Not all tumors are malignant. Many are benign, but they can cause serious trouble within the bowel by obstructing the lumen or tube. Fairly large non-malignant tumors may develop whit in the stomach, causing a chronic loss of blood. Although not malignant, these can be serious Polyps in the colon are also common and are often responsible for blood in the stools.
Over half of those who have polyps in the colon eventually develop cancer as well. For this reason, all polyps should be removed, either through the sigmoidoscope, if they are low enough down, or by resecting the involved portion of the colon. Before doing so, the patient should have a thorough medical examination, and also X-ray of the large and small bowel.

This is particularly important in all cases of rectal bleeding or unexplained loss of blood from the stomach or bowel.