When Small Cells Matter: Diagnosing Bladder Small Cell Carcinoma on Urine Cytology.

Small cell carcinoma of the urinary bladder (SCCB) is a rare and highly aggressive neuroendocrine neoplasm that poses significant diagnostic challenges, particularly on urine cytology. We report a case of a 68-year-old man with painless hematuria in whom SCCB was initially detected on voided urine cytology. Cytologic evaluation revealed small malignant cells with high nuclear-to-cytoplasmic ratios, nuclear moulding, finely granular chromatin, necrotic background and apoptotic debris, raising suspicion for a neuroendocrine carcinoma. Immunohistochemical studies performed on the urine cell block supported this diagnosis. Subsequent transurethral resection demonstrated a predominant high-grade neuroendocrine carcinoma admixed with conventional high-grade urothelial carcinoma, confirming primary bladder origin. This case highlights the importance of careful cytomorphologic assessment, the diagnostic value of cell block immunohistochemistry and the limitations of the Paris System in classifying non-urothelial malignancies encountered in urine specimens.