Metastatic prostate carcinoma mimicking primary anal cancer.

A 71-year-old man with a history of castration-refractory prostate cancer was initially treated with hormonal therapy. He responded with a decreasing prostate-specific antigen level and improved symptoms. Chemotherapy was initiated later, after an increasing prostate-specific antigen level and findings of distant metastases. Nine months after his initial diagnosis, he presented with a large multinodular perianal mass that was suspicious for primary anal cancer. Biopsy revealed poorly differentiated metastatic prostate carcinoma. The patient died 2 months after the initial presentation with perianal skin metastasis.