Colorectal cancer is a malignant tumour that starts in cells of the colon or rectum. Malignant means it is a cancerous tumour that can spread, or metastasize, to other parts of the body.
The colon and rectum are parts of the large intestine and the digestive system. The colon absorbs water and nutrients and passes waste (stool or feces) to the rectum. Colon and rectal cancers are grouped together as colorectal cancer because these organs are made of the same tissues and there isn’t a clear border between them.
Cells in the colon or rectum sometimes change and no longer grow or behave normally. These changes may lead to benign tumours such as hyperplastic and inflammatory 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 adenomas and hereditary colorectal syndromes.
Colorectal cancer most often starts in gland cells that line the wall of the colon or rectum. These gland cells make mucus that helps stool move through the colon and rectum. This type of cancer is called adenocarcinoma of the colon or rectum.
Rare types of colorectal cancer can also develop. These include small cell carcinoma and squamous cell carcinoma.