Carcinoid tumor arising in a horseshoe kidney.

Carcinoid tumors are common neoplasms developing in gastrointestinal and respiratory tract. They are rarely found in the kidney. To date, approximately 40 cases of primary renal carcinoid tumors have been reported, with less of a third of them occurring within a horseshoe kidney. These cases appear to have a better prognosis, even in the presence of distant metastases, compared to those arising in normal kidneys.