Lumpectomy Cavity Delineation Based On MRI And Set-up Error Based On CBCT For Patients After Breast-Conserving Surgery In Prone Radiotherapy

ObjectiveTo assess the feasibility of delayed-enhancement magnetic resonance(MR)imaging in contouring the lumpectomy cavity(LC)for patients with invisible seroma or a low cavity clarity score(CVS)≤2 in the excision cavity after breast-conserving surgery(BCS);The purpose of this part was to investigate the trends in magnitude of inter-fraction motion and the correlation between set-up errors(SE)and the parameters of targets matching for patients underwent simultaneous-integrated boost intensity-whole breast modulated radiation therapy(SIB-WBIMRT)in prone position based on cone-beam CT(CBCT).The correlation between SE and radiation induced lung injury(RILI)was also analyzed.Methods1.Application of delayed-enhancement MRI in lumpectomy cavity delineation for patients after breast-conserving surgery in prone radiotherapy using deformable image registration:Twenty-six patients who were suitable for radiotherapy in prone positions after BCS were enrolled,and both computed tomography(CT)and delayed-enhancement MR simulation scans were acquired.The LCs were countered manually by the three radiotherapy physicians on each simulation scans.The LC delineated based on titanium surgical clips on CT images was denoted as LCCT.The LC delineated based on the signal of cavity boundaries on fat-suppressed T2-weighted imaging(T2WI)and multiphase delayed-enhancement T1-weighted imaging(DE-T1WI),which was performed at 2 min,5 min and 10 min post injection,were denoted as LCT2,LC2T1,LC5T1 and LC10T1,respectively.Afterwards,DIR was performed to compare the volumes and locations of the LCs with MIM software.The generalized conformity index(CIgen)of inter(intra)observer(Inter-CIgen and Intra-CIgen)was also used to explore the inter(intra)observer variation for LC delineation on each image modality.2.Correlation between set-up error and parameters of target for SIB-WBIMRT in prone position based on CBCT:Thirty-one breast cancer patients suitable for SIB-WBIMRT in prone position were enrolled.Prone CT images and CBCT were acquired for all patients and rigid image registration(RIR)between them was performed based on MIM software.The clinical target volume(CTV)of ipsilateral breast contoured on CT images and CBCT were denoted as CTVCT and CTVCBCT,respectively.The plan target volume(PTV)was CTV plus 0.5cm.Three dimensional inter-fraction motion based on CBCT scans were quantified.The system SE(∑)and random error(a)were calculated for each patient based on multiple outline scanning based on CBCT.The CT images of patients after radiotherapy were retrospectively analyzed to obtain images of patients with RILI,and the location and physical parameters of RILI were also recorded.Results1.Application of delayed-enhancement MRI in lumpectomy cavity delineation for patients after breast-conserving surgery in prone radiotherapy using deformable image registration:The volumes of LCCT,LCT2,LC2T1,LC5T1 and LC10T1 were 9.96cm3、12.28cm3、12.16cm3、11.45cm3、14.61cm3,respectively.The volumes of LCCT were significantly positive correlated with those of LCT2,LC2T1,LC5T1 and LC10T1,respectively(r=0.904,0.852,0.888,0.929,all P<0.05).Although the volumes of LCT2,LC2T1,LC5T1 and LC10T1 was larger than those of LCCT,there was no statistical significance between LCCT and those of LC10T1 or LCT2(Z=-1.714,-1.810,both P>0.05).While a statistical significance was found between the volumes of LCCT and those of LC2T1 or LC5T1,respectively(Z=-3.054,-2.845,both P<0.05).The conformal index(CI),degree of inclusion(DI),dice similarity coefficient(DSC)and the distance between the center of mass of the targets(COM)of LCCT-LC10T1 were better than those of LCCT-LCT2,LCCT-LC2T1 and LCCT-LC5T1,however,there was no statistical difference among them(all P>0.05).The time in the delineation of LCT2,LC2T1,LC5T1 and LC10T1 was obviously shorter than that of LCCT(Z=-3.517,-3.518,-3.516,-3.516,all P<0.05).The agreement of LC delineation was strongest for 10th min DE-TIWI(coefficient of variation,COV=2.30%,Inter-CIgen=87.06%,Intra-CIgen=92.64%).2.Correlation between set-up error and parameters of target for SIB-WBIMRT in prone position based on CBCT:The magnitude variation of SE in three dimensional directions was moderately increased(<1mm)as the CBCT scanning increase.The conformal index(CI),degree of inclusion(DI),dice similarity coefficient(DSC)and the distance between the center of mass of the targets(COM)of CTVCT-CTVCBCT and PTVCT-PTVCBCT were significantly correlated with the set-up error(SE)of vertical(Vrt)direction(all P<0.05).There was no significant correlation between RILI and inter-fraction motion of three dimensional directions(P>0.05).The V5,V10,V15,V20,V25,V30,V35 of ipsilateral lung tissue for patients with RILI were significantly higher than those for patients without RILI(all P<0.05).Conclusions1.For patients with a low CVS(CVS≤2)after BCS,it is feasible to contour the LC based on prone DE-MR simulation images.Furthermore,the LC derived from prone DE-T1WI at 10 min was found to be most similar to that derived from prone CT simulation scans using titanium surgical clips regardless of the volume and location of the LC.Inter(intra)variability was minimal for the delineation of the LC based on DE-T1WI of 10-minute.2.It is feasible to apply SIB-WBIMRT in the prone position after BCS.The SE of three dimensional direction can be kept relatively stable during the whole course of treatment.Although the SE will not have a significant impact on the occurrence of RILI,the implementation of SIB-WBIMRT in prone position should be closely monitored by CBCT to accurate irradiation for the targets and the surrounding normal tissue.