Redefining the standard of care for low-grade serous ovarian cancer.

Low-grade serous carcinoma is a rare epithelial ovarian cancer subtype with distinct clinical, histologic, and molecular features. Improved understanding of this disease subtype has prompted recent advances in treatment options. Although low-grade serous carcinoma historically has been treated following a high-grade serous carcinoma paradigm, new data have called into question the utility of platinum retreatment, addressed the possibility of first-line hormonal treatment, and brought forth therapeutic options targeting the MAPK pathway and cyclin D kinase in low-grade tumors. Ongoing research efforts seek to leverage the unique features of low-grade serous carcinoma to refine treatment options for patients with this rare tumor subtype.

Nivolumab for mismatch-repair-deficient or hypermutated gynecologic cancers: a phase 2 trial with biomarker analyses.

Programmed death-1 (PD-1) inhibitors are approved for therapy of gynecologic cancers with DNA mismatch repair deficiency (dMMR), although predictors of response remain elusive. We conducted a single-arm phase 2 study of nivolumab in 35 patients with dMMR uterine or ovarian cancers. Co-primary endpoints included objective response rate (ORR) and progression-free survival at 24 weeks (PFS24). Secondary endpoints included overall survival (OS), disease control rate (DCR), duration of response (DOR) and safety. Exploratory endpoints included biomarkers and molecular correlates of response. The ORR was 58.8% (97.5% confidence interval (CI): 40.7-100%), and the PFS24 rate was 64.7% (97.5% one-sided CI: 46.5-100%), meeting the pre-specified endpoints. The DCR was 73.5% (95% CI: 55.6-87.1%). At the median follow-up of 42.1 months (range, 8.9-59.8 months), median OS was not reached. One-year OS rate was 79% (95% CI: 60.9-89.4%). Thirty-two patients (91%) had a treatment-related adverse event (TRAE), including arthralgia (n = 10, 29%), fatigue (n = 10, 29%), pain (n = 10, 29%) and pruritis (n = 10, 29%); most were grade 1 or grade 2. Ten patients (29%) reported a grade 3 or grade 4 TRAE; no grade 5 events occurred. Exploratory analyses show that the presence of dysfunctional (CD8+PD-1+) or terminally dysfunctional (CD8+PD-1+TOX+) T cells and their interaction with programmed death ligand-1 (PD-L1)+ cells were independently associated with PFS24. PFS24 was associated with presence of MEGF8 or SETD1B somatic mutations. This trial met its co-primary endpoints (ORR and PFS24) early, and our findings highlight several genetic and tumor microenvironment parameters associated with response to PD-1 blockade in dMMR cancers, generating rationale for their validation in larger cohorts.ClinicalTrials.gov identifier: NCT03241745 .

Molecular and pathologic data to guide selection of patients with endometrioid endometrial cancer for ovarian preservation.

