The position and volume of the small bowel during adjuvant radiation therapy for rectal cancer

Abstract

Purpose: The rate of small bowel toxicity from adjuvant pelvic radiation therapy (RT) for rectal cancer has been reported to be lower for patients treated preoperatively (Preop). This was probably due to a lesser volume of irradiated small bowel; however, studies of postoperative treatment reported that patients with an abdominoperineal resection (APR), who likely have the largest volume of small bowel in the pelvis, had less acute and chronic toxicity than those with a low anterior resection (LAR). In this study, three-dimensional treatment planning techniques were used to characterize the position and volume of small bowel in the pelvis and compare these to repeat studies obtained during the typical 5-week course of treatment to attempt to explain the above observations. Methods and Materials: Treatment planning CT scans were obtained in 30 patients with rectal cancer (10 Preop, 10 LAR, 10 APR), including 12 patients with weekly CT scans during RT (65 scans). The position of the small bowel was measured by the distance to the nearest small bowel from the bones of the posterior pelvis and by the volume of small bowel within four anatomically defined regions of the pelvis. The motion of the small bowel was expressed as the standard deviation of the small bowel position measured with both the distance and the volume in the 12 patients with repeat studies. Results: Contrast-containing small bowel was found an average 2.9 cm more anterior than small bowel without contrast below the sacral promontory. The position of the small bowel in Preop patients was significantly more anterior ( p ≤ 0.01) with less volume ( p ≤ 0.04) in the pelvis than postoperatively treated patients. The small bowel was also more anterior for patients with an LAR vs. APR ( p ≤ 0.03) but with similar volume in all pelvic regions. Small bowel motion, expressed as the standard deviation of the distance from the bones of the posterior pelvis to the closest small bowel, was 2.9 cm, 1.4 cm, and 0.2 cm for the Preop, LAR, and APR group, respectively. The LAR group had a considerable degree of motion in the posterior pelvis. Increased bladder volume was associated with reduced small bowel volumes, although this benefit decreased during treatment. Conclusion: Because treatment planning CT scans can detect small bowel that does not contain contrast, they may be more accurate than the traditional small bowel series. The Preop patients had significantly less pelvic small bowel supporting the clinical observation of better tolerance to therapy. The higher small bowel toxicity reported for LAR vs. APR patients may be explained by the greater variability of both the position and volume of the small bowel in the posterior pelvis for LAR patients. This finding suggests that a single planning study may not be accurate for the block design used for boost treatment of LAR patients. Bladder-filling techniques were useful for Preop and LAR but not APR patients, and decreased in benefit over time. This study suggested that treatment planning CT scans were more useful than a small bowel series and that more than one treatment planning CT may be obtained in any patient receiving > 45 Gy for rectal cancer. However, further research will be necessary to determine the optimal timing and total number of repeat studies.