Technical advances in the treatment of patients with rectal cancer

Abstract

During the 1980s and 1990s, prospective randomized trials in the United States established the value of postoperative pelvic irradiation with concurrent and maintenance 5-FUbased chemotherapy for patients with resected Stages II and III rectal cancer. Since the publication of the National Institutes of Health Consensus Statement in 1990, all patients with resected Stages II and III rectal cancer have been advised to receive adjuvant therapy (1). Although important improvements in local control and survival have been achieved by this therapy, acute and late treatment-related morbidity has also been observed. In an important intergroup trial of adjuvant therapy for rectal cancer, 24% of patients receiving concurrent pelvic irradiation and 5-FU by peripheral venous infusion experienced severe or life-threatening diarrhea (2). In addition to acute toxicity, there may be significant late effects as well. In an analysis of 306 rectal cancer patients treated with postoperative irradiation from the Mayo Clinic from 1981 to 1990, the 10-year probability of developing chronic bowel injury was 25% (3). Besides the standard endpoints of acute and late bowel injury, there is also increasing awareness of changes in quality of life induced by adjuvant therapy. Investigators from the Mayo Clinic have reported major long-term detrimental effects on the bowel function of patients undergoing low anterior resection and postoperative chemoirradiation compared with patients undergoing low anterior resection alone (4). Given the increasing use of this therapy in the care of patients with rectal cancer and its potential morbidity, efforts to minimize treatment-related complications would be invaluable. These efforts have evolved along many fronts. Surgical measures to exclude the small bowel from the radiation field—absorbable mesh, ectopic implant, cystopexy, and omental flap—have shown encouraging results in reducing the amount of bowel in the radiation field and thus reducing the incidence of small bowel injury. Pharmacological approaches (the use of sucralphate) during pelvic radiotherapy may decrease the early and late symptoms of radiation enteritis. In the past 20 years, there has been a progressive improvement in radiation planning and techniques in the treatment of patients with rectal cancer resulting in less acute and late toxicity. The use of three or four pelvic field techniques, prone positioning, bladder distention, belly board, and oral contrast to visualize the small bowel to guide field design and dose selection have been important steps in reducing treatment-related sequelae (5). The report of Koelb and colleagues represents a further step in the evolution of improved treatment planning and technique (6). In this study, 20 patients with resected rectal cancer underwent pelvic CT scans following a uniform protocol in the supine position and prone position using a belly board technique. Comparative treatment planning was then obtained by a 3-D commercially available system. For each axial CT slice, the investigators defined a “typical standard planning target volume (PTV) for postoperative irradiation” as well as relevant normal tissue volumes (bladder and small bowel). They further defined the intersection between the PTV with bladder and small bowel volumes. A three-field technique (posterior and right and left obliques) treating the posterior pelvis was then employed using “beam’s eye view” for creating individual collimated blocks. Dose–volume histogram analysis of bladder and small bowel was performed comparing prone to supine position. Based on these data, the authors also calculated normal tissue complication probabilities by established radiobiologic models and tolerance data. Pertinent observations of this study were as follows: 1) a statistically significant less volume of intersection of the PTV with bladder and small bowel in the prone versus supine position; 2) statistically significant less median dose to the bladder and small bowel in the prone versus supine position; 3) statistically less bladder and small bowel volume within the 90%, 80%, 60%, and 40% isodose distributions in the prone versus supine position; 4) by the radiobiologic model employed in this study, the normal tissue complication probability was significantly lower in the prone than in the supine position.