Carcinoid tumors are low-grade malignant tumors with neuroendocrine differentiation that are occurring less frequently in the urogenital system. Since the first report of primary renal carcinoid tumor by Resnick in 1966 , about 70 cases have been documented in the literature to date. The size of renal carcinoid tumor was 2–30 cm (mean 9.6 cm) in diameter [6, 7]. Only 13 cases of primary carcinoid tumor within a horseshoe kidney were found till today in the world. Here, we report a case of 4.7 mm primary micro neuroendocrine tumor (also called carcinoid tumors) arising within the horseshoe kidney. To our knowledge, this is a case of minimum primary renal neuroendocrine tumor presents on the horseshoe kidney.
The primary renal Neuroendocrine tumors have been reported to arise most commonly in the setting of acquired and congenital renal abnormalities (as in the case herein presented), such as horseshoe kidney (18–26 %) [8, 9], renal teratoma or teratoid malformation (15 %) , and polycystic kidney disease (2 %). Krishnan et al. calculated a relative risk of 62 times for the occurrence of a renal carcinoid tumor in association with a horseshoe kidney . The patient did not found any more tumors in the other organs, which sufficiently confirming the primitivity of this lesion. Accordingly, we concluded that it was another case of renal neuroendocrine tumor of minimum in size found in China.
Neuroendocrine tumors are thought to arise from neuroendocrine cells (NECs) or APUD cells. NECs have been described in the urinary bladder, urethra, and the renal pelvis, but NECs are not found within normal renal parenchyma . So many hypotheses have been proposed for the coexistence of primary renal neuroendocrine tumors, including entrapped neural crest cells in the metanephros during embryogenesis , neuroendocrine differentiation of a primitive totipotential stem cell [6, 7], and hyperplasia of preexisting neuroendocrine cells within metaplastic or teratomatous epithelium [12–14]. The most popular hypothesis is the totipotent cell hypothesis, that primary renal neuroendocrine tumor arises from totipotential primitive stem cells capable of neuroendocrine, mesenchymal and epithelial differentiation. A recent study showed that primary renal neuroendocrine tumor was absence of the expression of PAX-2 (paired box gene 2) and PAX-8 (paired box gene 8), which are thought to be renal cell lineage specific transcription factors. It may support the theory that these are derived from non-nephrogenic elements .
Carcinoid tumorlet has been discribed as a precursor lesion for diffused idiopathic neuroendocrine cell hyperplasia of pulmonary neuroendocrine tumors (typical carcinoid tumors that measurement ≤5 mm), In this case, the tumor was 4.7 mm in maximum diameter, and the patient had no lymph node metastases and systemic endocrine syndrome. Therefore, we considered that this case met the diagnosis of renal carcinoid tumorlet.
The primary treatment for the renal neuroendocrine tumor within a horseshoe kidney is the complete surgical resection, which is curative for localized disease. There was no evidence of recurrence at 8-month follow-up. Further follow-up is needed to evaluate the prognosis of the patient.