Localized extra focal dose collimator angle dependence during VMAT: An out-of-field Monte Carlo study using PRIMO software

Abstract

PurposePatients undergoing 3DCRT on TrueBeam linac equipped with HD_120_MLC are subjected to localized extra-focal dose (LEFD) in a specific region out of the treatment field. The dose in this region is up to 160% of the mean transmitted dose through MLC. Although VMAT provides a more conformal dose to PTV and better sparing to OAR, it implies more monitor units and irradiating more healthy tissues outside the treatment field. One aims to investigate the dose distribution due to LEFD during VMAT and the collimator rotation angle optimization to minimize this extra dose. MethodsEight VMAT plans for a hypothetical abdominal tumor were contoured on an anthropomorphic pediatric phantom and calculated on Eclipse TPS. The plans were optimized to the same dosimetric objectives at PTV and OAR with different collimator rotation angles. They were also simulated using PRIMO Monte Carlo software. The dose out of the treatment field was evaluated in eight spherical structures distributed at different distances from the isocenter. To validate the results, QA verification plans were created, were measured by EPID and simulated in a virtual phantom of the EPID. Measured and simulated QA plans were compared using gamma-index methodology. Point dose measurements were also used to verify Monte Carlo simulations accuracy out of the treatment field. ResultsBoth measured and simulated QA dose distributions showed that each treatment arc has two LEFD regions out of the treatment field which were not calculated by TPS. The simulated treatment plans showed LEFD as cylindrical dose distributions, 10 cm height, with the diameter of X-jaws aperture. At 0° rotation the two LEFD regions corresponding to the two-treatment arcs are superimposed. As rotation angle increases the two cylindrical distributions start to drift apart but their intersection remains localized along the central axis until they are completely separated. ConclusionThe LEFD can be avoided during VMAT without compromising the treatment plan objectives by increasing the collimator rotation angle to relocate LEFD out of the body.