Method Of Virtual Source Modeling For External Photon Radiotherapy And Its Clinical Application In Dose Checking

Patient specific QA has become a critical part of radiotherapy.Due to the complexity and uncertainty of treatment planning techniques,the dose distribution generated by TPS needs to be validated before delivery.The dose verification methods can be divided into two categories,namely measurement-based dose validation and calculation-based dose validation.Measurement-based dose validation methods cannot take the heterogeneous anatomy of the actual patient into account and are limited by the measurement equipment.The objective of this paper is to develop a virtual source model for external photon beam radiotherapy and integrate it into the dose verification software ArcherQA.To achieve this goal,this project accomplished the following tasks:(1)constructing a flexible virtual source model for Varian TrueBeam as well as Elekta Axesse,and performing automatic optimization of model parameters using measurement data;(2)constructing a model of beam limiting devices(JAW,MLC)and developing fast pho-ton transport calculation methods;(3)integrating these models into the ArcherQA dose calculation framework and validating ArcherQA using clinical radiotherapy plans.The initial parameters of the virtual source model are extracted from the Varian TrueBeam phase space file published by the manufacturer,and then the model parameters of Varian TrueBeam and Elekta Axesse are tuned automatically based on the measurement data;The photon transport in beam limiting devices are based on the mass attenuation coefficient.The corresponding results are(1)For Varian TrueBeam,the VSM depth doses agree within 2%with the measurements,and the lateral doses are within 3%.For Elekta Axesse,the VSM depth doses agree within 3%with the measurements,and the lateral doses are within 3%.(2)The dose profiles of Archer and EGSnrc are consistent with the film measurements,while the profile of Monaco deviates significantly from the film measurements.(3)For clinical cases,ArcherQA and Eclipse have a better degree of compliance.The 3mm/3%gamma passing rates are above 96%and the 2mm/2%gamma passing rate are above 90%.However,ArcherQA has a lower gamma passing rate compared with Monaco,For most cases,3mm/3%passing rates are around 90%and 2mm/2%passing rate are only 80%.In conclusion,the speed and accuracy of ArcherQA,which integrates the virtual source and beam limiting device model developed in this paper,can meet the clinical requirements,making a breakthrough of Monte Carlo independent dose validation software in China.