Oligometastatic deposits of prostate cancer found within the sigmoid pericolic fat that was resected for colonic adenocarcinoma: a case report

BACKGROUND: Prostate cancer may rarely metastasize to the colon and colonic lymph nodes, and local treatment of oligometastatic deposits may improve oncological outcomes. Immunohistochemical stains are used to determine the most likely source of metastatic deposits when they are seen within surgical specimens. The aim of this case report is to illustrate how such techniques were used to identify unexpected prostatic metastases within the pericolic fat of a sigmoid colon resection specimen following elective curative surgery for colorectal cancer. To our knowledge, this is the first report of complete excision of oligometastatic deposits of prostate cancer found incidentally within the specimen of another cancer. CASE REPORT: An 89-year-old Caucasian man underwent sigmoid colectomy for an obstructing colorectal cancer in the sigmoid colon with some mesenteric lymphadenopathy. He had previously received radical radiotherapy for prostate cancer 10 years earlier. When the specimen was examined by the histopathologist, it was noted that the pericolic fat adjacent to the colorectal adenocarcinoma contained some metastatic deposits. Positive immunohistochemical staining for prostate-specific antigen and prostate-specific acid phosphatase with negative staining for CDX2 and CK20 revealed these to be prostatic metastases rather than colonic. Since these were completely excised, and there were no other metastases, this represented a serendipitous, curative excision of oligometastatic deposits of an additional cancer to the one that was being treated. CONCLUSIONS: This case illustrates how immunohistochemical staining may be used to distinguish the source of metastatic deposits based on the likelihood of primary tumor from a careful and thorough patient history.