Papillary mucinous cystadenoma of probable renal pelvic origin in a horseshoe kidney.

A 59-year-old woman underwent a partial nephrectomy to remove a 7-cm cystic mass involving the left portion of a horseshoe kidney. Microscopically, the cyst was lined by tall columnar mucosecretory cells resembling endocervical cells. The lesion is believed to represent the benign counterpart of mucinous adenocarcinoma of the renal pelvis.