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30 Background: The use of immune checkpoint inhibitors (ICI) in MSI-H/MMRd CRC patients has profoundly changed the treatment landscape in the metastatic setting; the most recent NICHE-2 trial showed 100% response rate in MSI-H/MMRd locally advanced CRC patients treated with neoadjuvant ICI, further highlighting the importance of identifying these patients accurately to enhance patient care. Here we report the concordance of NGS-MSI and IHC-MMR from a very large cohort of CRC tumors and study the molecular characteristics and clinical outcomes of these patients. Methods: A total of 28,105 CRC tumors were analyzed by NGS (592 genes, NextSeq or WES, NovaSeq) and IHC. MMRd was defined as complete loss of ≥1 IHC stains (MLH1, MSH2, MSH6, or PMS2) and proficient (MMRp) as any positive staining for all four proteins. MSI status was determined from over 7,000 target microsatellite loci covered by NGS. Central pathology review (CPR) of MMR IHC was done on 73 cases with discordant MMR/MSI results. Real-world overall survival was obtained from insurance claims and calculated from either tissue collection or treatment start to last contact; Kaplan-Meier estimates were calculated for molecularly defined patients. Results: For CRC tumors with NGS and IHC, 28031/28105 were concordant (99.74%), 0.09% were MMRd/MSS, and 0.2% were MMRp/MSI-H. After CPR, 23/24 (96%) of MMRd/MSS cases and 46/49 (94%) of MMRp/MSI-H were confirmed. Of the 46 post-CPR MMRp/MSI-H CRC samples, 52% had ≥1 pathogenic missense mutation in including MLH1/PMS2/MSH2/6 or MLH3, MSH3, PMS1 or POLE. Of the 23 post-CPR MMRd/MSS CRC samples, 38% had MLH1 loss, 91% PMS2 loss, no MSH2 loss and 9% MSH6 loss. When comparing clinical outcomes with concordant MSS/MMRp tumors, concordant MSI-H/MMRd patients had significantly longer OS (HR=1.131 [95% CI: 1.02-1.254], p28,000 CRC tumors that the concordance of IHC-MMR/NGS-MSI is 99.74%. Clinical outcome from a large cohort of patients show NGS is not inferior compared to IHC in identifying patients with MSI-H/MMRd. The additional lens that NGS-MSI offers is of value in identifying CRC patients who may benefit from ICI therapy. [Table: see text]

e17622 Background: 50% of endometrial atypical hyperplasia (AH) cases on endometrial sampling have concurrent endometrial cancer (EC) on hysterectomy. Little data exists regarding molecular signatures that can predict the presence of concurrent endometrial cancer in patients with AH on biopsy. Therefore, we set to characterize molecular landscapes of endometrial samples diagnosed with AH with EC on the final hysterectomy specimen, using matched controls. Methods: Karmanos Cancer Institute pathology database was searched for patients with AH on endometrial curettage or biopsy who had a hysterectomy within 6 months. Only samples with adequate tissue for next generation sequencing were included. In total, 59 tissue samples from 34 patients were included: there were 15 AH endometrial biopsy samples, 18 AH final hysterectomy samples (13 matched pairs), 13 EC endometrial biopsies and 13 samples that were EC on final hysterectomy samples (12 matched pairs) that were analyzed using next-generation sequencing (WES) and/or whole transcriptome sequencing (WTS) (NovaSeq). TMB was measured by counting all somatic mutations found per tumor (TMB-high: >10 mutations per MB). Immune cell infiltrates were calculated by Quantiseq. Significance was determined using Fisher exact, Chi-square and Mann-Whitney U test and adjusted for multiple comparisons: p < 0.05 but q > 0.05 was considered a trend. Results: Clinical analysis demonstrated 15/34 patients with endometrial AH on initial sampling and EC on the final hysterectomy. 2 out of 15 patients were > stage II at time of surgery. Molecularly, fewer PTEN mutations were found between AH (2/8) and EC (14/17) on final hysterectomy (25% vs 82.4%, p = 0.01). Samples of EC origin were MSI-H by NGS-MSI (3/18) and had no mutations in PPP2R1A (0/16). Comparing hysterectomy samples, EC trended toward increased CTLA4 (FC: 6.97-fold) expression and immune cell infiltration of Macrophage M1 (+1.19%), NK cells (+2.13%), CD8+ T Cells (+1.27%), regulatory T cells (+2.17%) and Dendritic cells (+2.87%) compared to AH samples (all p < 0.05). Similarly, when comparing endometrial biopsies, EC samples trended toward increased expression of CTLA4 (12.6-fold), HAVCR2/TIM3 (FC: 2.59-fold) and IFNG (FC: 17.8-fold) immune checkpoint genes, as well as increased immune cell infiltration of Neutrophils (+11.7%), CD8+ T cells (+2.39%) and regulatory T cells (+2.43%) compared to AH samples (all p < 0.05). Conclusions: There are molecular and tumor microenvironment differences seen between AH endometrial biopsies that have concurrent EC compared to those that do not on the final hysterectomy specimens. These differences may lead to advances in identifying appropriate patients for fertility sparing treatments, versus those that can be managed surgically by a benign gynecologist or a gynecologic oncologist. Analyses with larger sample sizes are needed and are ongoing.

