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Causes
Colorectal cancer generally begins as a polyp—a growth or a tumor that is not cancerous. Some stay benign, but others, known as adenomatous polyps or adenomas, may become cancerous if not removed. Nearly all colorectal cancer begins as one of these adenomas. (Growth is slow, so if detected as a benign polyp, colorectal cancer can be arrested before it begins.) Chronic inflammation, as found in inflammatory bowel diseases like ulcerative colitis and Crohn's disease, can also set the stage for colorectal cancer through a process known as dysplasia. Dysplasia involves changes within cells that may lead to cancer.
When a polyp becomes cancerous, it can, like all cancers, invade the surrounding tissues and spread malignant cells through the body via blood or lymph vessels.
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