Primary carcinoid tumor in a horseshoe kidney.

A case of primary carcinoid tumor arising within a horseshoe kidney in a 51-year-old woman is reported. The tumor was found incidentally by computed tomography (CT) during a check-up for a suspected gall bladder polyp. Histologic, immunohistochemical and electron microscopic analyses of this tumor revealed features typical of carcinoid tumor. Primary carcinoid tumor of the kidney is extremely rare and only 32 cases were previously reported, including five cases in horseshoe kidneys. None of these five cases in horseshoe kidneys demonstrated any evidence of local or distant metastases and all were alive at the time of reporting without evidence of disease after up to 3 years of follow up. The present case, even with accompanying lymph nodal metastasis, also has had no evidence of local recurrence or distant metastasis for 3 years post operation. Primary carcinoid tumor arising within horseshoe kidneys appear to be more benign than those within non-horseshoe kidneys.