Feasibility of placing a dose constraint to peritoneal space instead of bowel loop in protection of the small intestine during intensity-modulated radiotherapy for rectal cancer

Abstract

Objective To compare the dose volume and normal tissue complication probability (NTCP) of small intestine between intensity-modulated radiotherapy (IMRT) with dose constraints to the peritoneal space (PS) and the bowel loop (BL) in the treatment of rectal cancer, and to investigate the feasibility of placing a dose constraint to the PS instead of the BL in protection of the small intestine. Methods A total of 24 patients with rectal cancer undergoing postoperative adjuvant radiotherapy were enrolled as subjects. In the 24 patients, 12 were treated in supine position and 12 in prone position. The weekly computed tomography (CT) scans from pre-treatment to weeks 1-4 of treatment were defined as Plan, 1 W, 2 W, 3 W, and 4 W. Contours of PS and BL were delineated on all CT images. Based on the Plan CT images, two IMRT plans, PPS and PBL, were designed with dose constraints to the PS and BL, respectively. The method was applied to 1-4 W CT images. For each CT scan, the dose volume and NTCP of the small intestine were evaluated in PPS and PBL. Results A total of 109 sets of CT images were acquired from 24 patients, and 218 plans were designed and copied. The median volume of the PS and BL was 1339.28 and 250.27 cm3, respectively. For the Plan CT scans, V15 values of the PS in PPS plan and the BL in PBL plan were 918.96 and 199.57 cm3, respectively. For all CT scans, the dose volume of the small intestine in PPS, in most cases, was lower than that in PBL, while V15 values in PPS and PBL were 170.07 and 178.58 cm3(P=0.000). The dose volume of the small intestine was correlated with V15(P=0.000). PPS had significantly lower NTCP of chronic and acute adverse reactions than PBL (2.80% vs. 3.00%, P=0.018; 57.32% vs. 58.64%, P=0.000). In patients with prone and supine treatment positions, most of the dose volume and NTCP of the small intestine in PPS were significantly lower than those in PBL (P<0.05 for V10, V15, V30, and NTCP of acute adverse reactions). Conclusions It is feasible to place a dose constraint to the PS instead of the BL for protection of the small intestine during IMRT for rectal cancer. V15<830 cm3 can be referred to as the objective function of dose restraint. Key words: Rectal neoplasms/intensity modulated radiotherapy; Peritoneal space; Small bowel; Normal tissue complication probability