Case Report: A rare case of serous cystadenocarcinoma of the ovary with a benign teratoma in the other ovary.

Synchronous ovarian tumors of different histogenesis are rare. High-grade serous carcinoma (HGSC) is an aggressive epithelial malignancy that primarily affects postmenopausal women, whereas mature cystic teratoma is a benign germ cell tumor usually seen in younger patients. Their occurrence in contralateral ovaries poses significant diagnostic challenges. A 57-year-old postmenopausal woman presented with abdominal distension, discomfort, and ascites. Imaging revealed a solid-cystic right ovarian mass with peritoneal involvement and a left adnexal lesion showing fat-fluid levels suggestive of a dermoid cyst. CA-125 was markedly elevated, and ascitic cytology confirmed malignant epithelial cells. She underwent a hysterectomy with bilateral salpingo-oophorectomy and omentectomy. Histopathology showed high-grade serous carcinoma in the right ovary with tubal and cervical stromal invasion, while the left ovary contained a benign mature cystic teratoma. Immunohistochemistry supported Müllerian origin (WT-1+, PAX8+, p53 mutant pattern, and high Ki-67). The tumor was staged as FIGO IIIC. Postoperative platinum-based chemotherapy resulted in significant clinical improvement, CA-125 reduction, and no recurrence at 9 months. This synchronous presentation underscores the importance of correlating imaging, cytology, extensive sampling, and immunohistochemistry to distinguish independent tumors from bilateral carcinoma. Accurate diagnosis enables appropriate oncologic management while avoiding overtreatment of benign lesions.