Neuroendocrine Tumors in Horseshoe Kidneys: A Review of the Literature With Report of a Novel Finding Expanding Their Morphologic Spectrum.

Horseshoe kidney is a rare congenital anomaly with an unusually higher frequency of neuroendocrine tumors. Symptoms are rare, and, in most of the cases, are incidentally diagnosed. The clinical behavior of these tumors is heterogeneous and can be difficult to predict based on histology alone. Necrosis and percentage of Ki-67 may have a role in prognosis. Almost all tumors are carcinoids (well-differentiated neuroendocrine tumors) observed at an early age and with no sex dominance. It is not known the reason for the higher frequency of neuroendocrine tumors in horseshoe kidneys and the histogenesis is unknown. One of the hypotheses supports that renal carcinoid tumors may arise from neuroendocrine cells within foci of metaplastic or teratomatous epithelium within the kidney. With consonance with this hypothesis, there are reports of carcinoids in horseshoe kidneys associated with a cystic lesion lined by the intestinal epithelium, with mucinous differentiation and osseous metaplasia, arising in a mature teratoma of the kidney, arising within mature teratoma and clear cell renal cell carcinoma, with a mucinous cystadenoma element, and arising within mature cystic teratoma synchronous with primary adenocarcinoma. There is only one reported large cell neuroendocrine carcinoma of a horseshoe kidney in a 57-year-old Chinese woman. Herein, we report a patient that to the best of our knowledge is the first case of a combined well-differentiated neuroendocrine tumor and large-cell neuroendocrine carcinoma with rhabdoid features in horseshoe kidney. The histologic component of rhabdoid features expands the morphologic spectrum of neuroendocrine tumors in the horseshoe kidney. We provide a comprehensive review of the literature summarizing pertinent key clinical and pathologic aspects.