Malignant Transformation of Endometriosis into Endometrioid Ovarian Carcinoma with a Contralateral Borderline Seromucinous Tumour: A Rare Case Report

ABSTRACT Background Endometriosis-associated ovarian cancer is uncommon and is most often represented by endometrioid and clear cell carcinomas. Synchronous contralateral borderline ovarian tumours are uncommon and may be missed when gross intraoperative assessment is reassuring. Case A 43-year-old woman with a confirmed diagnosis of endometriosis for approximately 5 years presented with worsening pelvic pain and menstrual symptoms. A left ovarian endometrioma increased from 6.1 × 6.8 × 5.2 cm in April 2024 to 8.8 × 9 × 9.5 cm in April 2025, with subsequent MRI showing a complex mass with solid components (O-RADS MRI 5). CA-125 was 408.6 U/mL and CA 19-9 was 643.58 U/mL; RMI-I was 1225.8 (U=3, M=1, CA-125=408.6). Staging laparotomy was performed. Histopathology demonstrated grade 2 endometrioid adenocarcinoma arising in an endometriotic cyst of the left ovary, FIGO stage IC1, while the grossly normal-appearing right ovary contained a borderline seromucinous tumour with endometrioid differentiation. The omentum, peritoneal cytology, and all 31 sampled lymph nodes were negative for malignancy. Immunohistochemistry supported endometrioid carcinoma. Conclusion This case highlights the importance of interval change and suspicious imaging features in an endometrioma, the limitations of gross intraoperative assessment of the contralateral ovary, and the value of complete histopathological examination in identifying synchronous bilateral ovarian pathology.