Abstract
This study aims to model an extra-focal source for the scattered radiation from multi-leaf collimators (MLCs), namely an MLC scatter source, and to correct in-air output ratio (Sc) calculated using the conventional dual source model (DSM) to achieve better accuracy of point dose calculation. To develop the MLC scatter source, a 6 MV photon beam from a Varian Clinac® iX linear accelerator with millennium 120 MLCs was used. It was assumed that the position for the MLC scatter source was located at the center of the MLC, consisting of line-based and area-based sources to consider the characteristics of the scattered radiation from the MLCs empirically. Based on the measured Sc values for MLC-defined fields, optimal parameters for the line-based and area-based sources were calculated using optimization process. For evaluation of proposed method, measurements were taken for various MLC-defined square and irregular fields. The Sc values calculated using the proposed MLC scatter source and conventional DSM were compared with the measured data. For MLC-defined square fields, the measured Sc values showed better agreement with those calculated using the MLC scatter source (the mean difference was − 0.03%) compared with those calculated using the DSM (the mean difference was 0.18%). For MLC-defined irregular fields, the maximum dose differences between measurements and calculations using the MLC scatter source and DSM were 0.54% and 1.45%, respectively. The developed MLC scatter source could improve the accuracy of Sc calculation for both square and irregular fields defined by MLCs.
Highlights
Following the advances in radiotherapy techniques, intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) have been widely adopted forThere were three important factors to determine Sc: (1) Scattered radiation from the linear accelerator head to the phantom (2) Backscattered radiation from the jaws to the monitor chamber, and (3) Source obscuring effect for small1 3 Vol.:(0123456789)Australasian Physical & Engineering Sciences in Medicine (2019) 42:719–731 field sizes [5,6,7,8,9]
It stated that the scattered radiation from multi-leaf collimators (MLCs) is no longer negligible for Sc calculation, when small and irregular fields are used in IMRT and VMAT plans [9]
This study aims to model an extra-focal source for the scattered radiation from MLCs, namely an MLC scatter source, and to correct Sc calculated using the conventional dual-source model (DSM) to increase the accuracy of point dose calculation
Summary
Following the advances in radiotherapy techniques, intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) have been widely adopted forThere were three important factors to determine Sc: (1) Scattered radiation from the linear accelerator head to the phantom (2) Backscattered radiation from the jaws to the monitor chamber, and (3) Source obscuring effect for small1 3 Vol.:(0123456789)Australasian Physical & Engineering Sciences in Medicine (2019) 42:719–731 field sizes [5,6,7,8,9]. The American Association of Physicists in Medicine (AAPM) Task Group (TG) 74 report clearly updated the definition of Sc as the change in the scattered radiation of the output beam due to the primary collimator, flattening filter, jaws, and MLCs [9] It stated that the scattered radiation from MLCs is no longer negligible for Sc calculation, when small and irregular fields are used in IMRT and VMAT plans [9]. Zhu et al developed an integrated extra-focal source involving MLC scatter and other components [18, 19] These studies improved the calculation accuracy of Sc by accounting for the MLC scatter effect, residual discrepancies between the measured and calculated Sc values remained. To the best of our knowledge, no attempt has been made to explicitly model an individual extra-focal source accounting for only scattered radiation from MLCs
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