Endometrial/Endometrioid Stromal Tumors With Extensive Whorling and CTNNB1 Translocation: A Report of 3 Cases.

Endometrial/endometrioid stromal tumors are rare and morphologically heterogenous, and their diagnosis may be challenging. We identified 3 endometrial/endometrioid stromal tumors with identical and previously undescribed histologic features and herein report their morphologic, immunohistochemical, and molecular profiles. Patients were 53, 62, and 79 years. Tumors were well-circumscribed, tan-yellow solid masses measuring 10.0, 11.0, and 18.7 cm, and were intramyometrial (n=2) or in the broad ligament (n=1). All showed small, tight whorls of epithelioid to slightly spindled tumor cells with minimal cytoplasm and negligible mitoses, multifocally associated with hyalinization and myxoid change set in a loose fibroblastic background with small, delicate vessels. This morphology was seen throughout in 1 tumor and in ∼20% and 70% of the 2 others with the remaining areas showing sex cord-like differentiation. Tumor cells expressed CD10 (3/3, 1 focal), calretinin (3/3 diffuse), WT1 (3/3 diffuse), estrogen receptor (1/1, diffuse). RNA-sequencing was successful in 1 tumor and revealed a GREB1-CTNNB1 in-frame fusion. All 3 tumors harbored a CTNNB1 translocation by fluorescence in situ hybridization correlating with nuclear β-catenin expression. Whole-genome DNA methylation analysis classified all 3 tumors within the low-grade endometrial stromal sarcoma reference class with flat copy number profiles. One patient (79-y-old) died of unrelated causes 2 months after surgery and the other 2 were alive without disease after 13 and 75 months. We have described a rare subset of endometrial/endometrioid stromal tumors with extensive whorling and a CTNNB1 translocation, expanding the morphologic and molecular spectrum of these neoplasms.