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.

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 ± 99 cm3 ; supine: 201 ± 162 cm3 ) and V30 (prone: 78 ± 71 cm3 ; supine: 105 ± 106 cm3 ). At higher doses, the trend reversed as exemplified by the small bowel V50.4 (prone: 20 ± 28 cm3 ; supine: 10 ± 14 cm3 ). 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.