title: Horseshoe kidney is associated with an increased relative risk of primary renal carcinoid tumor.
abstract: OBJECTIVE
  Carcinoid tumor is a rare neoplasm of the kidney with an unknown histogenesis. Of only 31 cases previously reported in the literature 4 arose within horseshoe kidneys. We report a case of primary carcinoid tumor arising within a horseshoe kidney and discuss the unique insight it provided into the pathogenesis of this tumor.
  
  
  METHODS
  We reviewed in detail all 31 reported cases of renal carcinoid tumor and, using reported incidence rates of horseshoe kidney, we calculated the relative risk of renal carcinoid tumor arising within a horseshoe kidney. Immunohistochemical staining for neuroendocrine related markers was performed on tissue sections from the present carcinoid tumor, the adjacent kidney and 5 control samples of normal renal parenchyma.
  
  
  RESULTS
  Of the reported tumors 15.6% occurred in horseshoe kidneys, yielding a calculated relative risk of 62. The present tumor was multifocal, arising from the wall of a cystic lesion and possibly representing a dilated calix within the isthmus. Intestinal epithelium lining the cyst cavity exhibited multifocal neuroendocrine cell hyperplasia with an immunohistochemical profile identical to that of the carcinoid tumor cells.
  
  
  CONCLUSIONS
  The relative risk of renal carcinoid tumor developing in a horseshoe kidney is markedly greater than that for Wilms tumor or transitional cell carcinoma. The clinical course of renal carcinoid tumor arising within a horseshoe kidney appears to be more benign than that of the nonhorseshoe variant. Our observations support the hypothesis that renal carcinoid tumors may arise from neuroendocrine cells within foci of metaplastic or teratomatous epithelium within the kidney.
authors.affiliation: Department of Pathology, Veterans Affairs Medical Center, Houston, Texas, USA.
authors.forename: B
authors.initials: B
authors.lastname: Krishnan
authors.forename: L D
authors.initials: LD
authors.lastname: Truong
authors.forename: G
authors.initials: G
authors.lastname: Saleh
authors.forename: D M
authors.initials: DM
authors.lastname: Sirbasku
authors.forename: K M
authors.initials: KM
authors.lastname: Slawin
mesh_terms: D002276:Carcinoid Tumor / Q000453:epidemiology / Q000209:etiology* / Q000473:pathology; D006801:Humans; D007668:Kidney / Q000002:abnormalities*; D007680:Kidney Neoplasms / Q000453:epidemiology / Q000209:etiology* / Q000473:pathology; D008297:Male; D008875:Middle Aged; D012306:Risk