Despite undergoing complete surgical removal of rectal cancer, some patients may experience recurrence of their cancer. It is important to realize that some patients with rectal cancer already have small amounts of cancer that have spread outside the rectum and were not removed by surgery. These cancer cells are referred to as micrometastases and cannot be detected with any of the currently available tests. The presence of these microscopic areas of cancer causes the relapses that follow treatment with surgery alone. External beam radiation therapy and chemotherapy can be used to cleanse the body of micrometastases in order to improve the cure rate achieved with surgical removal of the cancer