Metastatic Adenocarcinoma of the Prostate Diagnosed in a Colon Polyp: A Unique Clinicopathologic Scenario

Adenocarcinoma of the prostate is the second most common cause of cancer-related deaths among males in the U.S. Metastatic disease commonly involves the bones, lymph nodes, lungs, liver, and brain. Rarely, colonic involvement is seen and it is generally due to direct extension to the rectum. It is exceedingly uncommon for distant metastasis to occur in the right colon and small bowel. We present a case of prostatic adenocarcinoma metastasizing to the appendiceal orifice in a 78-year-old male. Our patient had a history of adenocarcinoma of the prostate diagnosed four years prior to presentation. He also had a history of adenocarcinoma of the distal colon 30 years prior which resulted in a partial colectomy and permanent diverting colostomy. Prior to his presentation, follow-up colonoscopies failed to reveal disease progression or additional malignancy. During routine colonoscopy, he was found to have a 2.5 cm polyp near the appendiceal orifice. Histologically the polyp demonstrated colonic mucosa with an infiltration of the lamina propria by individual cells with abundant cytoplasm and round nuclei with prominent nucleoli. The neoplastic cells were strongly positive for PSA and negative for CK7, CK20, and CDX2 supporting a diagnosis of metastatic prostatic adenocarcinoma. Metastatic disease of extracolonic origin arising in a polyp is extremely uncommon, but metastases have been reported to involve breast, ovary, stomach, esophagus, and kidney. This case contributes to the scarce information available regarding metastatic spread of prostate cancer to the ascending colon and enlightens the community of pathologists, surgeons, gastroenterologists, and urologists about this unusual presentation of a common carcinoma.