To Verify The Movement Dosimetry Impact Of The Modified VMAT Technique Applied To Whole Breast Radiotherapy For Breast Cancer Through 4D-CT

Objective To discuss the dosimetric advantage of the modified volumetric modulated arc therapy(VMAT)technique compared with the hybrid intensity modulated radiation therapy(H-IMRT)technique and convensional VMAT technique when applied to breast conserving postoperative radiotherapy of left sided breast cancer,and to reduce the impact of respiratory movement on dosimetry.Methods 20 cases of patients with left sided breast cancer after breast-conserving surgery in Shenzhen People’s Hospital from were selected.In the case of placed and not placed 15 mm virtual bolus on the left sided breast at supine position,each patient was underwent four-dimensional computed tomography(4D-CT)scan under spontaneous breathing respectively and two group of CT image sequences which varied with respiratory phase were obtained.On the two groups of MIP images,the radiotherapist contoured the left whole breast and the organs at risk including heart and lung.For each patient,the radiologist formulated an modified VMAT plan on MIP images with placed bolus,and H-IMRT plan and conventional VMAT plan on MIP images without placed bolus.The completed plans were copied and applied to each other respiratory phases to calculate and obtain the cumulative dose of the target volume and organs at risk for the entire respiratory phases.IBM SPSS Statistics 26 statistical software was used for statistical analysis.Measurement data were expressed as χ± S,and the paired t test was used for comparison between groups.P<0.05 indicate that the difference is statistically significant.Results Compared with conventional VMAT group,the volume coverage rate of PTV V50 of the modified VMAT group and H-IMRT group increased by 10.20% and10.40%,respectively,and the differences were statistically significant(P<0.001).The volume coverage rate of PTV V50 of the modified VMAT group was similar to that of the H-IMRT group,and the difference was not statistically significan(P=0.597).Compared with H-IMRT group,the heart Dmean and left lung Dmean in the modified VMAT group decreased by 1.61 Gy and 2.26 Gy,respectively,with statistical significance(P<0.001).In terms of the percentage of the volume of heart heart and lung irradiated in the high-dose area,V20 and V30 of heart of the modified VMAT group were 6.7% and 7.4% lower than those of the H-IMRT group,respectively,the differences were statistically significant(P<0.001),and V20,V30 and V40 of left lung of the modified VMAT group were 4.5%,6.46% and 9.18% lower than those of the H-IMRT group,respectively,the differences were statistically significant(P<0.001).In terms of the percentage of the volume of heart heart and lung irradiated in the low-dose area,compared with H-IMRT group,V5 of heart and of left lung of the modified VMAT group were increased by 8.13% and 5.98%,respectively,with statistical significance(P<0.001).Conclusions When the modified VMAT technique was applied to breast conserving postoperative radiotherapy of left sided breast cancer,compared with H-IMRT technique,it further reduced the irradiated volume of organs at risk including heart and lung in the high-dose area.The higher the dose is,the more obvious the decreasing trend of the exposed volume is.And compared with conventional VMAT technique,it improved the prescription dose coverage of the target volume,and reduced the influence of respiratory movement on dosimetry of target volume.