cystic nephroma as the benign end of the pathologic spectrum with cystic partially differentiated nephroblastoma and Wilms tumor, whereas others have considered it a mixed epithelial and stromal tumor. We hypothesize that cystic nephroma, like the pleuropulmonary blastoma in the lung, represents a spectrum of abnormal renal organogenesis with risk for malignant transformation. Here we studied DICER1 mutations in a cohort of 20 cystic nephromas and 6 cystic partially differentiated nephroblastomas, selected independently of a familial association with pleuropulmonary blastoma and describe four cases of sarcoma arising in cystic nephroma, which have a similarity to the solid areas of type II or III pleuropulmonary blastoma. The genetic analyses presented here confirm that DICER1 mutations are the major genetic event in the development of cystic nephroma. Further, cystic nephroma and pleuropulmonary blastoma have similar DICER1 loss of function and 'hotspot' missense mutation rates, which involve specific amino acids in the RNase IIIb domain. We propose an alternative pathway with the genetic pathogenesis of cystic nephroma and DICER1-renal sarcoma paralleling that of type I to type II/III malignant progression of pleuropulmonary blastoma.

Embryonal rhabdomyosarcoma of the uterine cervix is an uncommon presentation of the most common soft-tissue sarcoma in the first decades of life. Unlike embryonal rhabdomyosarcoma in other anatomic sites, in which 70-80% of cases present before 9 years of age, the average age in our series of 14 cervical cases was 12.4 years (median, 13 years), with an age range of 9 months to 32 years at diagnosis. Of the 14 cases, 12 presented as a polyp at the cervical os; two patients had an infiltrative mass in the cervix without a botryoid polyp. The polyps measured 1.5-5 cm and all had the histopathological pattern of the sarcoma botryoides variant of embryonal rhabdomyosarcoma, with condensations of primitive and differentiated rhabdomyoblasts beneath the surface epithelium and around endocervical glands. Nodules of benign-appearing cartilage were present in the stroma of six cases (43%). One of the embyronal rhabdomyosarcomas from the youngest patient, 9 months old, also had a distinctive microscopic focus of immature tubular profiles in a primitive stroma; these tubules expressed epithelial and neuroendocrine markers. Two patients had a pleuropulmonary blastoma, one diagnosed 9 years before the embryonal rhabdomyosarcoma of the cervix and the other recognized synchronously. This latter 9-year old had a DICER1 germline mutation. One patient presented with hirsutism and had a Sertoli-Leydig cell tumor, an incidentally detected cervical embryonal rhabdomyosarcoma, and nodular hyperplasia of the thyroid. Although a pleuropulmonary blastoma was not documented in the latter patient, ovarian sex-cord stromal tumors and nodular hyperplasia of the thyroid are manifestations of the pleuropulmonary blastoma family tumor and dysplasia syndrome (OMIM 601200). Embryonal rhabdomyosarcoma of the cervix must be distinguished from other rare entities, including adenosarcoma, malignant mixed Mullerian tumor and low-grade stromal sarcoma, as the former has a better prognosis; 12 of our 14 patients remain disease-free following conservative surgery and chemotherapy. Our study suggests that cervical embryonal rhabdomyosarcoma may be another pathological manifestation in the spectrum of extrapulmonary pathology in the setting of pleuropulmonary blastoma., Competing Interests: Disclosure/conflict of interest The authors declare no conflict of interest.

Squamous papillomas of the lung are an uncommon feature of recurrent respiratory papillomatosis, occurring in fewer than 1% of cases. We describe a 23-year-old patient with pulmonary papillomas who developed a fatal squamous cell carcinoma of the lung. PCR-based human papillomavirus (HPV) typing showed the presence of HPV 11 DNA in both benign papillomas and invasive carcinoma. A review of the literature reveals four reports of malignant transformation of juvenile-onset recurrent respiratory papillomatosis in which HPV typing was performed. Similar clinical features are noted in all of the reports; specifically, each case has arisen in a young adult man with a history of papillomatosis since childhood. In each of the cases, HPV 11 was identified in association with the squamous cell carcinoma. Although HPV 11 is uncommonly associated with the development of invasive carcinoma at other sites, these findings suggest that it is correlated with malignant transformation in the setting of juvenile-onset recurrent respiratory papillomatosis.

Extramedullary hematopoiesis (EMH) after fetal development is uncommon and is most often seen in patients who have hematologic disorders. EMH unassociated with hematologic disease is rare. After the recent observation of EMH in a myocardial infarct, we sought to determine the frequency and clinicopathologic setting of EMH in myocardial tissues submitted for pathologic examination. Hematoxylin and eosin (H&E)-stained sections from 805 consecutive myocardial samples (207 surgical specimens, 598 autopsy specimens) were examined retrospectively. The presence of immature erythroid or myeloid cell clusters in intramyocardial capillaries or stroma was considered sufficient for the diagnosis of EMH. Immunoperoxidase studies confirming the nature of the hematopoietic cell infiltrate were performed in selected cases. Foci of EMH (often multiple) were identified in 15 of 207 surgical hearts (7.2%) and in 22 of 598 autopsy hearts (3.7%). Patient ages (exclusive of premature infants) ranged from 2 weeks to 73 years (median, 13 years). Twenty-four of 37 (65%) EMH-positive cases were associated with infarcts in various stages of repair (accounting for 11 of 68 [16.2%] of all infarcts in surgical specimens and 13 of 86 [15.1%] of infarcts in autopsy specimens). Acute infarcts less than 72 hours old, excluding those with acute extension, were not associated with EMH. Viral myocarditis and myocardial hypertrophy with fibrosis accounted for primary diagnoses in the nonischemic, EMH-positive surgical cases, whereas seven of nine nonischemic, EMH-positive autopsy cases involved premature or term infants with no obvious myocardial disease. Another autopsy patient had sarcoidosis with myelophthisic involvement of her bone marrow and represented one of only two cases overall in which a hematopoietic disorder was coexistent or suspected. Myocardial EMH is relatively common after myocardial infarct but is rarely encountered in normal or nonischemic myocardium. Its presence in healing but not early acute stages of infarct suggests that EMH results from inflammation- or repair-associated trophic factors, not from ischemia itself.