Endometriosis-Associated Ovarian Carcinosarcoma Featuring Well-Differentiated Adenocarcinoma and Fetal Rhabdomyoma-Like Mesenchymal Components: An Unusual Case Report-With Molecular Analysis.

Ovarian carcinosarcoma typically comprises high-grade carcinoma and sarcoma components. We report a case of a woman in her 40s with endometriosis-associated ovarian carcinosarcoma exhibiting an unusual presentation of well-differentiated adenocarcinoma and rhabdomyosarcoma. The patient underwent bilateral salpingo-oophorectomy and hysterectomy for tumors in both ovaries. Histologically, with a background of endometriosis, the epithelial component exhibited features of a borderline tumor with an intraepithelial carcinoma composed of Müllerian-type epithelium in both ovaries. In the left ovary, a very minor area of clear cell carcinoma (less than 5%) was identified. In contrast, the mesenchymal component was a well-differentiated rhabdomyosarcoma resembling fetal rhabdomyoma, constituting more than 60% of the left ovarian tumor. One year later, pulmonary metastases of the sarcomatous component were detected. Molecular analysis using next-generation sequencing identified PIK3CA H1047R (c.3140A > G) and CSF1R (c.∗1841TG > GA) in both the epithelial and mesenchymal components, indicating a clonal origin and supporting a diagnosis of primary ovarian carcinosarcoma. The patient died 29 months after surgery despite receiving platinum-based chemotherapy. This report highlights the challenges of diagnosing rare ovarian carcinosarcomas arising in endometriosis and with unusually low-grade histology, and it emphasizes the need for comprehensive pathological assessment, including molecular analysis, for accurate diagnosis and optimal management.