OBJECTIVES: To investigate the association of molecular and pathologic factors with concurrent or recurrent ovarian disease to guide ovarian preservation in endometrioid endometrial cancer. METHODS: Patients with endometrial cancer ≤50 years of age at diagnosis were grouped by elective oophorectomy versus ovarian preservation at staging (January 2010 to June 2021). Tumors were stratified by molecular sub-type and CTNNB1 mutational status with next generation sequencing and immunohistochemistry. Germline data identified patients with Lynch syndrome. Associations between molecular/pathologic features and concurrent ovarian disease in patients electing oophorectomy were compared with the Wilcoxon rank-sum and Fisher's exact tests. Associations with isolated ovarian recurrences in patients who chose ovarian preservation were examined using survival analyses. RESULTS: Among 317 patients with endometrial cancer who underwent bilateral oophorectomy, 27 (9%) had malignant ovarian tumors, of whom 11 (41%) had no gross ovarian involvement on intra-operative survey. For patients with sequencing, concurrent malignant ovarian tumors were diagnosed in 0/14 (0%) POLE, 2/48 (4%) copy number-low/no specific molecular profile, 10/22 (45%) microsatellite instability-high, and 3/6 (50%) copy number-high/TP53abnormal patients (p<0.001). Concurrent malignant ovarian tumors were present in 1/30 (3%) hotspot CTNNB1-mutated versus 10/60 (17%) wildtype/CTNNB1 non-hotspot mutated endometrial cancer patients (p=0.11) and 7/28 (25%) Lynch versus 7/74 (9%) non-Lynch syndrome patients (p=0.06). Concurrent malignant ovarian tumors were present in patients with higher grade endometrial cancer (5% grade 1 vs 20% grade 2 and 24% grade 3; p<0.001), present versus absent lymphovascular space invasion (20% vs 6%; p=0.004), positive versus negative pelvic washings (28% vs 7%; p=0.016), and ≥50% versus <50% myoinvasion (24% vs 7%; p=0.004). Of 103 patients who chose ovarian preservation, four had isolated ovarian recurrences (two had high-risk pathologic features and two had high-risk molecular features). CONCLUSIONS: The integration of molecular and pathologic data may improve risk stratification of pre-menopausal patients with endometrial cancer and enhance candidate selection for ovarian preservation.

Pathogenic germline variants in patients with endometrial cancer of diverse ancestry.

BACKGROUND: Racial disparities in outcomes exist in endometrial cancer (EC). The contribution of ancestry-based variations in germline pathogenic variants (gPVs) is unknown. METHODS: Germline assessment of ≥76 cancer predisposition genes was performed in patients with EC undergoing tumor-normal Memorial Sloan Kettering Cancer Center Integrated Mutation Profiling of Actionable Cancer Targets sequencing from January 1, 2015 through June 30, 2021. Self-reported race/ethnicity and Ashkenazi Jewish ancestry data classified patients into groups. Genetic ancestry was inferred from Memorial Sloan Kettering Cancer Center Integrated Mutation Profiling of Actionable Cancer Targets. Rates of gPV and genetic counseling were compared by ancestry. RESULTS: Among 1625 patients with EC, 216 (13%) had gPVs; 15 had >1 gPV. Rates of gPV varied by self-reported ancestry (Ashkenazi Jewish, 40/202 [20%]; Asian, 15/124 [12%]; Black/African American (AA), 12/171 [7.0%]; Hispanic, 15/124 [12%]; non-Hispanic (NH) White, 129/927 [14%]; missing, 5/77 [6.5%]; p = .009], with similar findings by genetic ancestry (p < .001). We observed a lower likelihood of gPVs in patients of Black/AA (odds ratio [OR], 0.44; 95% CI, 0.22-0.81) and African (AFR) ancestry (OR, 0.42; 95% CI, 0.18-0.85) and a higher likelihood in patients of Ashkenazi Jewish genetic ancestry (OR, 1.62; 95% CI; 1.11-2.34) compared with patients of non-Hispanic White/European ancestry, even after adjustment for age and molecular subtype. Somatic landscape influenced gPVs with lower rates of microsatellite instability-high tumors in patients of Black/AA and AFR ancestry. Among those with newly identified gPVs (n = 114), 102 (89%) were seen for genetic counseling, with lowest rates among Black/AA (75%) and AFR patients (67%). CONCLUSIONS: In those with EC, gPV and genetic counseling varied by ancestry, with lowest rates among Black/AA and AFR patients, potentially contributing to disparities in outcomes given implications for treatment and cancer prevention. PLAIN LANGUAGE SUMMARY: Black women with endometrial cancer do worse than White women, and there are many reasons for this disparity. Certain genetic changes from birth (mutations) can increase the risk of cancer, and it is unknown if rates of these changes are different between different ancestry groups. Genetic mutations in 1625 diverse women with endometrial cancer were studied and the lowest rates of mutations and genetic counseling were found in Black and African ancestry women. This could affect their treatment options as well as their families and may make disparities worse.

