Types of Colorectal Cancer
Adenocarcinomas account for 90 to 95 percent of all large bowel tumors. They typically originate in the mucosa from a benign growth or adenoma. Adenomatous polyps look like grapes on the surface of the bowel's inner wall. The larger their size and the greater the degree of dysplasia (abnormally developed cells), the more likely the polyps are to progress to cancer.
On the right side of the colon near the cecum, cancers usually grow into the space within the colon. They can become large enough to be painful and are likely to cause bleeding. In these cases anemia from chronic blood loss is often the first sign and is why a stool test for occult, or hidden, blood is important (guaiac).
Most polyps and cancers appear on the left side of the colon. In the left or descending colon, where the channel is narrow, the cancer usually grows around the colon wall and encircles it. Left-sided colon cancer typically constricts the bowel channel, causing partial blockage. Typical symptoms include constipation, change in bowel habits, and narrow, ribbon-shaped stool when a cancer is low in the rectum.
In general, colorectal cancers tend to be slow growing, gradually enlarging and eventually penetrating the bowel wall. When they do spread, it is usually through invasion of nearby lymph nodes. In fact, cancer cells may enter a lymph node even before the tumor penetrates through the intestinal wall. The most common sites of distant metastasis are the liver, lungs, and brain.