Bowel cancer. Coloured MRI (magnetic resonance imaging) scan of a frontal view through a 62-year- old man's rectum (red). The bowel wall around the rectum has thickened, indicating the presence of cancer. The cancer has spread (metastasised, blue) beyond the boundaries of the bowel wall, making the cancer harder to treat. Bowel cancer is one of the most common cancers in North America and Western Europe. It is thought to be connected with a high fat, low fibre diet. Treatment is usually surgery, followed by radiotherapy or chemotherapy.