The Study On Planning Organs At Risk And Variation Of Dose-volume Parameters On Cardiac In Radiotherapy For Breast Cancer

In this study,we quantitative analyzed the displacement of the coronary artery main bifurcation,the variation of location,volume,morphology and dose-volume parameters on left anterior descending branch and left ventricular myocardium caused by heartbeat in radiotherapy for breast cancer based on electrocardiography-gated.Then,determined the margin of coronary artery main bifurcation,and the margin of planning organs at risk on left anterior descending branch and left ventricular myocardium,and the error range of dose evaluation index on them.A total 37 famale patients were enrolled in the study on margin of the major coronary artery bifurcation.A total 15 famale patients were enrolled in the study on the left anterior descending branch and left ventricular myocardial caused by heartbeat.All patients received electrocardiography-gated 4DCT scanning performed in the state of peacefully inspiratory breath-hold.The image reconstruction was conducted at 5%intervals during the cardiac cycle,and a total of 20 phase images（0%,5%...90%,95%）were reconstructed.We contoured the coronary artery main bifurcation,the left anterior descending branch and left ventricular myocardium slice by slice on the cross-sectional images of 20 time phases in each patient using MIM Maestro 6.7.6 software,the displacement,volume and morphology of left anterior descending branch and left ventricular myocardium were quantified,and the margin of planning organs at risk on them were calculated.The radiation treatment planning of left-side breast cancer was designed on 0%phase images of each patient and rigid registration was performed between 5%-95%and 0%phase images,and the dose assessment of left anterior descending branch and left ventricular myocardium in 20 time phases in the cardiac cycle were simultaneously included.The dose-volume parameters of left ventricular myocardium and left anterior descending branch were mean dose D_mean,V₁₀,V₂₀,V₃₀ and V₄₀.We found that:（1）The margins of coronary artery main bifurcation in the directions of left-right,cranio-caudal,anterior-posterior were:the left main coronary artery 3mm,3mm,3mm;the first diagonal branch 6mm,3mm,3mm;the second diagonal branch 3mm,3mm,3mm;the tip of left anterior descending branch 4mm,4mm,4mm the obtuse marginal branch 4mm,6mm,5mm;the first right ventricular artery 6mm,8mm,7mm;the acute marginal branch 6mm,8mm,7mm.（2）The margin of plannning organs at risk on left ventricular myocardium and left anterior descending branch in the directions of left-right,anterior-posterior,cranio-caudal were 2mm,3mm,2mm;3mm,3mm,3mm,respectively.（3）The average variation rate of D_mean on left ventricular myocardium was（18.74±9.32）%and the maximum variation rate was 41.95%.The average variation rate of D_mean on left anterior descending branch was（58.88±29.10）%,and the maximum variation rate was 130.14%.The average variation rate of the D_mean on left anterior descending branch was significantly higher than that of left ventricle myocardium.（4）The difference between the maximum and minimum values of V₁₀,V₂₀,V₃₀,V₄₀0 on left ventricular myocardium and left anterior descending branch caused by heartbeat were（10.06±9.21）%,（9.24±9.63）%,（8.45±9.66）%,（6.86±10.00）%;（28.52±12.11）%,（29.35±12.65）%,（28.84±13.74）%,（26.35±15.89）%,respectively.The difference values of V₁₀,V₂₀,V₃₀,V₄₀ on left anterior descending branch were significantly greater than those of the left ventricular myocardium.The results indicated that the heartbeat would significantly affect the the location,volume,morphology and dose assessment index on left anterior descending branch and left ventricular myocardium in radiotherapy for breast cancer.These variation suggested us there were significant error on the dose-volume parameters of left anterior descending branch and left ventricular myocardium based on static treatment planning,which lead to the uncertainty of prediction on radiation-induced heart disease.Therefore,it is necessary to give the appropriate margin to left ventricular myocardium and left anterior descending branch as planning organs at risk to participate in the dose assessment during the radiotherapy of breast cancer.