Effect of prone and supine treatment positions for postoperative treatment of rectal cancer on target dose coverage and small bowel sparing using intensity-modulated radiation therapy

BACKGROUND: To evaluate the influence of patient positioning on target dose coverage and dose distribution to the small bowel in intensity-modulated radiation therapy (IMRT) for rectal cancer patients. METHODS: Twenty-four consecutive rectal cancer patients undergoing postoperative radiation were selected and set up in either a prone or supine position. All patients received computed tomography (CT) scans before and during treatment (1–4 weeks). The CT images were defined as plan, 1W, 2W, 3W, or 4W. The plan, 1W, 2W, 3W and 4W CT images were fused. The clinical target volume (CTV) and planning target volume (PTV) delineated on the plan CT were copied onto the 1–4W CT. The treatment plans based on the plan CT were also copied onto the 1–4W CT. The target dose coverage rate was assessed. The couch-position data of the linear accelerator were acquired. RESULTS: Failure rates of the CTV and PTV target dose coverage were higher in the prone group than in the supine group (18.60% vs. 0% and 69.76% vs. 53.65%) during the treatment. The total couch-position deviation for the prone group (1.23±0.76 cm) was significantly greater than that of the supine group (0.28±0.18 cm; P=0.001). Compared with the supine group, the prone group had significantly less irradiated small bowel volume at V5 (P=0.003) and V10 (P=0.004). CONCLUSIONS: The supine position maintained better target dose coverage and setup reproducibility in rectal cancer patients treated with IMRT. The prone position combined with the belly board can reduce the dose received by the small bowel.