Molecular subtyping in endometrial cancer: A promising strategy to guide fertility preservation.

OBJECTIVES: To investigate the association of molecular subtype with progesterone response in patients with endometrial cancer (EC) or atypical endometrial hyperplasia (AEH). METHODS: Premenopausal patients aged ≤48 years with tumor-normal sequencing data who received progesterone for EC/AEH from 1/1/2010-6/30/2021 were identified. Tumors were classified as POLE-ultramutated, microsatellite instability-high (MSI-H), copy number-high (CN-H), or copy number-low (CN-L) molecular subtype. Best response to progesterone was compared by subtype. Appropriate statistical tests were performed. RESULTS: Of 20 patients, 7 (35%) had AEH and 13 (65%) had EC. Sixteen tumors (80%) were CN-L, 3 (15%) were MSI-H, and 1 (5%) was POLE-ultramutated. Median time on progesterone was 22 months (range, 3-115). Ten patients (50%) had complete response (CR); median time to CR was 9 months (range, 3-32). Four patients (20%) had stable disease (SD) and 6 (30%) had progressive disease (PD). For CN-L tumors, 10 patients (62%) had CR, 3 (19%) had SD, and 3 (19%) had PD. For MSI-H tumors, 1 patient (33%) had SD and 2 (66%) had PD. For POLE-ultramutated tumors, 1 patient had PD. Median follow-up was 48 months (range, 12-123). Four of 10 patients (40%) with CR recurred; median time from CR to recurrence was 16 months (range, 5-102). CONCLUSION: Molecular subtype may be associated with progesterone response in patients with EC/AEH. CN-L tumors had the best response, and MSI-H tumors had the poorest. Recurrence after CR is common, and close surveillance is warranted. Larger studies investigating the role of molecular classification in medical management of EC/AEH are needed.

Leave it in the past - primary treatment modality for high-grade epithelial ovarian cancer is not associated with secondary cytoreduction outcomes: A Memorial Sloan Kettering Cancer Center Team Ovary study.

OBJECTIVES: To compare outcomes of patients with high-grade epithelial ovarian cancer (EOC) who underwent secondary cytoreduction surgery (SCS) after up-front treatment with neoadjuvant chemotherapy followed by interval debulking surgery (NACT-IDS) versus primary debulking surgery (PDS). METHODS: Patients with high-grade EOC who underwent SCS from 2/1/2004-10/31/2021 were classified by up-front treatment. Clinical and treatment characteristics were compared between cohorts. Progression-free survival (PFS2) and overall survival (OS2) following SCS were compared using a Cox model adjusted for stage, age at SCS, and number of years between end of chemotherapy and SCS. RESULTS: Of 374 patients, 62 (17%) underwent NACT-IDS and 312 (83%) PDS. Justification for NACT was disease extent (n = 57, 92%), comorbidities (n = 3, 5%), and thromboembolism (n = 2, 3%). The NACT-IDS cohort had a higher median age at SCS (64 years [IQR: 56-70] vs 59 years [IQR: 53-66]; P = .03), higher proportion of stage III/IV disease (100% vs 81%; P < .001), and shorter median interval between end of chemotherapy and SCS (1.5 years [IQR: 1.1-2.3] vs 1.9 years [IQR: 1.3-3.1]; P = .01). Achievement of complete gross resection at SCS did not differ between NACT-IDS and PDS (84% vs 88%; P = .18). PFS2 (HR: 1.19, 95% CI: 0.83-1.71) and OS2 (HR: 0.96, 95% CI: 0.57-1.63) did not vary by primary treatment modality after adjusting for clinically relevant covariates. CONCLUSIONS: Despite more extensive disease at presentation, patients with high-grade EOC who recur after NACT-IDS seem to have similar surgical and survival outcomes after SCS compared to patients who recur after PDS, suggesting that prior NACT-IDS should not preclude SCS.

The highs and lows of serous ovarian cancer.

