Female Adnexial Tumor of Probable Wolffian Origin (FATWO) without Ki-67 Expression Reflecting Low Malignant Potential in a 55-Year-Old Woman

Female adnexial tumors of probable Wolffian origin (FATWO) are rare tumors derived from the remnants of the mesonephric duct. These tumors generally exhibit a low malignant potential, but recurrence and or metastasis is possible during the course of the disease according to the tumors' possible malignant potential. We report a case of FATWO without estrogen and progesterone receptors and with negative immunostaining for Ki-67 (a proliferation marker) as a probable low-malignant-potential tumor. A 55-year-old woman presented with a complaint of heavy menstrual bleeding and pelvic pain. Pre-operative ultrasonographic evaluation revealed an intramural uterine leiomyoma of 4 cm in diameter and a right adnexial solid mass measuring 5 cm in diameter. Following total abdominal hysterectomy and bilateral salpingo-oophorectomy, immunostaining based on the labeled streptavidin-biotin method was performed on sections from representative blocks of paraffin-embedded tissues sampled from the mass, revealing a low mitotic index with negative Ki-67 immunostaining. Immunohistochemical staining with promising new markers and pathological investigation of the entire tumor are needed to determine the malignant behavior of an individual FATWO. Ki-67 is a helpful marker for determining Wolffian duct tumors' potential malignant behavior.