Transitional cell-like morphology in ovarian endometrioid carcinoma: morphologic, immunohistochemical, and behavioral features distinguishing it from high-grade serous carcinoma.

Transitional cell-like growth has been reported as a morphologic variant of endometrioid adenocarcinoma in the uterus but is not well-described in the ovary. We report the clinicopathologic features of a series of ovarian endometrioid adenocarcinomas with transitional cell-like morphology, emphasizing the distinction from its mimics, including high-grade serous carcinoma, transitional cell carcinoma, and granulosa cell tumor. Among a cohort of 71 ovarian endometrioid adenocarcinomas surgically staged at our institution, 10 tumors (14%) exhibited transitional cell-like morphology. Patient age ranged from 39 to 79 years (mean, 52 y). Five tumors were stage I, 2 were stage II, and 3 stage III. The tumors ranged from 8.5 to 23 cm, and the transitional cell-like component occupied from 5% to 90% of the overall tumor, with the remainder being conventional endometrioid adenocarcinoma. The most compelling findings to support that this tumor pattern represents a morphologic variant of endometrioid adenocarcinoma are that the transitional cell-like components (1) merged directly and seamlessly with the conventional endometrioid component; (2) contained areas of mature or immature squamous differentiation; (3) lacked WT1 immunoexpression; (4) lacked the characteristic p53/p16 immunophenotype of high-grade serous carcinoma; and (5) did not appear to independently affect patient outcome. Two patients (20%) whose tumor contained transitional cell-like morphology died, whereas 14 patients (23%) lacking this morphology died. Although uncommon, transitional cell-like morphology appears to be a variant growth pattern of ovarian endometrioid adenocarcinoma that does not affect behavior and that should be distinguished from high-grade serous carcinoma and conventional ovarian transitional cell carcinoma.