Prostate Metastasis Presenting as Primary Colon Cancer

Abstract

Prostate cancer is the second most common cancer, and the second leading cause of cancer death in US men. Prostate cancer has high metastatic potential, with 20% having metastatic disease upon diagnosis. Here we describe a rare case of a metastatic prostate cancer to the rectosigmoid initially presenting as a colon polyp. A 74 yo man with a history of prostate cancer and radical prostatectomy 15 years ago was referred for evaluation of colon cancer and pericolonic abscess. He had mild suprapubic pain and constipation. He was diagnosed with subacute diverticulitis and pericolonic abscess after CT Abdomen and Pelvis showed a sigmoid/pericolonic fistula extending to the bladder. Diagnostic flexible sigmoidoscopy found a sigmoid polyp which was positive for poorly differentiated carcinoma of undetermined origin. He was then directed to our facility for a second opinion.His vitals and physical exam, including rectal exam, and PSA were normal. He was still taking Leuprolide every 6 months. A CT Chest Abdomen and Pelvis showed a 5 cm mottled mass between the sigmoid and the bladder with clouding of omental fat suggesting an inflammatory vs peritoneal metastatic disease. There was thickening of the ileocecal valve and focal segments of the distal colon. On colonoscopy there were several small adenomatous/hyperplastic polyps. However, a rectosigmoid polyp was positive for mucosal adenocarcinoma with stains consistent with metastatic prostate adenocarcinoma, not primary colon adenocarcinoma as originally thought. The tumor cells stained positive for AE1/3, PSA, PSAP, and mucicarmine, and negative for CD56, synaptophysin, chromogranin S-100, and Melan A. The case presented here highlights the importance of thoroughly evaluating masses found in the colon and the importance of specialized GI pathologists. Rectal involvement by prostatic cancer is rare and only occurs in approximately 4% of patients. Of those 4%, only 3% present as separate metastasis to the rectosigmoid, as shown in this case. Upon literature review, few accounts of metastatic spread to the colon have been published, and even rarer is the presentation of prostate cancer as a colon polyp. While rare, it is still essential to confirm the diagnosis by histopathology with appropriate immunohistochemical stains. Distinguishing between primary and metastatic tumors is essential as major surgical intervention may be avoided in favor of less invasive treatments, such as hormonal or radiological therapies.1519_A Figure 1. Rectosigmoid polyp positive for metastatic prostate adenocarcinoma.1519_B Figure 2. CT scan showing mass with central mottled contents between the bladder and sigmoid.