The Set-up Errors And Impact Of The Errors On Dose Distribution In Volumetric Modulated Arc Therapy For Cervical Cancer Based On 3DVH System

Objective:To retrospectively investigate the set-up errors in volumetric modulated arc therapy?VMAT?for cervical cancer,and using 3DVH sys-tem to analyze the impact of set-up errors on dose distribution of tumor target and organ at risk.Methods:45 cervical cancer patients treated for radical VMAT radiotherapy in our department were randomly selected to research the trend of the Set-up Errors,and 12 cases among those were selected by clinical stage to research the impact on dose distribution.All pantients got the CBCT scans prior to be performed treatment delivery at first three days.After registration to planning CT image by employed XVI medical image processing system and bony algorithm,translational and rotational shifts were calculated and corrected online.Then the influence of set-up errors on dose distribution was simulated by shifting the ArcCHECK phantom isocenter,and meanwhile the difference between the orignal and comparing dose distribution were obtained by measuring and analysing with 3DVH dosimetry system,but not changed the planning parameters and dose's delivery.Results:The registration results form 135 scans in 45 cervical cancer patients show that three translational shifts and rotational shifts were?-0.94±2.49?mm,?3.10±3.75?mm,?0.93±1.91?mm,and?0.02±1.80?°,?0.27±0.93?°,?-0.41±1.33?°,respectively.The PTV margins in three axis were 7.2mm,10.1mm and 5.2mm respectively.The Gamma pass rate decreased significantly When the impact of set-up errors were be simulated.Compared with the orginal center,the Gamma pass rate?Gp%?of 2D dose?absolute dose and relative dose?and 3D dose decreased 24%,24%,2%,respectively;The proportion of GTV and PTV D_95%dose decreased more than 1%in DVH parameter were 13.89%,8.33%,respectively,and no dose decreased more than 5%occurred;The proportion of bladder and rectum V_45Gy volume decreased more than 1%were 77.8%,80.56%,respectively,and that decreased more than 5%were 50%,38.89%,respectively.The proportion of bladder and rectum V_45Gy>50%were 41.7%,33.3%,respectively.The 2D Gp%change?absolute dose and relative dose?,the 3D Gp%change,the D_95%dosage change of GTV and PTV were significantly related to Y axis set-up errors,and the regression coefficients were-46.931,-59.176,-13.402,-0.652 and-0.609 respectively.PTV D_95%dosage change have orientational different,which in Y-axis positive direction was 0.19Gy more than the negative direction.Conclusion:There exists some set-up errors in cervical cancer VMAT that would significant impacted dose distribution,especially in organs at risk,and that would adverse effected on the therapeutic effect.There were some statistically significant negative correlations between Y-axis setup errors and dosage change,so we should pay attention to that PTV D95%dosage change have directional different.The application of IGRT technology would reduced effectively the setup errors in VMAT for cervical cancer to achieve precision radiotherapy.