Rectal cancer. Endoscope view of cancer of the rectum. The condition is normally confirmed in this way and by taking a biopsy. Symptoms include rectal bleeding, pain, and diarrhoea and constipation. If untreated the cancer may grow until it blocks the intestine. Rectal cancer can also spread to organs in the pelvis. It is most common in people aged between 50 and 70, and is thought to be linked to diet and genetic factors. Usually the cancerous region is surgically removed.