Small Cell Neuroendocrine Carcinoma of the Urinary Bladder with Admixed Squamous Differentiation: A Case Report

Introduction: Small cell neuroendocrine carcinoma (SCNEC) of the urinary bladder is a rare and highly aggressive malignancy, accounting for less than 1% of bladder tumors. The presence of mixed histological components such as squamous differentiation poses diagnostic challenges, particularly in limited biopsy specimens. Case Presentation: We report a case of a 75-year-old male presenting with painless hematuria. Imaging revealed asymmetric thickening of the urinary bladder wall involving the left lateral and posterior wall. Transurethral resection of bladder tumor (TURBT) was performed. Histopathological examination showed a malignant neoplasm predominantly composed of small cell neuroendocrine carcinoma with an admixed squamous component. Immunohistochemistry demonstrated diffuse CD56 positivity, focal chromogranin expression, high Ki-67 proliferation index, and focal cytokeratin and p40 positivity in squamoid areas. The tumor infiltrated the muscularis propria.Conclusion: Recognition of mixed histology in bladder SCNEC is crucial for accurate diagnosis and appropriate management. Immunohistochemistry plays a key role in distinguishing tumor components and avoiding diagnostic pitfalls.