A Study Of Dose Accumulation In The Viscera Of Left Breast Cancer IMRT Radiotherapy Center On Deformation Registration

In this study,deformation registration technology was used to explore two ways to accurately evaluate the cardiac dose,respectively,the cardiac dosimetry changes caused by the changes in the target area and the cardiac dosimetry changes caused by the heartbeat movement.In the study of cardiac dosimetry changes caused by changes in target area,CT1 series of 16 patients with left breast cancer and CT2series in the middle and later stages of treatment were analyzed retrospectively.Plan 1 was made on CT1,plan2 was made on CT2.The two-time positioning CT series were used for CT rig/CT def registration using MIM Maestro 6.6.9（MIM）software,and the dose was accumulated accordingly to obtain plan-rig/plan-def.The clinician delineated the heart,left lung,right lung and target area on all images.The dosimetric differences of target areas and organs in four plans were compared.In the study of cardiac dosimetry changes caused by heartbeat movement,15 cases of electrocardiographic gated four-dimensional computer tomography（ECG 4D-CT）series were analyzed retrospectively under the condition of deep inspiration breath holding（DIBH）.The images of 5%⁹5%20 phases were reconstructed at intervals of 5%of cardiac cycle.The ECG 4D-CT series is reconstructed with MIM software to get the maximum intensity projection（MIP）,minimum intensity projection（MinIP）,average intensity projection（AIP）and sum intensity projection（SIP）images.On all images,left ventricular muscle（LV）and left anterior descending coronary artery（LAD）were delineated one by one.Based on the 0%time phase,the left breast cancer intensity adjustment plan was designed,and 20 time phase accumulative doses(Dose_-acc)were obtained by deformation registration.The dose distribution of 0%phase was registered on MIP,MinIP,AIP and SIP images respectively.The difference of dose-volume index between LV and LAD based on different CT series was compared.The main results were as follows:（1）The volume of CTV in breast cancer patients decreased by 8.5%on average compared with that in CT1;（2）The dose volume indexes of the heart,left lung and right lung in plan2,plan-rig,plan-def and plan1 groups had no significant difference（P>0.05）;（3）After deformation,the dice similarity coefficients（DSC）of heart,left and right lungs were lower than those before deformation（0.94±0.01 vs 0.89±0.05;0.96±0.01 vs 0.91±0.03;0.96±0.01 vs 0.92±0.03,respectively）.The difference was statistically significant（P<0.05）;（4）The volume of LV decreased in ECG 4D-CT,MinIP,AIP,MIP and SIP images,which were（54.55±12.68）cm³,（53.36±12.08）cm3,（52.05±13.31）cm³,（50.92±11.10）cm³ and（42.37±9.72）cm³,respectively;Compared with ECG 4D-CT,LV dose volume indexes were less than 5%,including V₅,V₃₀,V₄₀,D_max,D_mean（3.8%,2.0%,0.9%,3.8%,1.7%）on MIP images and D_max on AIP,SIP,MinIP images;（5）The volume of LAD decreased in MIP,AIP,SIP,MinIP and ECG 4D-CT images,which were（3.40±0.94）cm³,（2.24±0.81）cm³,（1.97±0.86）cm³,（1.67±0.47）cm³,and（1.45±0.24）cm³,respectively;In the evaluation of LAD’s dose-volume index,only D_max in four special reconstructed images had a change rate of less than 5%compared with ECG 4D-CT,but the difference was statistically significant（P>0.05）.The results of this study showed that the target area and crisis organ changed little in the course of radiotherapy for left breast cancer,and rigid registration was recommended when multiple planned doses were added;the target area and Organs at Risk dose-volume index changed relatively little,and the first plan could be used to evaluate the exposure of both lungs and heart.The volume of LV had little difference between the special reconstruction image and the original ECG 4D-CT image.In the evaluation of LV reception,V₃₀,V₄₀ and D_mean of MIP image can be used to replace ECG 4D-CT image for rapid evaluation;for lad,there is no special reconstruction image to replace ECG 4D-CT.