Abstract
BACKGROUND: Primary renal neuroendocrine tumors (PRNETs) are exceptionally rare neoplasms, with only limited cases documented in the literature. Their occurrence in association with horseshoe kidney represents an even rarer clinical scenario. This report presents a case of well-differentiated neuroendocrine tumor arising in a horseshoe kidney and discusses its clinical management and prognostic implications. CASE PRESENTATION: A 58-year-old female was admitted after an incidental discovery of a left renal mass during a routine health examination. Imaging studies, including enhanced CT and MRI, revealed a horseshoe kidney with a 5.5 × 4.7 cm cystic-solid mass in the mid-lower portion of the left kidney, classified as Bosniak IV. The patient underwent laparoscopic partial nephrectomy via a retroperitoneal approach. Histopathological and immunohistochemical analyses confirmed a well-differentiated neuroendocrine tumor, with positive staining for Syn, CD56, SSTR2, and focal CgA, and a low Ki67 index (3%). The postoperative course was uneventful, and no recurrence or metastasis was observed during the 12-month follow-up. CONCLUSION: PRNETs, though rare, should be considered in the differential diagnosis of renal masses, particularly in patients with congenital renal abnormalities such as horseshoe kidney. Nephron-sparing surgery may be a feasible option in selected cases, with favorable short-term outcomes. Long-term follow-up is essential due to the potential for delayed metastasis. Immunohistochemical profiling plays a critical role in diagnosis and therapeutic planning, especially regarding the potential use of somatostatin analogs.