Radiation therapy of colorectal cancer.

Radiation therapy has been used with increasing frequency in patients with carcinoma of the rectum and rectosigmoid. After surgical resection alone, local failure rates of 20 to 60% are noted, with the rate increasing with degree of penetration of tumor through the bowel wall and with presence of lymph node involvement. Preoperative or postoperative radiation therapy has been shown to decrease the local failure rate significantly, and may improve survival rates. In patients who present with locally unresectable disease, preoperative irradiation has been shown to convert a large percentage of these tumors to resectable lesions, with eventual cure in perhaps 30% of patients. Newer techniques such as intraoperative electron beam radiation therapy, may be effective in decreasing the high local failure rate in these patients, by allowing the radiation therapist to increase safely the dose to the tumor volume. Even without surgical resection, external radiation therapy alone can cure approximately 10% of these patients. For small, superficial, well-differentiated tumors within 10 cm of the anal verge, endocavitary radiation therapy is an effective method of locally controlling these tumors, while avoiding general anesthesia and avoiding colostomy.