Dedifferentiation in Breast Metastasis of Endometrial Carcinoma: A Diagnostic Dilemma.

Most breast tumors are primary to this site; breast metastasis of endometrial origin is extremely rare. Low-grade endometrioid endometrial carcinomas can undergo dedifferentiation to undifferentiated carcinoma but such transformation at a metastatic site has been reported previously in only 2 cases. We report a case of dedifferentiation occurring in an isolated solitary breast metastasis of a low-grade endometrioid endometrial carcinoma. A 64-yr-old woman presented with a breast mass 2 yr after initial diagnosis of a grade 1 FIGO stage IIIA endometrioid endometrial carcinoma. Ultrasound guided biopsy of the breast mass showed a grade 1 endometrioid carcinoma which was diffusely estrogen receptor and PAX8-positive, consistent with metastasis from the previous endometrial carcinoma. The tumor initially responded to Letrozole therapy but then abruptly increased in size. Mastectomy revealed a poorly differentiated malignant tumor with morphology and immunophenotype (including loss of ARID1A and ARID1B immunoreactivity) consistent with undifferentiated endometrial carcinoma with no residual low-grade component. Awareness of the phenomenon of dedifferentiation of endometrial carcinoma in a metastatic site is important to avoid misdiagnosis as a primary breast cancer or metastasis from another primary site.