A collision tumor is a condition in which two distinct tumors exist within the same organ, and the interface does not exhibit overlapping histological features.

We present a case of a 59-year-old postmenopausal woman who experienced abdominal distension for six months. After a comprehensive evaluation that included her medical history, physical examination, laboratory tests, and imaging studies, two potential diagnoses were considered: either early-stage epithelial ovarian cancer or a collision tumor. She underwent a right adnexectomy, followed by a frozen section. The frozen section histopathology indicated two differentials: a mature cystic teratoma and a serous cystadenoma with focal xanthogranulomatous changes. Considering the ORADS 4 lesion and her age, surgery was preceded by peritoneal wash cytology, total hysterectomy with left adnexectomy, pelvic and paraaortic lymph node sampling, and infracolic omentectomy. The rest of the pelvic and abdominal systemic examination was normal. Final histopathology revealed a case of a sero-mucinous tumor alongside a mature dermoid cyst.

This case represents a rare instance of true mixed and collision tumors occurring within the same ovary.