Metastatic Prostate Cancer Presenting as a Malignant, Obstructing Rectal Mass

Prostate cancer is a prevalent clinical entity that uncommonly metastasizes to the rectum. We present the case of an 87-year-old man with previously diagnosed prostate cancer and Crohn’s disease, who presented to our Emergency Department with constipation and was found on Computed tomography (CT) to have thickening of the proximal and mid rectum. Magnetic resonance imaging (MRI) confirmed the presence of a rectal tumour, with pelvic sidewall nodal involvement. Flexible sigmoidoscopy demonstrated an inflammatory stricture in the mid-rectum, with mucosal ulceration proximal to the stricture. Histopathology was consistent with adenocarcinoma of prostatic origin, and immunohistochemistry (IHC) was positive for prostate-specific antigen and NKX3.1. Rectal tropism is uncommon in prostate cancer, but the differential should be considered in patients who present with rectal tumors, especially those with a history of prostate cancer. Endoscopy and serological markers are unreliable for distinguishing between lesions, and histopathology and IHC should be utilized to confirm the diagnosis and guide management.