Dependence of Collimator Angle on Prostate VMAT: A Treatment Planning Study

Abstract

This study investigated the dose-volume dependences of planning target volume (PTV) and organs-at-risk (OARs) on prostate volumetric modulated arc therapy (VMAT), when the collimator angle is varied. Single-arc VMAT plans at different collimator angles (0°, 15°, 30°, 45°, 60°, 75° and 90°) were created systematically based on a Harold pelvis phantom and the 6 MV photon beams. The conformity index (CI), homogeneity index (HI), gradient index (GI), machine monitor units (MUs), dose-volume histogram and mean and maximum dose of the PTV were calculated and analyzed (Table 1). The dose-volume histogram and mean and maximum doses of OARs such as bladder, rectum and femoral heads for different collimator angles were determined from the plans (Table 2). From our dosimetric results, there was no significant difference, according to the planned dose-volume evaluation criteria, found in the VMAT optimizations for all studied collimator angles. However, a higher CI (0.53) and lower HI (0.064) resulted from the 45° collimator angle. The 15° collimator angle provided a lower value of HI similar to the 45° angle while collimator angles of 75° and 90° were found to be good for rectum sparing. In addition, collimator angles of 75° and 30° worked well for sparing the right and left femur. The PTV dose coverage for each plan was comparatively independent of the collimator angle. The dosimetric results in this study provide support and guidance in the collimator angle selection for prostate VMAT to improve the PTV coverage and OARs sparing.