Study On Dose Audit Technology Of Volumetric Modulated Arc Therapy

Objective:Volumetric modulated arc therapy(VMAT)is a new type of intensity modulated radiotherapy that integrates a high-precision accelerator and a reverse optimization plan design system.The fundamental goal of VMAT is to give the tumor a precise radiation dose,improve the tumor control rate,reduce the organ at risk(OAR)dose and decrease the complications of normal tissues.Quality control of radiotherapy is the key to ensuring the accuracy of radiotherapy planning,improving the effect of radiotherapy and preventing radiotherapy accidents.Thermoluminescence dosimeters(TLDs)and optically stimulated luminescence dosimeters(OSLDs)are practical and accurate solid-state detectors for point dose measurement in medical physics.At present,TLDs are used for point dose audit of radiotherapy in China.However,TLD powder is too complicated and cumbersome in packaging,measurement and annealing.TLD can not be repeated measurement,it is sensitive to heat and chemical factors,so the operation has a certain challenge.OSLD has similar performances to TLD,such as good environmental stability,high measurement efficiency,repeatable measurement,and has been used in radiation therapy dose audit abroad.The aim of this study is to reduce the workload of surveyors in the preparation and measurement,improve measurement efficiency and establish a method suitable for large scale VMAT dose quality audit.The purpose is to verify the performances of OSLD,commissioning relevant correction factors,and further the study end-to-end dose audit method of VMAT.Methods:In this study,nanoDot OSLDs combined with solid phantoms were used to perform batch irradiations and verify the performances under SSDL radiation level 60Co radiation field.The researche was determined the Element Correction Factors(ECF),signal depletion per reading,signal fading,linearity,angular dependence and confirmed the possibility of OSLD for VMAT dose quality audit.EBT3 film measured the 2D dose distribution,TLD and OSLD audited the point dose of PTV and OAR.The end-to-end dose delivery audit of intensity modulated radiation therapy(IMRT)and VMAT was conducted in this work to analyz the results,evaluate the measurement efficiency and accuracy of TLD and OSLD,and to explore the application prospect of OSLD in VMAT radiotherapy dose audit.Results:The results showed that the sensitivity,signal depletion per reading,signal fading,linearity and angular dependence of OSLD were meet the requirements of VMAT dose audit.The ECF of OSLD was distributed around 1.OSLD lost 0.029%of its original signal per readout.Due to the logarithmic signal fading,the singal of OSLD were extremely unstable in the first few minutes after irradiation.After 15 minutes,the counts were relatively stable and could be measured.The signal fading of OSLD at 2 Gy dose was 6.94%in 34 days.OSLD showed a linearity(R2=0.998)below 5 Gy.When the OSLDs are perpendicular to the beam,the response is-6.76%-+1.5%,the difference is statistically significant(F=34.493,P<0.001).Whereas,when the OSLDs are parallel to the beam,the angular response is-1.74%—+1.67%,the difference is statistically nonsignificance(F=1.263,P>0.05).The±3mm/3%γ pass rates of EBT3 film of 2D dose distribution were all over 90%in dose audit.And TLD results generally consistented with the requirement.The audit results of planning target volume(PTV)measured by OSLD mostly consistented with the requirements,while the results of organs at risk were largely unqualified.Conclusions:From what has been discussed above,TLD technology has been very mature,but it is difficult to carry out large scale and efficient development at present.OSLD as a new detector,its performances can meet the requirements of VMAT dose audit.When measuring the dose of PTV,the accuracy of OSLD is equivalent to TLD.However,OSLD can greatly reduce the workload and time and improve the measurement efficiency.Therefore,OSLD is suitable for large scale VMAT dose audit.The OAR dose measured by OSLD has a large deviation from the TPS plan,and further explorations are needed in the experimental methods and evaluation indexes.In this study,the performances of OSLD were verified,and the method of OSLD for radiotherapy dose audit was explored.This research provides a basis for OSLD to replace TLD in radiotherapy dose verification.It to some extent fills in the margin of OSLD in radiotherapy dose audit in China,and supplies a preliminary guidance on method of application.