Clear cell carcinoma arising in endometriosis of the rectum following progestin therapy.

A 40-year-old woman presented at our hospital with rectal stenosis. Computed tomography demonstrated a conglomerate tumor in the pelvis and malignant look-alike lesions in the liver and in both lungs. A palliative deep rectum resection was done. Histologically, clear cell carcinoma with a small area of endometrioid carcinoma was diagnosed. Severe endometriosis had been diagnosed 13 years earlier. The patient had been treated with medroxyprogesterone acetate (MPA) for two years; no estrogen therapy had been given. The association between unopposed estrogen replacement therapy and malignant transformation of endometriosis is documented, but malignant transformation following progestin therapy has not been reported previously. It appears that radical surgery is the only means of preventing malignant transformation of the lesion in patients with endometriosis.