Primary Renal Neuroendocrine Tumor in a Horseshoe Kidney: A Case Report of an Indolent Course

Background/Objectives: Primary renal NETs are exceptionally rare neoplasms that occur disproportionately in horseshoe kidneys. Their rarity and non-specific clinical and radiological features make preoperative diagnosis particularly challenging. Methods: Herein, we report the case of a 60-year-old woman presenting with a painful left lumbar mass associated with dysuria and pollakiuria. Results: Computed tomography revealed a large heterogeneous tumor arising from the left moiety of a horseshoe kidney. Histopathological examination of the resected specimen demonstrated a 14 cm well-differentiated NET (grade 1) with diffuse chromogranin A and synaptophysin expression, a mitotic count of 1 per 2 mm2, and a Ki-67 proliferation index of <1%. The histopathological and immunohistochemical findings, together with the absence of an extrarenal primary site on staging investigations, supported the diagnosis of a primary renal NET. The postoperative course was uneventful, and the patient remained disease-free after two years of follow-up. Conclusions: This case highlights the diagnostic challenges posed by this rare entity and underscores the importance of including primary renal neuroendocrine tumors in the differential diagnosis of renal masses arising in horseshoe kidneys.