The Study On The Target Movement And Dosimetric Variance In Whole Breast IMRT During Free Breathing Based On4DCT

Objective: To investigate respiration-induced whole breast displacement togetherwith the respiratory cycle during free breathing (FB) following breast-conserving surgeryand to explore the relationship between the displacement of the whole breast target(CTV)and the displacement of a selected skin marker, the nipple and a selected surgical clip usingfour-dimensional CT (4DCT), and the correlation between the CTV motion and volumevariation with the dosimetric variation. To explicit the dosimetric variance in intensitymodulated radiotherapy (IMRT) based on4DCT and3DCT after breast conserving surgery.Methods: Patients after breast conserving surgery underwent the3DCT simulationscans followed by respiration-synchronized4DCT simulation scans during FB. The CTimage data sets were exported to the Varian Eclipse treatment planning system, and basedon the same delineating criteria, the CTV, nipple, superior clip, metal marker and organ atrisk(OAR) were delineated on each CT images by the same radiotherapist. Thedisplacement of the delineated targets in the lateral (LR), anteroposterior (AP) andsuperoinferior (SI) directions were achieved, and the displacement of the CTV relative tothe displacement of the clip, nipple and skin marker were analyzed. The treatment planningconstructed using the end inspiration(EI) scan, then copied and applied to the otherrespiratory phases and3DCT, the dose distribution was calculated separately. To evaluatethe dose-volume histograms (DVH) parameters for the PTV and OAR.Results: The CTV exhibited the maximal displacement in SI direction during FB.There was no relationship between the three-dimensional (3D) displacement of the CTVand the volume of the whole breast, and there was no relationship between thedisplacement of the CTV and the volume change of the ipsilateral lung (r=0.346, P=0.246).Furthermore, there was no relationship between the displacement of the CTV and thedisplacement of the nipple, skin marker or superior clip in the cavity along the LR and SIdirections. Along the AP direction,8/13,7/11and9/13of the patients showed displacementof the CTV relative to the displacement of the nipple, skin marker and superior clip,respectively. However, according to a population-based analysis, the displacement of the CTV was only significantly associated with the displacement of the superior clip (r=0.657,P=0.015).During FB, the CTV motion vector was (2.09±0.74)mm, and the volume variation was(3.05±0.94)%. There was no correlation between the volume variation of CTV andtarget/OAR dosimetric variation (|r|=0.39~0.48). In AP, SI and vector directions, the CTVmovement correlated well with the PTV mean dose, conformal index (CI), and the lungvolume receiving high dose (V20, V30, V40, and V50)(|r|=0.651~0.975)；in SI and vectordirections, the CTV displacement only correlated with the heart volume receiving>5Gy(V5)(r=-0.795,0.687). The lung volume variation and the lung volume receiving high dosecorrelated reasonably well (r=0.655~0.882), and the correlation only exist between the heatvolume variation and the V5of heart(r=-0.701).Mean dose of PTV at EE was lower than EI(t=2.87, P=0.011), but there were nostatistic significance between3D and EI、EE(t=1.06、-1.59, P=0.304、0.132). Thehomogeneity index(HI) at EI、EE、3D plans were0.156±0.02、0.162±0.02、0.161±0.02,respectively, and difference only between EI and EE(t=-2.56, P=0.021). The highest CI wasat EI phas（et=4.55、2.70, P=0.000、0.016），and there was no significant difference betweenEE and3D(t=0.04, P=0.967). The V20、V30、V40、V50and Dmeanof ipsilateral lung at EEphase were lower than EI(t=2.39~5.54, P=0.000~0.030).There were no significantdifferences in all the indexes for heart(t=-1.77~1.40, P=0.128~0.693).Conclusions: The clip registration is more credible and sensitive than a skin markeror the nipple for measuring and correcting for displacement of the whole breast targetduring radiotherapy.During free breathing, the breast deformation may be disregarded in whole breastIMRT. PTV dose distribution was changed significantly between EI and EE phase, and thedifferentiation of the lung high dose area between EI and EE phase may induced by thoraxexpansion. Because target movement correlated well with the target/OAR dosimetricvariation and combined with the difference between the3D and4D treatment planning,3Dtreatment planning is sufficient for whole breast IMRT, but4DCT scans assist with respiratory gating ensure precise delivery of radiation dose.