Mixed Neuroendocrine Carcinoma and Urothelial Carcinoma of the Upper Urinary Tract: A Case Report and Literature Review.

We describe a rare case of high-grade divergent urothelial carcinoma (UC) of the renal pelvis with neuroendocrine differentiation, specifically a small cell carcinoma (SCC) component. A 70-year-old male who presented with frank hematuria underwent a thorough clinical workup including computed tomography (CT) scan, which showed a large, contrast-enhancing obstructive right renal mass. The mass, when analyzed microscopically, showed two distinct components: high-grade urothelial carcinoma and SCC. Immunohistochemistry analysis confirmed primary dual morphological subtypes and ruled out a metastatic source. Mixed SCC and UC of the renal pelvis are extremely rare diagnoses, and staging of these tumors is difficult, highlighting the importance of integrated diagnostic approaches for an accurate characterization of complex renal tumors.