The large bowel may be the site of metastasis or extension from a number of tumors. It may also be involved by direct invasion or extension, intraperitoneal seeding, hematogenous dissemination, lymphatic metastasis, or intraluminal or intramural dissemination. The most common mode of secondary colonic involvement is peritoneal seeding; therefore, it is most common to see metastatic nodules deriving from either ovarian cancers (810,811) or mucinous colonic or appendiceal neoplasms. The anterior wall of the rectum at the Douglas pouch is probably the most common site for the discovery of peritoneal seeding (812). Other common locations include the sigmoid colon, transverse colon, and along both paracolic gutters (813). The rectum may also be affected by direct extension of tumor from the prostate or cervix (Fig. 14.152) (811). When prostate cancers invade the rectum, patients may present with signs and symptoms of rectal disease and an absence of genitourinary symptoms. Secondary involvement by extension along mesenteric reflections occurs in the transverse colon, where neoplasms of either the stomach or pancreas extend into the colon (Fig. 14.152) (814). Anal tumors, particularly melanomas, can extend into the rectum. Malignant mesotheliomas may metastasize and present as colonic polyps (815). The lesions may closely resemble primary adenocarcinomas.