What are the most common primary tumor sites that can show colon metastases?
These include lung, stomach, breast, ovary, endometrium, and melanoma. These tumor cells creep under the surface epithelium or form submucosal nodules of varying sizes. More than one focus is generally seen. The surface epithelium lacks dysplasia (expected with primary colon adenocarcinomas). IHC may be helpful in poorly differentiated neoplasms. Usually primary colonic adenocarcinomas show immunoreactivity with cytokeratin 20 (95%) and CDX2 (intestinal epithelium marker). Difficulty arises in some poorly differentiated tumors that have lost antigenicity or show lineage infidelity.