Nature of Colon Cancer – Natural History

Most colon cancers were reported to arise in the distal bowel. The rubric that rectal examination plus rigid proctosigmoidoscopy finds 75% of all colon cancers is no longer valid . Approximately 25% of all polyps can be reached by the rigid proctoscope and 60% can be reached with the 60-cm flexible sigmoidoscope. Although no satisfactory explanation for the change in the segmental distribution of colon and rectal carcinoma is conclusive, there are numerous possibilities. First, because the more proximal colon is more accessible through the wider application of endoscopic techniques and because there is greater use of double-contrast barium studies, more right-sided or proximal bowel lesions may be diagnosed that were always there but were less frequently diagnosed. Second, there may actually be multiple environmental and genetic risk factors that determine right-sided or proximal bowel lesions that are distinct from the causes of left-sided or distal bowel tumors. Numerous formalized treatment protocols of bowel cancer demonstrate that the natural history of right-sided lesions differs from that of left-sided lesions. The natural history of sigmoid cancers differs from that of more proximal colonic tumor. The natural history of rectal cancer is different from that of colon cancer. These differences are reflected in patterns of recurrence and include responsiveness to multimodality adjuvant therapy.