Dosimetric Impact Of Hysteresis On Lung Cancer Tomotherapy

Background and Objectives:Hysteresis is an individual but common phenomenon in patients.For disease affected by respiratory motion such as lung cancer,hysteresis is closed connected with the precise delivery of radiotherapy and therefore plays a role in considerations of tumor motion behavior.Researches about the dosimetric impact of hysteresis on radiotherapy were seldom performed,and the dosimetric impact of hysteresis on lung cancer tomotherapy has thus far been examined in no studies yet.This study investigates the dosimetric impact of hysteresis on lung cancer tomotherapy,and provides suggestion for lung cancer tomotherapy.Methods:MapCheck was sandwiched between the array calibration fixture and the film alignment template and then fixed on a XY4D motion simulation table to form the phantom combination.Phantom plans were based on the megavoltage computed tomography image of the phantom combination.The target,OARs and optimization parameters were from patient plans,which ensured that the phantom plans were similar to clinical situations.Each plan was measured three times with a random starting phase in the static state and five hysteresis states(hysteresis 0,?/32,?/16,?/8,and ?/4);the average of the three values was used in the analyses.The SNC Patient software imported the DICOMRT structure file modified by an in-house-developed MATLAB program and displayed and analyzed globally,target and OARs in the measurement plane individually.The gamma index analysis with relative dose mode,distance-to-agreement 3 mm,and dose difference 3%were used in comparisons of the measured static data and the planning data.The absolute dose difference global normalization with dose difference 3%was adopted in the analysis of each hysteresis vs.static state.The thresholds were 10%for all analyses.The Wilcoxon signed rank test was used for the statistical analysis,and the significance level was 0.05.The Spearman's rank correlation coefficient was used for the correlation analysis,and the significance level was 0.05.For simulating various 3D organ motions,especially the hysteresis phenomenon,an in-house-developed organ motion simulation system was designed.It was composed of three parts:anthropomorphic thoracic phantom,target motion simulating device and organ motion simulating software.And above methods were also used in the representative case measurement on the in-house-developed 3D organ motion simulation system.Results:For the gamma index analysis of the static measured data vs.the planning data,the passing rate for globally and target were 99.11%±1.51%and 98.97%±2.35%respectively.For OARs,the lowest passing rate was 98.91%±3.69%,recorded for the left lung.For the dose difference analysis of each hysteresis vs.static state,the passing rate for globlally,target,left lung,right lung,heart and chest wall were 34.54%,29.11%,26.28%,49.56%,34.75%and 32.19%respectively.And it was similar between different hysteresis states for same object,except for significant differences in the comparison of hysteresis ?/32 relative to static vs hysteresis ?/16 relative to static for the left lung,24.87±10.97%and 26.12%± 10.6%respectively.In target and OARs around target,the shape of dose difference distribution of motion vs static changed with hysteresis increment.Since the major components of anthropomorphic thoracic phantom were not achievable,the organ motion simulation system contained only target motion simulating device and organ motion simulating software.The software drived three programmable stepper moters to achieve 3D motion in Anterior-Posterior,Left-Right and Superior-Inferior directions with max range 120mm,300mm and 200mm separately.There is no metal in the measurement range of the carbon fiber carrier,the impact of metal component for phantom and detector in imaging and delivering is eliminated.Comparing with the data from XY4D,the data from 3D organ motion simulation system was different with passing rate value slightly,but same with changing trend.The max period difference and max amplitude difference between 3D organ motion simulation system and XY4D was 0.11s(2.77%)and 0.84mm(5.54%)respectively.Conclusions:The high passing rate for gamma index analysis of the static measured data vs.the planning data showed the impact from other factors than motion,for instance the metal in phantom combination and the angular dependence of diode,were limited for all plans in this study.The impact of hysteresis for each hysteresis vs.static state differed depending on the object but was similar between different hysteresis states for same object.And the dose difference of motion vs static in target and OARs around target changed with hysteresis increasing,particularly for small OARs adjacent to target.This method,in which the dosimetric impact is evaluated object by object,provides an individual and comprehensive approach to studies on the dosimetric impact of organ motion,and has thus far been adopted in no studies yet.The in-house-developed 3D organ motion simulation system met the experimental needs of this subject,and it could provide sophisticated 3D organ motion simulation.And it is the first radiotherapy dosimetric research with 3D organ motion simulation domestically.