Incidental Primary Ovarian Squamous Cell Carcinoma Arising in Endometriosis Presenting As Adenomyosis-Related Abnormal Uterine Bleeding

Adenomyosis often coexists with endometriosis, which primarily involves the ovary and can lead to endometriosis‑associated ovarian cancers, most commonly endometrioid and clear‑cell carcinomas. Ovarian squamous cell carcinoma (OSCC) arising in endometriosis is exceedingly rare, lacks established treatment guidelines, and carries a poor prognosis despite only sporadic reports in the literature. We describe a 45‑year‑old multiparous woman with a three‑month history of abnormal uterine bleeding. Speculum examination revealed a bleeding polypoid cervical mass, and transvaginal ultrasound confirmed adenomyosis. Persistent bleeding refractory to polypectomy and resultant severe anemia prompted total abdominal hysterectomy with bilateral salpingo‑oophorectomy. Unexpected histopathology and immunohistochemistry revealed a primary, poorly differentiated OSCC of the left ovary arising from endometriosis, extending into the left fallopian tube; the uterus, cervix, right adnexa, and ovarian capsule were uninvolved. The tumor was staged International Federation of Gynecology and Obstetrics IIA, and the patient received six cycles of carboplatin plus paclitaxel. At 15 months postoperatively, she remains disease‑free. This case underscores how adenomyosis can mask an underlying ovarian malignancy and illustrates the rare occurrence of OSCC in endometriosis. Awareness of such presentations should prompt thorough histological evaluation, and we emphasize the urgent need to develop treatment protocols for rare, aggressive histotypes. We also review the existing literature to contextualize this uncommon pathology and its clinical implications.