Background: Malignant transformation may occur in up to 1% of women with endometriosis and only 20% of these cases occur at extragonadal pelvic sites. Similarly, urinary tract endometriosis is rare and occurs in only 1% of all endometriotic lesions.
Case: An endometrioid carcinoma with squamous differentiation arising from periureteral endometriosis presented as a pelvic mass encasing the right ureter. The patient had a history of total hysterectomy and bilateral salpingo-oopherectomy 8 years earlier because of bilateral serous benign cysts with right hemorrhagic corpus luteum and uterine fibroids, and has since been under unopposed estrogen replacement therapy for 5 years.
Conclusion: This is the second case of a malignancy arising in endometriosis presenting as an obstructive ureteral mass and the first case of a patient with this condition whose endometriosis is not consistent with a gonadal origin. An analysis of the case and related literature is presented together with the discussion of possible pathogenetic mechanisms.