The effect of prone and supine treatment positions for the pre-operative treatment of rectal cancer on organ-at-risk sparing and setup reproducibility using volumetric modulated arc therapy

Abstract

Background and purpose: To compare organ-at-risk doses and setup reproducibility using the prone and supine orientations in volumetric modulated arc therapy (VMAT) for rectal cancer. Materials and methods: Seventeen consecutive rectal cancer patients undergoing preoperative radiation were selected and setup in either the prone (N=8) or supine (N=9) position. All patients were treated using posteriorly-applied VMAT. Bladder and small bowel dose and cone beam CT (CBCT) reproducibility metrics were retrospectively collected. Results: Dose metrics for bladder and small bowel did not show significant differences between the prone and supine orientations. The prone data had a trend for smaller irradiated volumes than supine for the small bowel at lower doses-V20 (prone: 135±99cm3; supine: 201±162cm3) and V30 (prone: 78±71cm3; supine: 105±106cm3). At higher doses, the trend reversed as exemplified by the small bowel V50.4 (prone: 20±28cm3; supine: 10±14cm3). CBCT data showed that rotational errors in pitch and roll were significantly larger for the prone vs. supine orientation (pitch: 2.0°±1.3° vs. 0.8°±1.1° p<0.001; roll: 1.0°±0.9° vs. 0.3°±0.5°, p<0.001). Conclusions: Bladder and small bowel doses were not significantly different when comparing VMAT plans developed for the prone and supine orientations. The supine orientation demonstrated improved setup reproducibility.