Evaluation of Small Bowel Motion and Feasibility of using the Peritoneal Space to Replace Bowel Loops for Dose Constraints during Intensity-Modulated Radiotherapy for Rectal Cancer
Abstract Background The goal of this study was to assess small bowel motion and explore the feasibility of using peritoneal space (PS) to replace bowel loops (BL) via the dose constraint method to spare the small bowel during intensity-modulated radiotherapy (IMRT) for rectal cancer. Methods A total of 24 patients with rectal cancer who underwent adjuvant radiotherapy were selected. Weekly repeat CT scans from pre-treatment to the fourth week of treatment were acquired and defined as Plan, 1W, 2W, 3W, and 4W. BL and PS contours were delineated in all of the scans. Two IMRT plans called PPS and PBL were designed on Plan CT using two dose PS and BL constraint methods, respectively, and then copied to CT 1 ~ 4W. The shift%, dose volume, and NTCP of the small bowel in PPS and PBL during treatment were evaluated. Results Overall, 109 sets of CT scans from 24 patients were acquired, and 218 plans were designed and copied. The PS and BL volumes were 1339.28 cc and 250.27 cc. The BL and PS shift% V15 was 28.48% and 11.79% (p = 0.000), which was less in the prone position than in the supine position (25.24% vs 32.10%, p = 0.000; 9.9% vs 14.85%, p = 0.000). On all of the CT scans, most PPS small bowel dose volumes were less than from PBL. V15 was 170.07 cc vs 178.58 cc (p = 0.000), and they had a significant correlation. The NTCP of chronic and acute side effects from PPS was significantly less than PBL (2.80% vs 3.00%, p = 0.018; 57.32% vs 58.64%, p = 0.000). Conclusions This study indicated that small bowel motion may lead to uncertainties in its dose volume and NTCP evaluation during IMRT for rectal cancer. The BL movements were significantly greater than PS, and the prone position was significantly less than the supine position. Using PS instead of BL can spare the small bowel. V15 < 830 cc is the dose constraint standard.