The Assessment Of Cardiac Motion Using ECG-gated DSCT And Their Radiation Dose Reduction In Intensity Modulated Radiotherapy For Left-sided Breast Cancer

Part Ⅰ The assessment of cardiac and coronary movement using ECG-gated DSCTPurpose:Information concerning the amount and nature of target motion is essential for the determination of internal margin size in planning of radiotherapy. The heart, anterior myocardial territory (AMT), left ventricle (LV) and coronary arteries will be irradiated partially and unavoidably during breast radiotherapy (RT). However, there are few published reports outlining the motion with heart and aortic pulsation. We measure their motion in three dimensional (3D) directions for the margins of planning risk volume (PRV) by using electrocardiographically (ECG) gated spiral scanning with dual-source CT system. We describe a new approach to visualize internal organ motion resulting from cardiac motion with ECG gated dual source computed tomography (DSCT).Methods and Materials:The DSCT images of17patients who had undergone CT-based coronary angiography with retrospective electrocardiographic gating were included in this study. The reconstructed image data of the diastolic and systolic phases were analyzed. For the heart, AMT and LV, the displacements in anterior (A), posterior (P), left (L), right (R), superior (S) and inferior (I) were measured. For the coronary arteries, we measured the displacements in AP, LR and SI directions. With retrospective ECG gating, only data acquired within a predefined interval of the cardiac cycle are used for image reconstruction. The shift of the internal organs between the cardiac cycles was evaluated. The amount of organ movement between the diastolic and systolic images was measured in the ventrodorsal and medial-lateral directions using reconstructed coronal and sagittal images.Results:For the heart, AMT and LV, the largest extent of displacements was in L, P and I direction.4-7mm displacements in majority of directions were found. The displacements of AMT and LV were of3-5mm in A and R direction and5-8mm in L and P direction and5-6mm in S and I direction. For the coronary arteries, the displacements were of in4mm in AP,4-7mm in LR and4-6mm in SI direction.Conclusions:The cardiac motion is asymmetric, which the large extent of motion was in the left, posterior direction and in the lower portion. For the heart including AMT, LV and the coronary arteries, majority of displacements in all directions are at the range of4-7mm which might provide the referrence to the extra margins of PRV to the OARs. Also, replacing the heart by AMT as one of OARs in breast IMRT could reduce the radiation dose not only to AMT itself, also to the heart, LV as well as LAD without the significant increase of other coronary arteries such as LCA, RCA, LCxA and RMA. Part II Decreasing the radiation dose to the heart and coronary arteries using heart motion compensation in intensity modulated radiotherapy for left-sided breast cancerPurpose:To reduce the radiation dose of the heart and coronary arteries, the heart, AMT, LV, LCA, RCA, LAD, LCxA and RMA were contoured as the OARs in IMRT plan, and we used the movement in three dimensional (3D) directions of heart and coronary arteries measured in part I as the margins of planning risk volume (PRV).Methods and Materials:21patients with early-stage left-sided breast cancer who received breast-conserving surgery (BCS) were included in the study. All patients underwent CT simulation in the treatment position. Slices of2.5mm were obtained from the neck to the upper abdomen, and then transferred to the in-house treatment planning system. For consistency, all contours were delineated by a single radiation oncologist. The clinical target volume (CTV) was defined as the palpable breast tissues, and anterior margin of the CTV was parallel to but5mm inside the skin. To compensate for the variability of setup error and breathing motion, the PTV was extended peritherally by8mm to the CTV. The heart, AMT, LV, LCA, RCA, LAD, LCxA and RMA were difined as OARs. We chose IMRT (H) and IMRT (AMT) plan which the heart and AMT was used to be the OAR respectively. The latter is our recommendation that the AMT might replace the heart as OAR in the breast IMRT. Both plans were generated with the similar optimization (Varian Eclipse8.6) and the prescribed doses were50Gy in25factions. After we obtained the displacements in three dimensions of all the OARs, extra margins with these displacements measured in Part I were added automatically by the treatment planning system (TPS) as the planning risk volume (PRV). Then the dose-volume parameters such as the mean dose (Dmean) and D2％were obtained from TPS. D2%means the dose at the2%volume of OARs or PRVs as the near maximum dose. SPSS statistical software package version16.0was used to analyze the differences between the two IMRT plans. The differences of radiation dose between the two IMRT plans were compared with paired t-test with p≤0.05considered to indicate statistically significant differences.Results:According to the displacements of OARs (including the heart, AMT, LV, LCA, RCA, LAD, LCxA and RMA), extra margins were added automatically to all the OARs as the corresponding PRV and the margins were shown in table2.1. For the heart, AMT, LV, compared with the volume of OARs, the PRV increased50.6%±7.9%(95%CI:33%-61%),194.9%±36.4%(95%CI:149.0%-290.0%),76.7%±9.8%(95%CI:56%-91%) respectively. For LCA, RCA, LAD, LCxA and RMA, it did18.7-42.6times as shown in table2.2.The Dmean and D2％of all the OAR in IMRT(H) and IMRT(AMT) were shown in table3.Compared the Dmean to OARs with their PRVs of the heart, it increased5.52%(P＜0.05) and5.81%(P＜0.05) in IMRT(H) and IMRT(AMT) plan respectively. That to PRV of the AMT did8.35%(P＜0.05) as show in figure2.1A and2.1C. Similarly, compared with the D2%to OARs such as LV, LCA, RCA, LAD, LCxA and RMA increased10%-21%approximately in IMRT(H) plan (all the P＜0.05) as shown in figure2.1B and that to the heart, AMT, LV and LAD did22%-43%approximately in IMRT(AMT) plan (all the P＜0.05) as shown in figure2.1D.When the mean dose and D2%of IMRT(AMT) plan compared with those of IMRT(H) plan, the Dmean to the heart, AMT, LV and their PRV decreased14-16%,26%-33%,19%-23%approximately with all the all the P＜0.05. And Dmean to LAD and its PRV did15%and14%(P＜0.05) respectively as shown in figure2.1E and2.1G. Similarly, D2%to the heart, AMT, LV, LAD and their PRVs decreased13%-26%,18%-23%18%-31%,3%-15%approximately respectively(all the P<0.05) as shown in figure2.1F and2.1H. In addition, the Dmean to the PRV of LCxA decreased13.7%(P＜0.05) as shown in figure2.1G. Compared the breast IMRT plans with the heart and AMT as OAR, the Dmean and D2%to the OAR including the heart, AMT, LV, left anterior descending (LAD) as well as their PRVs decreased15%-33%and13%-31%respectively with statistical difference (all P＜0.05).Conclusions:The displacement of heart, AMT, LV, LCA, RCA, LAD, LCxA and RMA might provide the referrence to the extra margins of PRV to the OARs. Using heart motion compensation and replacing the heart by AMT、LCA、LAD、RCA as OARs in breast IMRT could reduce the radiation dose not only to AMT itself, but also to the heart, LV as well as LAD without the significant increase of other coronary arteries such as LCA, RCA, LCxA and RMA.