CT and MR findings of a retrorectal cystic hamartoma confused with an adnexal mass on ultrasound.

We describe the imaging features of a tailgut cyst mistaken for an adnexal mass. A pelvic ultrasound in a 28-year-old woman showed a 10-cm hypoechoic left pelvic mass. Having not seen the left ovary, the radiologist concluded that the mass was an endometrioma. CT disclosed a retrorectal cystic lesion with wall calcifications and internal septa. MR confirmed the extra-ovarian location of the tumor, which was hyperintense on T2-weighted images and had an intermediate signal on T1-weighted images. Surgery revealed a retrorectal cystic hamartoma. Radiological diagnosis of a tailgut cyst requires first correct localization of the tumor and then differentiation from other retrorectal masses.