Dosimetric Study Of Cardiac Substructures During Breast-Conserving Radiotherapy For Left-Sided Breast Cancer

Objectives To observe the exposure dose of the heart,left ventricle（LV）and left anterior descending artery（LAD）in adjuvant radiotherapy after breast-conserving surgery for left breast cancer patients who were all treated with volumetric modulated arc therapy（VMAT）technique,and investigate the effect of LAD and LV as organs at risk on target coverage and dose volume distribution of organs at risk in the original radiotherapy plan.Methods A total of 20 patients who were treated in Hebei General Hospital from January2020 to October 2021 and who had previously received conventional fractionated tumor bed simultaneous radiotherapy after breast-conserving surgery for left-sided breast cancer were included in the study.To observe the radiation dose received by the patient’s heart and its important substructures under the VMAT radiotherapy technique.In addition,for these patients who had previously completed radiotherapy and whose LV and LAD were not defined as organs at risk（OAR）,the LV and LAD were delineated as new organs at risk（OAR）by the same radiographer on the same localized CT.A new radiotherapy plan was redesigned for each patient by a senior physicist based on VMAT technology.To ensure plan comparability,the radiotherapy plans for all patients were prepared by the same senior physicist,and all optimized parameters and the number of fields in the two radiotherapy plans of the same patient were the same.The whole breast prescription dose:5000Gy/25f,200c Gy/f,5f/week,and the tumor bed area was simultaneously boosted to 60Gy.The dose-volume histograms were used to collect the dose-volume parameters of the target volume and organs at risk in the radiotherapy plans of the two groups,the target volume-related optimization parameters such as conformity index（CI）and uniformity（HI）and the dose-volume parameters related to organs at risk were compared between the two groups.SPSS 26.0 statistical software was used for statistical analysis.Statistical methods used paired samples t test or Wilcoxon signed rank sum test.The difference was considered to be statistically significant at P<0.05.Results 1 In the left breast cancer postoperative radiotherapy based on VMAT technology,LAD in the treatment group was exposed to high radiotherapy dose,the LAD Dmax of the treatment group was 46.88±3.88Gy,the LAD V40 of the treatment group compared with the optimized group was（13.49±9.94 vs 0±0.01,P<0.001）,and the LV V5of the treatment group compared with the optimized group was（62.30±19.85 vs34.97±12.34,P<0.001）.2 In terms of target dosimetry comparison:both of the two groups met the target dosimetry requirement that 100%of the prescribed dose covered at least95%of the planned target volume,and the difference was not statistically significant.There was no statistical difference in the HI of PTV_TBand PTV_WBbetween the two groups.The CI of PTV_TBin the treatment group and the optimization group were 0.77±0.03 and0.76±0.03,P=0.044.Further analysis found that although the CI values of the two groups were statistically different,the CI values of the two groups were greater than 0.75.The actual difference is very small,and the target coverage of the two groups is satisfactory,and both can meet the actual clinical radiotherapy target dosimetry requirements.In PTV_TB,the D5%of the optimized group was slightly higher than that of the treatment group（64.33±0.55 vs 64.56±0.82,P=0.025）.There was no significant difference in other dose-volume-related parameters of PTV_TBand PTV_WBin the two groups.3 In terms of cardiac dose comparison:the optimized group can significantly reduce the Dmean of the heart,LAD and LV.The optimized group can significantly reduce the equivalent irradiation volumes V5,V10,V20,V25,V30,and V40 of the heart and LV,and the difference is statistically significant（P<0.05）.It can also reduce the equivalent irradiation volumes V20,V30,and V40 of LAD.Compared with the treatment group,the LAD Dmax of the optimized group decreased by 8.88Gy,and the difference was statistically significant（46.88±3.88 vs 38.40±1.41,P<0.001）.There was no significant difference in LADV5 and V10 between the two groups.4 In terms of organ at risk dosimetry comparison:The doses of the left and right lungs,right breast and spinal cord in the treatment group and the optimized group were similar,and the difference was not statistically significant.Conclusions 1 In the postoperative radiotherapy of left breast cancer,LAD and LV are still exposed to a relatively high level even if the more precise and conformal VMAT radiotherapy technology is used and the overall average dose to the heart is controlled within a reasonable range.And it is reasonable and necessary to delineate the LAD and LV as organs at risk and give appropriate dose restrictions.2 The coverage of the target area in the treatment group and the optimized group is basically close,and both can meet the actual clinical radiation therapy target area dosimetry requirements.3 Compared with the treatment group,the optimization group achieved a significant reduction in the radiation dose to the heart,LAD and LV,which is particularly important in radiotherapy centers without DIBH technology,and the optimization group did not increase other organs at risk（such as bilateral lungs,right breast,spinal cord）.Figure 3;Table 9;Reference 118...