Low-grade serous ovarian cancer was initially described as a distinct type of rare epithelial ovarian cancer 20 years ago; however, only recently have physicians begun to leverage the understanding of the clinical behavior and molecular profile of this disease for treatment. The use of routine next-generation sequencing has allowed a deeper understanding of the molecular drivers of this disease and shown how molecular alterations in mitogen-activated protein kinase pathway genes such as KRAS and BRAF can affect overall prognosis and disease behavior. The use of targeted therapies, including MEK inhibitors, BRAF kinase inhibitors, and other investigational targeted therapies are changing the way this disease is viewed and treated. In addition, endocrine therapy can provide prolonged disease stability with generally mild toxicity, as well as promising response rates in recent studies examining combination therapy with CDK 4/6 inhibitors in the upfront and recurrent setting. Once seen merely as a chemo-resistant form of ovarian cancer, recent studies have worked to harness the unique features of low-grade serous ovarian cancer to provide individualized treatment options for patients with this disease.

Prevention over screening for ovarian cancer in patients with high-risk germline mutations: Misinterpreting the findings of ALDO.

In November 2022, the findings of the Avoiding Late Diagnosis of Ovarian cancer (ALDO) study were published. Subsequent media coverage suggested that investigators had found a safe alternative to risk-reducing bilateral salpingoophorectomy (rrBSO) in patients with pathogenic BRCA1 and BRCA2 germline mutations who chose to decline or defer risk-reducing surgery. Unfortunately, this media coverage was largely misleading. Specifically, in the ALDO trial, 4 of 6 patients found to have ovarian cancer by the ALDO screening methodology were diagnosed with advanced-stage disease. The primary endpoint of the ALDO study was the rate of complete surgical cytoreduction, rather than stage at diagnosis or overall survival, which is an inappropriate surrogate for benefit in a population at risk of ovarian cancer. The ALDO trial again demonstrates that screening women at high-risk of ovarian cancer should not be considered a safe alternative to risk-reducing surgery, and can lead to false reassurance and the development of preventable cases of ovarian cancer. While we should continue to investigate new screening options, future efforts should largely focus on why patients decline rrBSO in the first place and how we can pivot our efforts to better address concerns related to rrBSO, including sequelae of surgical menopause. Furthermore, as we continue to understand the role of the fallopian tube in the epithelial ovarian cancer (EOC) disease process, we must identify the role of salpingectomy alone in prevention of EOC.

Chemotherapy response in low-grade serous ovarian carcinoma at a comprehensive cancer center: Readdressing the roles of platinum and cytotoxic therapies.

BACKGROUND: Data on platinum sensitivity of low-grade serous ovarian carcinoma (LGSOC) in the upfront setting is lacking, and there is limited and contradictory information on chemotherapy responses in recurrent disease. METHODS: Patients with LGSOC seen at a comprehensive cancer center from January 1, 1998 to September 30, 2021 were identified from institutional databases. Response to neoadjuvant chemotherapy (NACT) or adjuvant platinum-based chemotherapy and to second- to fifth-line regimens was retrospectively characterized by Response Evaluation Criteria in Solid Tumors (RECIST) v1.1. Wilcoxon rank-sum and two-tailed Fisher exact tests were employed. RESULTS: Of 50 patients, 12 received platinum doublets for suboptimal residual disease and 11 as NACT. Of 12 patients with suboptimal residual disease, seven (58%) achieved objective responses (five partial responses [PRs] and two complete responses); of the 11 patients who underwent NACT, one (9%) achieved a PR (p = .027). The 15 remaining patients had stable disease on first-line platinum chemotherapy. Of 44 patients who recurred, 20 had RECIST-evaluable responses to second-line and 27 to third-line chemotherapy. Objective response rates to platinum-based chemotherapy were 22% (two of nine) in the second line and 10% (one of 10) in the third. In second and third lines, highest response rates were observed with nonplatinum chemotherapy with bevacizumab, at 100% (two of two) and 30% (three of 10), respectively. CONCLUSIONS: Primary platinum-based chemotherapy has moderate activity in LGSOC and minimal activity in the recurrent setting, suggesting standard definitions of platinum sensitivity may not apply in LGSOC. In the second and third lines, nonplatinum chemotherapy/bevacizumab elicited the highest response rates.

