Abstract
PurposeThe purpose of this study was to investigate the role of beam collimator rotation in Volumetric Modulated Arc Therapy (VMAT) for craniospinal irradiation (CSI), and the impact on dose distribution in the beam junctions.MethodsSix adult patients were selected for the study. Six VMAT plans with different collimator angles were generated for each patient. The patients were treated in supine position with two beam isocenters. The plans were evaluated by analysis of Dose-Volume Histogram (DVHs) data for planning target volume (PTV) and organs at risk (OAR), and conformity index (CI) and homogeneity index (HI) for the target. Dose distributions in the beam junctions were examined carefully and experimentally validated in phantom, with measurement using an ion chamber array and film.ResultsThe mean values of HI and CI for the plans with different beam collimator angles were not significantly different. The numbers of segments, monitor units (MUs) and the delivery time of the plans with 45° beam collimator were obviously higher than those in plans with other beam collimator angles. When collimator angle for both sets of beams were set at 0°, there was a 1 mm low dose gap measured in the junction region.ConclusionsBy setting the collimator angle to 45°, only two isocenters were needed for the treatment of a target with the length up to 90 cm. The HI and CI of the plans were almost the same, regardless if the collimator angles were at 0°. The collimator angles for at least one set of beams should be off 0° in order to avoid a dose gap in the beam junction region.Electronic supplementary materialThe online version of this article (doi:10.1186/s13014-015-0544-z) contains supplementary material, which is available to authorized users.
Highlights
Craniospinal irradiation (CSI) is a standard therapy for some primary central nervous system (CNS) tumors like medulloblastoma and ependymoma [1,2,3,4,5]
In this study we investigated the volumetric modulated arc therapy (VMAT) technique for CSI incorporated with collimator rotation, in order to limit the number of field sets to two
There were no significant differences among the mean doses for the planning target volume (PTV) in all plans, and the organs at risk (OAR) doses, in general do not pose a concern
Summary
Craniospinal irradiation (CSI) is a standard therapy for some primary central nervous system (CNS) tumors like medulloblastoma and ependymoma [1,2,3,4,5]. The traditional CSI techniques are complicated; the patients in prone position are uncomfortable with poor reproducibility, and with supine position it is rather difficult to verify the field junctions. The poor dose uniformity in the region of field junction is often unacceptable. In this study we investigated the volumetric modulated arc therapy (VMAT) technique for CSI incorporated with collimator rotation, in order to limit the number of field sets to two. A beam with 45° collimator angle can cover the longest for a tube-like shape of target such as the spine, as shown in Additional file 1: Figure S1. The impact on dose distribution in the field junction region with various collimator angles, or directions of MLC leaf travel was evaluated using accurate Monte-Carlo based dose calculation
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