Carcinoid tumor originating in a horseshoe kidney.

An atypical carcinoid tumor originating in the horseshoe kidney of a 39-year-old man is reported. The tumor showed cytological atypia, increased mitotic rate and histological signs of mucinous differentiation and osseous metaplasia. Even though there were distant metastases, the patient survived partial nephrectomy including resection of the primary tumor for four years. The review of literature shows that carcinoid tumors originate some 60 to 85 times more often in horseshoe kidneys than in normal kidneys. The clinical course of renal carcinoid tumors is highly variable.