Endoscope image of cancer of the rectum, the lower gut. The cancer is the yellow, ulcer-like lesion in the centre of the image. The most common type of cancer of the rectocolon is the adenocarcinoma; development of this tumour is closely associated with existing (premalignant) polyps. Adenocarcinoma commonly presents as an ulcer-like mass, typically deep with an irregular edge. The surrounding mucosa (the rectal lining) is likely to be heaped up & red in appearance, with a tendency to bleed. Colonoscopy is used in screening programs to prevent rectocolonic cancers. Certain types of polyps may be removed (snared) via the endoscope.