Volumetric modulated arc therapy planning method for supine craniospinal irradiation

Background The purpose of this study is to propose a volumetric modulated arc therapy (VMAT) treatment planning technique for supine craniospinal irradiation (CSI) in order to improve dose conformity and homogeneity, as well as the reliability of the matching of abutting fields. Methods and materials CT datasets of a 9-year-old child and a 24-year-old adult treated with supine CSI were identified. The planning target volume (PTV) was contoured to include the whole contents of the brain and spinal canal with a uniform margin of 5 mm. RapidArc plans were generated with two partial arcs covering the brain and the superior portion of the spinal cord and a single partial arc covering the remaining inferior portion of the spinal cord. Conformity and heterogeneity indexes (CI and HI) and organs at risk (OAR) dose-volume histograms were evaluated. The effect of treatment inaccuracy was simulated with intentional patient shifts of +/- 3 mm. Results The CI for the pediatric and adult cases was 1.0 and 0.99, respectively, and the HI for the two cases was 12.7 and 11.8 %, respectively. The mean dose to the PTV was 102.5 and 102.1 %, respectively, while the mean doses to the OARs were all low. The simulated +/- 3-mm shift in patient position generated an error of less than +/- 10 % of the calculated dose in the spinal cord. Conclusions VMAT CSI was able to achieve highly conformal and homogeneous dose distributions, with minor doses to the surrounding normal tissues. Setup errors of +/- 3 mm were shown to have little effect on the intended dose distribution.