Endometrioid adenocarcinoma associated with endometrial stromal sarcoma: A rare, often unrecognized collision tumor

•Endometrioid adenocarcinoma and endometrial stromal sarcoma should be classified as collision tumors.•They should be differentiated from carcinosarcoma due to their different behavior and treatment.

We are reporting 3 cases of the uterine corpus with collision of endometrioid adenocarcinoma (EAC) with endometrial stromal sarcoma (ESS). The patients' ages ranged from 36 to 59 years old. The major clinical presentation was abnormal uterine bleeding. Microscopically, all 3 cases presented with 2 separate components, EAC Grade 1 and ESS (one low grade and two high grades). The EAC component ranged from 10% to 70%, and the ESS component ranged from 30% to 70% of total tumor volume. The EAC component was stage 1A in two cases and stage II in one case. The ESS component was stages IA, IIB, and IIIB. Adjuvant hormonal therapy was administrated to one patient while a second patient was treated with chemo/radiation therapy. Two patients were still alive with no evidence of disease at 4 years post-therapy. One patient was lost for follow-up. Collision tumor should be distinguished from carcinosarcoma due to its different treatment modality, outcome and, prognosis.