The malignant transformation of endometriosis within the ovary is a recognised condition. There is less literature surrounding the malignant transformation of extra‐ovarian endometriosis (MEOE).

We report our experience with MEOE in ten patients and present a review of the literature regarding this rare malignancy.

Materials and Methods
For this retrospective case series, patients were identified from a practice‐based database. Where required, operative notes and pathology reports were reviewed.

Ten patients diagnosed with MEOE between 1991 and 2014 were identified. In each case, the tumour was localised to the pelvis and centred on the pouch of Douglas, broad ligament, obturator fossa, parametrium and rectovaginal septum. Tumour histology was endometrioid adenocarcinoma (six), clear cell carcinoma (two), and adenosarcoma (two). Five patients had a history of endometriosis and four had received oestrogen‐only hormone replacement therapy after hysterectomy and bilateral salpingo‐oophorectomy. Treatments included surgery (one), surgery and radiotherapy (one), surgery and chemotherapy (one), surgery, radiotherapy and chemotherapy (three), and radiotherapy and chemotherapy (four). Maintenance hormonal therapy was also used in three patients. Curative doses of radiotherapy 45 Gy or more resulted in in‐field control in five patients. Six patients had no evidence of disease at a mean follow up period of 15 years (5.5–24 years). Severe G3 long‐term bladder morbidity occurred in three patients after radical surgery and radiotherapy.

MEOE is a rare condition for which treatment needs to be individualised. Multicentre studies and registries will hopefully define optimal treatment.