Introduction/Background Extraperitoneal endometriosis located in the abdominal wall is usually associated to a history of gynecological surgeries, with a reported incidence of malignant conversion of 0.7–1%. The most common histological types of endometriosis associated malignant transformation are endometrioid adenocarcinoma and clear cell adenocarcinoma. To our knowledge, only 30 cases of abdominal wall endometriosis with malignant transformation to clear cell adenocarcinoma have been reported. There are no published reports on borderline tumors originated from extraperitoneal endometriosis. The objective is to report a patient with abdominal wall endometriosis with malignant transformation to a borderline clear cells tumor.

Methodology 49 year old female with a history of a painful abdominal mass, with an abdominal-pelvic computed tomography that reported a multilobulated 6 cm mass with internal septums and mural nodules. A percutaneous biopsy of the mass reported endometriosis. She underwent laparotomy revealing a 7x5 cm solid mass which infiltrated the rectus abdominis sheat and muscle extending to the peritoneum and the anterior wall of the bladder. The final pathology reported endometriosis with extensive atypical tubular proliferation. Inmunohistochemistry stainings were consistent with a borderline clear cell tumor originating from endometriosis.

Results A laparoscopic hysterectomy, bilateral salpingoophorectomy and omentectomy was performed to rule out a primary ovarian neoplasia. Final pathology report was negative for malignancy. The patient has been under surveillance for 7 months without clinical findings indicative of recurrence.

Conclusion Extraperitoneal endometriosis located in the abdominal wall has an incidence of 0.3.-3.5% and it rarely undergoes malignant transformation. Borderline tumors are noninvasive epithelial tumors with significant celular atypia, high mitotic rate and high proliferation index but no stromal invasion. 20 to 40% of these tumors are associated to extraovarian implants. The non-invasive nature of these implants is an important histological landmark that defines the tumor´s behaviour and the lack of adyuvant treatment.