Rectal Mucinous Adenocarcinoma Mimicking Prostatic Urothelial Carcinoma: A Diagnostic and Therapeutic Challenge

Rectal mucinous adenocarcinoma rarely invades the prostate and can mimic primary urothelial carcinoma, risking misclassification. A man in his late 60s presented with urosepsis, and transurethral resection of papillary prostatic fossa tissue was initially read as high-grade urothelial carcinoma with adenocarcinoma differentiation. Pelvic imaging showed a rectoprostatic mass, and multidisciplinary review of histology, immunohistochemistry, and molecular profiling confirmed colorectal origin. After total neoadjuvant therapy, the rectal primary achieved clinical complete response, but residual prostate-bed disease persisted along a fistulous tract, requiring close surveillance. This case highlights integrated diagnostic assessment and careful response evaluation. • Rectal cancer invading the prostate mimicked primary urothelial carcinoma. • Transurethral biopsy initially suggested a high-grade urothelial tumor. • Integrated imaging, pathology, immunohistochemistry, and molecular profiling established colorectal origin. • Surgically altered pelvic anatomy and rectoprostatic fistulous extension contributed to diagnostic complexity. • Neoadjuvant therapy achieved clinical complete response of the rectal primary, but persistent prostate-bed disease required close surveillance.