Rectal cancer. Double contrast coloured X-ray of the abdomen of a patient with rectal cancer. A malignant (cancerous) tumour is blocking the rectum above the white mass of barium at lower centre. The rectum is at the end of the digestive system, and is where faeces is stored before it is expelled through the anus. Symptoms of rectal cancer include rectal bleeding and constipation. Treatment is with surgery to remove the affected area. A double contrast X-ray is obtained by introducing a solution of radio-opaque barium sulphate to the rectum, which is then distended by air.