Treatment of Mediastinal Endometrial Carcinoma Developed from Extragenital Endometriosis and Simultaneous Rectal Adenocarcinoma in a 55-year-old Woman.

Background/aim: Endometriosis is a common disorder in reproductive-age women leading to a broad range of symptoms and is associated with a higher risk for endometrioid ovarian carcinoma.

Case Report: We report the case of a 55 year-old woman with previously undiagnosed endometriosis presenting with a large mediastinal cancer of unknown primary (CUP) and synchronous Union Internationale Contre le Cancer (UICC) stage II rectal adenocarcinoma. Histopathologically the mediastinal tumor resembled endometrial carcinoma and laparoscopically endometriotic lesions on the patient's peritoneum were detected. The patient was treated with neoadjuvant carboplatin and paclitaxel, followed by resection of the mediastinal tumor. After recovery, the patient received neoadjuvant short-course radiation to the rectal adenocarcinoma, which was resected afterwards. No primary endometrial carcinoma was found in the uterus, leading to the most likely conclusion that the mediastinal tumor derived from an extragenital endometriotic lesion.

Conclusion: Although rare, cases of degeneration of endometriosis have been described. In this case not only the localization of endometriosis was uncommon, but also its malignant transformation and synchronous diagnosis of a rectal adenocarcinoma, complicating diagnosis, and treatment of the patient. This rare case highlights the importance of diagnosing and treating patients with CUP or multiple malignancies at large interdisciplinary centers to reach the best possible outcome.