POU2F3-positive neuroendocrine carcinoma of the urinary bladder showing basaloid morphology: expanding the morphologic spectrum of tuft cell-like carcinoma.

POU2F3 defines tuft cell-like carcinomas, yet POU2F3-driven neuroendocrine neoplasia in the urinary bladder remains incompletely characterized and can morphologically mimic basaloid carcinomas. We report a case of an octogenarian woman with a diverticular bladder tumor composed of conventional high-grade urothelial carcinoma juxtaposed with a sharply demarcated basaloid component. Although the basaloid component lacked classical small-cell morphology and showed scant conventional neuroendocrine marker expression, it was diffusely POU2F3-positive, underscoring a potential diagnostic pitfall with human papillomavirus (HPV)-associated basaloid squamous cell carcinoma. Both components showed concordant aberrant tumor-suppressor immunoprofiles (p53 overexpression and Rb loss), and high-risk HPV RNA in situ hybridization (RNAscope) was negative. This case expands the recognized morphologic spectrum of bladder neuroendocrine carcinoma to include POU2F3-defined non-small cell neuroendocrine carcinoma with basaloid architecture, supporting the practical value of incorporating POU2F3 into immunohistochemical panels for poorly differentiated basaloid bladder tumors.