Renal Neuroendocrine Tumor Mimicking Upper Tract Urothelial Carcinoma

ABSTRACT Introduction Renal pelvic or peripelvic neuroendocrine tumors are rare and may radiologically mimic invasive upper tract urothelial carcinoma. Case Presentation A man in his 70s presented with two episodes of painless gross hematuria and lower urinary tract symptoms from benign prostatic hyperplasia. Computed tomography urography and magnetic resonance imaging showed a 3.1‐cm enhancing mass centered at the right renal pelvis with apparent cortical invasion. However, flexible ureterorenoscopy showed intact urothelium, and renal pelvic wash cytology was negative. Upper tract urothelial carcinoma with parenchymal invasion was suspected, and single‐port robot‐assisted right nephroureterectomy with bladder cuff excision was performed. Histology revealed a well‐circumscribed, well‐differentiated neuroendocrine tumor beneath intact renal pelvic mucosa, with negative margins and a Ki‐67 index of 4.9%. Ten‐month postoperative imaging showed no recurrence or metastasis. Conclusion Submucosal renal neuroendocrine tumors should be considered when imaging suggests a renal pelvic mass but endoscopy shows no mucosal lesion.