One biopsy, two tumors: A case report of collision tumor consisting in breast and endometrioid carcinoma detected on bioptic sample of peritoneal carcinomatosis

Collision tumors are rare and very few cases were described in which collision was revealed in a metastatic lesion. Herein we report a case of woman with a peritoneal carcinomatosis underwent to bioptic procedure in correspondence of a nodule of Douglas peritoneum with clinical suspect of ovarian/uterine origin. Histologic examination revealed two different colliding epithelial neoplasms: an endometrioid carcinoma and a ductal breast carcinoma, the latter not suspected at the time of biopsy. Morphology and immunohistochemistry, in particular GATA3 and PAX8, defined clearly the two different colliding carcinomas.