5580 Background: HER2 is an emerging prognostic and therapeutic target in uterine serous carcinoma (USC). Testing algorithms and platforms in breast and gastric cancers are well studied and validated, but optimal HER2 testing in uterine cancer is not yet established. We aimed to assess the concordance of chromogenic in situ hybridization (CISH), immunohistochemistry (IHC), and next generation sequencing (NGS) platforms to aid in the development of USC specific testing guidelines. We also evaluated the rate of downstream mutations that may affect response to HER2 directed therapy. Methods: A total of 2,192 USC tumors were analyzed using NGS (NextSeq, 592 Genes and WES, NovaSEQ), a subset of 1,423 tumors were also tested by IHC and CISH (Caris Life Sciences, Phoenix, AZ). HER2 positivity through IHC (4B5, Ventana) and CISH (INFORM DUAL HER2 ISH Assay, Ventana) was determined based on 2007 and 2018 ASCO/CAP HER2 breast cancer guidelines. PD-L1 expression was tested by IHC using SP142 (Spring Biosciences) (positive cut-off >1%). Microsatellite instability (MSI) was tested by fragment analysis (FA), IHC and NGS. Tumor mutational burden (TMB) was measured by totaling somatic mutations per tumor (TMB-high cut-off > 10 mutations per Mb). Statistical significance was determined using chi-square. Results: Rates of HER2 positivity were comparable using the 2018 and 2007 breast cancer guidelines (19.5% vs 17.5%; p=0.25). Based on 2018 guidelines, the concordance between IHC and CISH was 98.9%. Specifically, 229/1423 patients (16%) were IHC+/CISH+, 5 patients (0.4%) were IHC+/CISH- and 11 patients (0.8%) were IHC-/CISH+ (Table). Common pathway alterations in HER2+ USC include TP53, RTK RAS, PI3K, NOTCH, chromatin remodeling and cell cycle genes. Single gene alterations in HER2+ tumors that may implicate HER2 therapy resistance (based on pathway analyses in other tumor types) included PI3K (36%), KRAS (2.6%), and PTEN (2.1%). HER2+ tumors had low immunotherapy biomarker profiles (0.3% MSI-H, 0.8% TMB, 17.1% PD-L1). Conclusions: High concordance rates were observed between CISH and IHC. Ultimately these testing platforms need to be validated by response to HER2 targeted therapies in order to develop USC specific HER2 testing guidelines.[Table: see text]

5040 Background: The Akt-mTOR pathway is an important regulator of cell proliferation and survival, shown to be deregulated in many cancers. This study was undertaken to determine the activation st...

10604 Background: Epithelial-mesenchymal transition (EMT) is a transient process occurring during carcinogenesis that is characterized by phenotypic and molecular alterations resulting in increased...

e15531 Background: Myeloperoxidase (MPO) is a heme protein synthesized during myeloid differentiation. It is an enzymatic mediator of several inflammatory cascades. A link between inflammation and ...

e15559 Background: The diagnosis of endometrioid carcinomas, mixed type, is based upon presence of a serous component varying from 10% to 25%. This range may introduce variation and subjectivity in...

e21101 Background: Determining ER, PR and HER2/neu expression status in microinvasive carcinoma is often hampered by the lack of adequate microinvasive component on the additional tissue sections u...

To evaluate the association of radiotherapy to subsequent second cancers in endometrial carcinoma survivors.Subjects with endometrial cancer as their first malignancy were identified from the Surveillance, Epidemiology and End Results (SEER) database from 1973 to 2004 and were divided into radiation (R) and No Radiation (NR) groups. Person years at risk (PYR), observed (O) and expected (E) counts and SIR (standardized incidence ratio) were determined. (%) increase in relative risk (RR) for R vs. NR group was calculated using Poisson regression. Absolute Excess Risk (AER) was 'O' minus 'E' per 10,000 PYRs.Of 90,502 subjects 31,643 (34.9%) were in R and 52,182 (57.6%) in NR. The radiation status of 6677 (7.3%) was unknown. In the R group 4203 developed second cancers vs. 5563 in NR group; relative risk for R group-1.25 (95% CI 1.20 to 1.29). A total of 17.11 excess tumors were observed in R group (AER). The relative risk increased with latency of exposure and peaked at (10 years), being 40% (p0.001). The statistically significant (%) increase in RR for individual organs was as follows: urinary bladder (124%), vagina (88%), vulva (83%), sarcoma (70%), colon and rectum (43%) and lung (29%).In the largest study reporting on association of radiotherapy to subsequent second neoplasms in endometrial cancer survivors, the increased risk of second cancers was most pronounced in the field of exposure and was affected by latency since exposure.

16000 Background: Although the majority of women with EC are postmenopausal, 25% are under the age of 50 years at the time of diagnosis. The aim of this study is to describe the demographic and survival data for women presenting with EC at age 35 years or younger (YOY). Design: Patients with a diagnosis of EC were identified from the Surveillance, Epidemiology, and End Results (SEER) cancer database from 1988 to 2003. Demographic and pathologic data were collected. Chi-square and t-tests were used to examine differences between the younger and older women, and survival analyses were performed using Kaplan Meier and Cox regression models. Results: Of the 61,364 women diagnosed with EC in the study period, 931 patients 1.5% were 35 YOY. Compared to women 36 years of age or older at diagnosis, the young-onset women were more diverse in terms of race/ethnicity (p0.0001). Younger women were more likely to be diagnosed in earlier stages and grades compared to their older counterparts (both p No significant financial relationships to disclose.