Phase I Study of a Multivalent WT1 Peptide Vaccine (Galinpepimut-S) in Combination with Nivolumab in Patients with WT1-Expressing Ovarian Cancer in Second or Third Remission.

We examined the safety and immunogenicity of sequential administration of a tetravalent, non-HLA (human leukocyte antigen) restricted, heteroclitic Wilms' Tumor 1 (WT1) peptide vaccine (galinpepimut-S) with anti-PD-1 (programmed cell death protein 1) nivolumab. This open-label, non-randomized phase I study enrolled patients with WT1-expressing ovarian cancer in second or third remission from June 2016 to July 2017. Therapy included six (every two weeks) subcutaneous inoculations of galinpepimut-S vaccine adjuvanted with Montanide, low-dose subcutaneous sargramostim at the injection site, with intravenous nivolumab over 12 weeks, and up to six additional doses until disease progression or toxicity. One-year progression-free survival (PFS) was correlated to T-cell responses and WT1-specific immunoglobulin (Ig)G levels. Eleven patients were enrolled; seven experienced a grade 1 adverse event, and one experienced a grade ≥3 adverse event considered a dose-limiting toxicity. Ten (91%) of eleven patients had T-cell responses to WT1 peptides. Seven (88%) of eight evaluable patients had IgG against WT1 antigen and full-length protein. In evaluable patients who received >2 treatments of galinpepimut-S and nivolumab, the 1-year PFS rate was 70%. Coadministration of galinpepimut-S and nivolumab demonstrated a tolerable toxicity profile and induced immune responses, as indicated by immunophenotyping and WT1-specific IgG production. Exploratory analysis for efficacy yielded a promising 1-year PFS rate.

Comprehensive analysis of germline drivers in endometrial cancer.

BACKGROUND: We sought to determine the prevalence of germline pathogenic variants (gPVs) in unselected patients with endometrial cancer (EC), define biallelic gPVs within tumors, and describe their associations with clinicopathologic features. METHODS: Germline assessment of at least 76 cancer predisposition genes was performed in patients with EC undergoing clinical tumor-normal Memorial Sloan Kettering-Integrated Mutation Profiling of Actionable Cancer Targets (MSK-IMPACT) sequencing from January 1, 2015, to June 30, 2021. In patients with gPVs, biallelic alterations in ECs were identified through analysis of loss of heterozygosity and somatic PVs. Clinicopathologic variables were compared using nonparametric tests. RESULTS: Of 1625 patients with EC, 216 (13%) had gPVs, and 15 patients had 2 gPVs. There were 231 gPVs in 35 genes (75 [32%] high penetrance; 39 [17%] moderate penetrance; and 117 [51%] low, recessive, or uncertain penetrance). Compared with those without gPVs, patients with gPVs were younger (P = .002), more often White (P = .009), and less obese (P = .025) and had differences in distribution of tumor histology (P = .017) and molecular subtype (P < .001). Among 231 gPVs, 74 (32%) exhibited biallelic inactivation within tumors. For high-penetrance gPVs, 63% (47 of 75) of ECs had biallelic alterations, primarily affecting mismatch repair (MMR) and homologous recombination related genes, including BRCA1,BRCA2, RAD51D, and PALB2. Biallelic inactivation varied across molecular subtypes with highest rates in microsatellite instability-high (MSI-H) or copy-number (CN)-high subtypes (3 of 12 [25%] POLE, 30 of 77 [39%] MSI-H, 27 of 60 [45%] CN-high, 9 of 57 [16%] CN-low; P < .001). CONCLUSIONS: Of unselected patients with EC, 13% had gPVs, with 63% of gPVs in high-penetrance genes (MMR and homologous recombination) exhibiting biallelic inactivation, potentially driving cancer development. This supports germline assessment in EC given implications for treatment and cancer prevention.

