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1525 Background: Universal testing of resected CRC for Microsatellite Instability (MSI) status has been widely advocated. While PCR-based MSI analysis is the current gold standard test, immunohistochemistry (IHC) of mismatch repair (MMR) proteins is more commonly performed. At present, it remains un-established whether PCR-based MSI analysis remains reliable when performed on post-neoadjuvant treated specimens. Our goal was to examine the effect of neoadjuvant therapy on MSI analysis using both in vitro and clinical data. Methods: MSI analysis was performed using the standard panel of 7 markers. In vitro, MMR deficient (HCT116) and proficient (HCT116+Ch3) isogenic cell lines were exposed to 3 cycles of single agent 5-Fluorouracil, Oxaliplatin and Irinotecan and MSI analysis was tested after each cycle. To evaluate the induction of genomic instability, 3 additional coding microsatellite tracts in ATR, BLM and CHK1 were assessed by fragment analysis. In parallel, clinical data from 238 CRCs that were resected after neoadjuvant chemoradiation (n=191) or chemotherapy (n=47) between 9/2009 and 8/2011 were queried for MSI results. MSI analysis performed on paired pre-treatment and post-treatment resection tissues was compared. Results: No changes in MSI status were detected and there was no increase in genomic instability post-treatment in cell line models. In the clinical setting, pre-neoadjuvant MSI testing was limited by the availability of pre-treatment biopsy tissue. 3 patients found to be MMR proficient based on MSI analysis performed on pre-treatment biopsy remained microsatellite stable when analyzed on post-treatment resection tissue. 7 patients were MMR deficient based on MSI analysis performed on post-treatment resection tissue but most had strong clinical evidence to expect MMR deficiency. Conclusions: Based on both preclinical in vitro and clinical data, results of the MSI analysis does not appear to be affected by neoadjuvant therapy for both MMR proficient and deficient cases. In addition, preclinical results do not suggest that neoadjuvant chemoradiation induces an increase in genomic instability. Therefore, MSI analysis remains a reliable gold standard test on post-treatment specimens.

4134 Background: AA is a rare malignancy ranging from well-differentiated to poorly differentiated carcinoma, including those with signet ring cells. Optimal therapy for low grade peritoneal disease is cytoreductive surgery (CRS) combined with heated intraperitoneal chemotherapy (HIPEC). However, some patients (pts) are suboptimal for CRS/HIPEC, and are considered for systemic chemotherapy (SC) alone, or SC + CRS. In light of our previously reported overall survival (OS) benefits for the role of SC in metastatic AA, here we explore the impact of surgical intervention on OS in these pts. Our aim was to clarify the OS benefit of multidisciplinary therapy (SC + CRS + HIPEC) in those pts with aggressive tumor biology. Methods: A retrospective chart review of AA pts registered in our tumor registry between Jan. 2005 to Dec. 2009 was undertaken to identify patients with AA who received SC. Electronic medical records (EMR) were reviewed for CRS, HIPEC, histology, SC, and OS. The K-M method and Log-Rank test were used for statistical analysis. Results: Of 143 AA pts, 52 (36%) pts were high grade with 33 (23%) having signet ring cells. After a median follow-up of 35M, high grade tumors were noted to have worse OS overall (24M vs 56M, p

199 Background: The natural history, clinical presentation, and impact of cytoreductive surgery (CRS) or systemic chemotherapy for high grade MAA has been poorly studied. Methods: A retrospective chart review of patients (pts) with moderate or poorly differentiated AA was completed from 1994-2010. Radiographic response was assessed semi-quantitatively form physician records and complete cytoreductive surgery (CRS) was defined as a completeness of cytoreduction score of 0 or 1. For CRS and chemotherapy subgroups efficacy endpoints were calculated from date of CRS or first chemotherapy dose, respectively. Results: 288 (136 moderate and 152 poor grade) pts were identified with a median age of 52 years (range: 25-82). Mucinous histology was present in 110 pts (38%) and differed by grade: 55% of moderate and 23% of poor. Stage IV presentation (n=163) occurred in 76% of poor grade but only 35% of moderate grade. Both median OS (4.5 vs. 2.7 yrs, P=0.04) and median OS for the stage IV subgroup (3 vs 1.9yrs, P=0.005) were improved for MAA vs. non-MAA. Moderate compared to poor grade had improved median OS (3.7 vs 1.9 yrs, P=0.002) for Stage IV disease, but this differed by mucinous histology (moderate mucinous 6.2yr vs. poor mucinous 1.6yr, P=0.001; moderate nonmucinous 1.4yr, poor nonmucinous 2yr). 39 MAA pts underwent a complete CRS (30 moderate and 9 poor) with a median time to recurrence of 1.5yrs and median OS of 9.2yrs. Improved OS was seen for pts with a complete CRS compared to pts with an incomplete CRS, Pth edition use of a single mucinous high-grade category. The use of systemic chemotherapy appears to be a viable treatment option for these pts and a complete CRS is associated with improved OS.

