Park So-Yeon, Kim Siyong, Sung Wonmo, Kim Sang-Tae
Institute of Radiation Medicine, Seoul National University Medical Research Center, Seoul, Republic of Korea.
Department of Radiation Oncology, Veterans Health Service Medical Center, Seoul, Republic of Korea.
Australas Phys Eng Sci Med. 2019 Sep;42(3):719-731. doi: 10.1007/s13246-019-00781-2. Epub 2019 Jul 22.
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 (S) 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 S 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 S values calculated using the proposed MLC scatter source and conventional DSM were compared with the measured data. For MLC-defined square fields, the measured S 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 S calculation for both square and irregular fields defined by MLCs.
本研究旨在为多叶准直器（MLC）的散射辐射建立一个焦外源模型，即MLC散射源，并对使用传统双源模型（DSM）计算的空气中输出比（S）进行校正，以提高点剂量计算的准确性。为了建立MLC散射源，使用了瓦里安Clinac® iX直线加速器产生的6 MV光子束以及millennium 120 MLC。假设MLC散射源的位置位于MLC的中心，由基于线的源和基于面积的源组成，以便根据经验考虑MLC散射辐射的特性。基于MLC定义射野的测量S值，通过优化过程计算基于线的源和基于面积的源的最佳参数。为了评估所提出的方法，对各种MLC定义的方形和不规则射野进行了测量。将使用所提出的MLC散射源和传统DSM计算的S值与测量数据进行比较。对于MLC定义的方形射野，与使用DSM计算的结果（平均差异为0.18%）相比，测量的S值与使用MLC散射源计算的结果（平均差异为-0.03%）显示出更好的一致性。对于MLC定义的不规则射野，使用MLC散射源和DSM进行测量与计算之间的最大剂量差异分别为0.54%和1.45%。所建立的MLC散射源可以提高MLC定义的方形和不规则射野S计算的准确性。
