What is colorectal cancer?

Colorectal cancer starts in the cells of the colon or rectum. A cancerous (malignant) tumour is a group of cancer cells that can grow into nearby tissue and destroy it. The tumour can also spread (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 poop) 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 non-cancerous tumours such as hyperplastic and inflammatory polyps.

Changes to cells of the colon and rectum can also cause precancerous conditions. This means that the abnormal cells are not yet cancer, but there is a chance that they may become cancer if they aren’t treated. The most common precancerous conditions of the colon and rectum are adenomas and hereditary colorectal syndromes.

But in some cases, changes to colon or rectal cells can cause colorectal cancer. Most often, colorectal cancer 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.