HORSESHOE KIDNEY IS ASSOCIATED WITH AN INCREASED RELATIVE RISK OF PRIMARY RENAL CARCINOID-TUMOR

Purpose: Carcinoid tumor is a rare neoplasm of the kidney with an unkn own histogenesis. Of only 31 cases previously reported in the literatu re 4 arose within horseshoe kidneys. We report a case of primary carci noid tumor arising within a horseshoe kidney and discuss the unique in sight it provided into the pathogenesis of this tumor. Materials and M ethods: We reviewed in detail all 31 reported cases of renal carcinoid tumor and, using reported incidence rates of horseshoe kidney, we cal culated the relative risk of renal carcinoid tumor arising within a ho rseshoe kidney. Immunohistochemical staining for neuroendocrine relate d markers was performed on tissue sections fi om the present carcinoid tumor, the adjacent kidney and 5 control samples of normal renal pare nchyma. Results: Of the reported tumors 15.6% occurred in horseshoe ki dneys, yielding a calculated relative risk of 62. The present tumor wa s multifocal, arising from the wall of a cystic lesion and possibly re presenting a dilated calix within the isthmus. Intestinal epithelium l ining the cyst cavity exhibited multifocal neuroendocrine cell hyperpl asia with an immunohistochemical profile identical to that of the carc inoid tumor cells. Conclusions: The relative risk of renal carcinoid t umor developing in a horseshoe kidney is markedly greater than that fo r Wilms tumor or transitional cell carcinoma. The clinical course of r enal carcinoid tumor arising within a horseshoe kidney appears to be m ore benign than that of the nonhorseshoe variant. Our observations sup port the hypothesis that renal carcinoid tumors may arise from neuroen docrine cells within foci of metaplastic or teratomatous epithelium wi thin the kidney.