Colorectal cancer is a malignant tumour that starts in cells of the colon or rectum. Malignant means that it can spread, or metastasize, to other parts of the body.
The colon and rectum are part of the digestive system. Together, the colon and rectum make up the large intestine, or large bowel. The colon takes up water and nutrients from food and passes waste to the rectum.
Cells in the colon or rectum sometimes change and no longer grow or behave normally. These changes may lead to benign tumours such as polyps, which are not cancerous.
Changes to cells of the colon and rectum can also cause precancerous conditions. This means that the cells are not yet cancer but there is a higher chance these abnormal changes will become cancer. The most common precancerous conditions of the colon and rectum are adenomatous polyps (also called adenomas) and polyposis syndromes. In some cases, changes to colon and rectum cells can cause colorectal cancer.
Most often, colorectal cancer starts in glandular cells, which make mucus and digestive fluids. These cells line the inside of the colon and rectum. This type of cancer is called adenocarcinoma of the colon and rectum.
Rare types of colorectal cancer can also develop. These include carcinoid tumour, lymphoma and sarcoma.