Colon cancer is a general term used to describe a group of malignant (cancerous) tumours that develop from cells normally found in the colon. A malignant tumour is made up of abnormal cells that have the ability to invade surrounding tissues and spread to other parts of the body.
The most common type of colon cancer is called adenocarcinoma and it starts from the cells normally found in the glands that cover the inside surface of the colon. Rare subtypes of adenocarcinoma include mucinous adenocarcinoma, signet-ring cell adenocarcinoma, medullary adenocarcinoma, serrated adenocarcinoma, micropapillary adenocarcinoma, adenoma-like adenocarcinoma, and adenosquamous adenocarcinoma.
Most types of colon cancer start from a small growth inside the colon called a polyp. Doctors perform a test called a colonoscopy to identify and remove polyps in the colon before they turn into cancer. Types of polyps that can change into cancer over time include tubular adenoma, tubulovillous adenoma, villous adenoma, and sessile serrated lesion (also called a sessile serrated adenoma). A hyperplastic polyp is a very common type of polyp that will not change into cancer over time.
For most patients, colon cancer is diagnosed after a small tissue sample is removed in a procedure called a biopsy or a polypectomy (if performed to remove a polyp). The tissue sample is then sent to a pathologist who examines it under the microscope. By examining the tissue under the microscope, the pathologist can determine the type of colon cancer and can order any additional tests that may help guide your care. Depending on the type of tumour, another procedure may be performed to remove the entire tumour in a procedure called a resection. Lymph nodes from the axilla (armpit) may be removed at the same time to look for any tumour cells that have spread to a lymph node.