Metastatic Prostate Ductal Adenocarcinoma Presenting as a Solitary Rectal Mucosal Polyp: A Case Report.

While prostate cancer is predominantly of the acinar subtype, ductal adenocarcinoma is a rare variant that typically presents at an advanced stage, often with low prostate-specific antigen (PSA) levels and rare gastrointestinal metastases. We report the case of a 75-year-old male patient with a history of prostate cancer status post-prostatectomy who, during routine surveillance, exhibited a solitary rectal mucosal polyp. Imaging and colonoscopy identified an invasive carcinoma, mimicking rectal adenocarcinoma, but histologically and immunohistochemically consistent with metastatic prostate ductal adenocarcinoma, isolated to the rectum without other site involvement. The metastatic tumor was surgically removed initially, but local recurrence occurred within a year, and the recurrent tumor was treated with radiation therapy due to inoperability. This case highlights the need to consider metastatic tumor in a patient with a previous history in the differential diagnosis of solitary colorectal lesions and underscores the role of immunohistochemistry in accurate diagnosis and management.