526 Background: Cyclooxygenase-2 (COX-2), an inducible isoenzyme, is upregulated in inflammation and involved in cancer progression by promoting angiogenesis, cell proliferation and migration and by inhibiting apoptosis. COX-2 is implicated in colorectal tumorigenesis and by extension COX-2 expression testing and therapeutic inhibition has been considered in appendiceal adenocarcinomas (AAs). However, its role in this setting has never been well studied. The purpose of our study was to investigate COX-2 expression in AAs and to evaluate its prognostic and predictive significance. Methods: We performed a retrospective review of 607 patients with AAs treated at MD Anderson Cancer Center between 2002 and 2010. Immunohistochemistry was performed for COX-2 expression (COX2 mAb Clone CX229, Cayman Chemical) in 49 (8%) patients. Thirty (61%) stained positive and 19 (39%) showed no staining. Kaplan-Meier product limit method and log-rank test were used to estimate overall survival (OS) and to determine association between OS, COX-2 expression and other characteristics. Results: Median age at diagnosis was 47 yrs (range 34-78 yrs). Grade (P = 0.003), completeness of cytoreduction score (P = 0.010) and stage (P = 0.023) were significantly associated with OS. Median OS for patients with COX-2 positive and COX-2 negative tumors was 58.3 and 42.8 months, respectively (P = 0.232). Nine COX-2 positive patients received celecoxib (selective COX-2 inhibitor) in combination with other chemotherapy. Mean treatment duration was 5.5 months with best radiographic response being stable disease in 7 patients. Reduction in tumor markers (CEA or CA19-9) was seen in 7 cases (median reduction of 30%). Similar treatment duration (6.6 months), rate of stable disease (6/8 pts) and tumor marker reduction (7/8 pts) was seen on the preceding non-celecoxib containing chemotherapy in these patients. Median OS, with and without COX-2 inhibition, was 55.7 and 57.6 months respectively (P = 0.57). Conclusions: In this cohort, COX-2 expression is not a significant prognostic factor in AAs. Benefit from COX-2 inhibition in COX-2 expressing AAs is unclear. Current data does not support the routine use of either COX-2 testing or COX-2 inhibition therapy in AAs.

575 Background: Microsatellite instability (MSI) testing in colorectal cancer (CRC) provides prognosis, predicts chemotherapy response, and guides diagnosis of hereditary nonpolyposis colorectal cancer (HNPCC) syndrome. MSI can be sporadic or hereditary, arising from somatic or germline mutations in DNA mismatch repair (MMR) genes respectively. The clinical implications of these distinct mechanisms are uncertain. Methods: Patients who underwent MSI testing for CRC between 2000 and 2011 were identified. MSI-high (MSH) CRCs were defined by: pathogenic mutation in MMR genes; >30% of markers with allelic shift in PCR-based MSI testing; or loss of expression in at least 1 MMR protein on immunohistochemistry. The subset with MLH1 gene promoter methylation, BRAF mutation, or EPCAM mutation was considered sporadic. Clinicopathologic features and disease-free survival (DFS) were examined in reference to microsatellite stable (MSS) CRCs. Results: MSH CRC’s, 92 germline and 49 sporadic, were compared with 105 MSS CRCs. Compared to MSS CRCs, both germline and sporadic MSH CRCs more commonly arose in the proximal colon (63% and 92%, vs. 30%; p Conclusions: Patients with sporadic MSH CRCs exhibit distinct clinicopathologic features compared to those with germline MSH tumors. Despite poor prognostic features, no apparent survival difference was observed. Further characterization of these distinct groups is warranted to explain the discordance between risk factors and outcomes.

402 Background: Though histological grade is known to have a major prognostic impact in metastatic mucinous appendiceal adenocarcinomas; the prognostic impact of grade in localized disease, and the validity of the AJCC Cancer Staging Manual 7th edition decision to combine moderately and poorly differentiated mucinous adenocarcinomas into a single mucinous high-grade category, is not known. Methods: Patients with adenocarcinoma of the appendix diagnosed between 1988-2007 were identified from the SEER database. Cancer-specific survival (CSS) stratified by histological subtype, stage and grade were calculated; and Cox proportional hazards regression analyses were performed. Results: We analyzed a total of 2,469 appendiceal adenocarcinomas, of which 1,375 had mucinous histology, 860 had non-mucinous histology, and 234 had signet-ring cell histology. Though overall CSS was similar for mucinous and non-mucinous subtypes, differences in stage distribution and stage-stratified CSS were seen. Female gender (57% vs.45%, P Conclusions: The strong prognostic impact of histological grade for mucinous adenocarcinomas is primarily restricted to stage IV disease. Stage IV moderately and poorly differentiated mucinous adenocarcinomas have distinctly different CSS and this data does not support the combination of these two histological grades in the recent AJCC 7th edition.