Predicting outcomes in female germ cell tumors using a modified International Germ Cell Cancer Collaborative Group classification system to guide management.

OBJECTIVE: We previously developed preoperative and pre-chemotherapy modified versions of the male International Germ Cell Cancer Collaborative Group (IGCCCG) prognostic model and assessed it in female patients with germ cell tumors (GCTs). We sought to validate these modified IGCCCG (mIGCCCG) models in a new cohort. METHODS: We queried institutional databases for female patients with GCTs treated at Memorial Sloan Kettering Cancer Center from 1/1/1990-6/1/2020. The mIGCCCG model classifies patients with non-dysgerminomas as good, intermediate, or poor risk based on tumor markers using male IGCCCG cutoffs and absence/presence of non-pulmonary/peritoneal visceral metastasis. In dysgerminomas, good- and intermediate-risk groups are defined by absence/presence of non-pulmonary/peritoneal visceral metastasis. Progression-free survival (PFS) and overall survival (OS) were estimated for each group in the validation and combined original and validation cohorts. Associations between individual clinical factors and outcomes were evaluated. RESULTS: Among 183 female patients with GCTs, clinical characteristics and outcomes were similar between the original (n = 93) and validation (n = 90) cohorts. In multivariable models, higher stage, older age, and non-dysgerminoma histology predicted worse PFS and OS (p < 0.05). Among 162 patients who received chemotherapy, preoperative and pre-chemotherapy mIGCCCG models were significantly associated with PFS and OS (p < 0.001 for all groups). With the preoperative model, 3-year PFS rates were 94%, 76%, and 50% in the good-, intermediate-, and poor-risk patients, respectively; OS rates were 96%, 86%, and 52%, respectively. Even within stage groups, mIGCCCG risk classifications were associated with clinical outcomes. CONCLUSIONS: A female-specific mIGCCCG risk model effectively stratifies patients and should be incorporated into clinical trials.

Validation of claims-based algorithms to identify non-live birth outcomes.

PURPOSE: Perinatal epidemiology studies using healthcare utilization databases are often restricted to live births, largely due to the lack of established algorithms to identify non-live births. The study objective was to develop and validate claims-based algorithms for the ascertainment of non-live births. METHODS: Using the Mass General Brigham Research Patient Data Registry 2000-2014, we assembled a cohort of women enrolled in Medicaid with a non-live birth. Based on ≥1 inpatient or ≥2 outpatient diagnosis/procedure codes, we identified and randomly sampled 100 potential stillbirth, spontaneous abortion, and termination cases each. For the secondary definitions, we excluded cases with codes for other pregnancy outcomes within ±5 days of the outcome of interest and relaxed the definitions for spontaneous abortion and termination by allowing cases with one outpatient diagnosis only. Cases were adjudicated based on medical chart review. We estimated the positive predictive value (PPV) for each outcome. RESULTS: The PPV was 71.0% (95% CI, 61.1-79.6) for stillbirth; 79.0% (69.7-86.5) for spontaneous abortion, and 93.0% (86.1-97.1) for termination. When excluding cases with adjacent codes for other pregnancy outcomes and further relaxing the definition, the PPV increased to 80.6% (69.5-88.9) for stillbirth, 86.6% (80.5-91.3) for spontaneous abortion and 94.9% (91.1-97.4) for termination. The PPV for the composite outcome using the relaxed definition was 94.4% (92.3-96.1). CONCLUSIONS: Our findings suggest non-live birth outcomes can be identified in a valid manner in epidemiological studies based on healthcare utilization databases.

Management of patients with early-stage ovarian clear cell carcinoma: risk stratification and fertility conservation.

