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.
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.
Malignant tumours are cancerous growths that have the potential to spread (metastasize) to other parts of the body.
Adenocarcinoma -About 90–95% of all colorectal cancers are adenocarcinomas. Adenocarcinomas are colorectal cancers that develop in the glandular cells in the inner lining (mucosa) of the colon or rectal wall. They can develop either on the wall itself or within a polyp.
Two uncommon types of adenocarcinomas are:
Mucinous (colloid) adenocarcinoma – characterized by large amounts of mucus outside the cells of the tumour
Signet ring adenocarcinomas – characterized by large amounts of mucus within the cell that shifts the nucleus of the cell, giving it a ringed appearance
Rare colorectal tumours -Rare colorectal tumours occur much less frequently (less than 5%) than adenocarcinomas. Rare colorectal tumours include: