Most bowel cancers are thought to develop from growths on the lining of the wall of the large bowel. These growths are called adenomas or polyps. Not all polyps develop into bowel cancer. The chance that a polyp will become cancer depends on the type of polyp, the size and number of polyps, and whether the polyp contains abnormal cells (dysplasia).^[1]  

The most common type of bowel cancer is called an adenocarcinoma, named after the gland cells in the lining of the bowel where the cancer first develops. When doctors talk about colorectal cancers, they are almost always talking about this type of cancer. There are some sub-types of adenocarcinoma, including mucinous or signet ring adenocarcinoma. 

Other rarer types include carcinoid tumours, gastrointestinal stromal tumours, sarcomas and lymphomas.^[2]