OBJECTIVE: We sought to describe clinicopathologic and treatment factors associated with oncologic outcomes in patients with early-stage ovarian clear cell carcinoma undergoing complete staging and in a sub-set of these patients undergoing fertility-conserving surgery. METHODS: We retrospectively identified patients with ovarian clear cell carcinoma initially treated at our institution from January 1, 1996 to March 31, 2020. Survival was estimated using Kaplan-Meier curves and compared by log-rank test. Survival-associated variables were identified by Cox proportional hazards regression. RESULTS: Of 182 patients, mismatch repair and p53 protein expression were assessed by immunohistochemistry on 82 and 66 samples, respectively. There were no significant differences in progression-free survival or overall survival between mismatch repair-deficient (n=6, including 4 patients with Lynch syndrome; 7.3%) and mismatch repair-proficient patients, whereas aberrant p53 expression (n=3; 4.5%) was associated with worse progression-free (p<0.001) and overall survival (p=0.01). Patients with stage IA/IC1 disease had a 95% 5-year overall survival rate (95% CI 88% to 98%); patients with stage IC2/IC3 disease had a similar 5-year overall survival rate (76%; 95% CI 54% to 88%) to that of patients with stage IIA/IIB disease (82%; 95% CI 54% to 94%). There was no difference in 5-year overall survival in patients with stage IA/IC1 undergoing chemotherapy versus observation (94% vs 100%). Nine patients underwent fertility-sparing surgery and none experienced recurrence. Of five patients who pursued fertility, all had successful pregnancies. CONCLUSIONS: In patients with completely staged ovarian clear cell carcinoma, those with stage IA/IC1 disease have an excellent prognosis, regardless of chemotherapy. Aberrant p53 expression may portend worse outcomes. Additional investigation is warranted on the safety of fertility conservation in patients with stage IA/IC1 disease.

Microsatellite Instability-High Endometrial Cancers with MLH1 Promoter Hypermethylation Have Distinct Molecular and Clinical Profiles.

PURPOSE: Microsatellite instability-high (MSI-H) endometrial carcinomas are underpinned by distinct mechanisms of DNA mismatch repair deficiency (MMR-D). We sought to characterize the clinical and genetic features of MSI-H endometrial cancers harboring germline or somatic mutations in MMR genes or MLH1 promoter hypermethylation (MLH1ph). EXPERIMENTAL DESIGN: Of > 1,100 patients with endometrial cancer that underwent clinical tumor-normal sequencing, 184 had MSI-H endometrial cancers due to somatic MMR mutations or MLH1ph, or harbored pathogenic germline MMR mutations. Clinicopathologic features, mutational landscape, and tumor-infiltrating lymphocyte (TIL) scores were compared among MMR-D groups using nonparametric tests. Log-rank tests were used for categorical associations; Kaplan-Meier method and Wald test based on Cox proportional hazards models were employed for continuous variables and survival analyses. RESULTS: Compared with patients with germline (n = 25) and somatic (n = 39) mutations, patients with MLH1ph endometrial cancers (n = 120) were older (P < 0.001), more obese (P = 0.001) and had more advanced disease at diagnosis (P = 0.025). MLH1ph endometrial cancers were enriched for JAK1 somatic mutations as opposed to germline MMR-D endometrial cancers which showed enrichment for pathogenic ERBB2 mutations. MLH1ph endometrial cancers exhibited lower tumor mutational burden and TIL scores compared with endometrial cancers harboring germline or somatic MMR mutations (P < 0.01). MLH1ph endometrial cancer patients had shorter progression-free survival (PFS) on univariate analysis, but in multivariable models, stage at diagnosis remained the only predictor of survival. For stage I/II endometrial cancer, two-year PFS was inferior for patients with MLH1ph endometrial cancers compared with germline and somatic MMR groups (70% vs. 100%, respectively). CONCLUSIONS: MLH1ph endometrial cancers likely constitute a distinct clinicopathologic entity compared with germline and somatic MMR-D ECs with potential treatment implications.