Extragonadal Giant Endometrial Cyst with Endometrioid Borderline Tumor

We describe an extremely rare case of a borderline tumor arising from an extragonadal giant endometrial cyst. A 41-year-old woman complaining of abdominal pain was referred to our hospital with a diagnosis of large ovarian tumor. Magnetic resonance imaging revealed a large cystic tumor approximately 27 cm × 9 cm in area. The cyst contents were largely removed by suction, and then the tumor was resected laparoscopically. Both adnexa were normal in size and location. The tumor did not originate from the ovaries, and it was adherent only to the bilateral uterosacral ligaments and uterine body. The postoperative histopathological evaluation confirmed the presence of endometrioid borderline tumor with transition from endometriosis. Staging laparotomy was performed, and no remnant tumor was detected. This case is extremely unusual because such a large cystic tumor originating from extragonadal endometriosis is very rare, as is endometrioid borderline tumor arising from endometriosis.