Comparison Of Postoperative CT-and Preoperative MRI-based Tumor Bed Contours In Prone Position For Radiotherapy After Breast-conserving Surgery

Purpose: To compare the target volume of tumor bed defined by postoperative computed tomography(post-CT)in prone position fused with or without preoperative magnetic resonance imaging(pre-MRI),as well with the further purpose of optimizing the tumor bed delineation for radiotherapy planning after breast-conserving surgery.Methods and materials: 22 patients were included with early-stage breast invasive ductal cancer,who have undergone breast-conservative surgery with titanium clips placed around the tumor bed,all of them received the pre-MRI and post-CT in prone position.The MRI sequences(T1W,T2 W,T2W-SPAIR-spectral attenuated inversion recovery,DWI,dyn-eTHRIVEdynamic-enhanced T1 high-resolution isotropic volume excitation,sdyn-eTHRIVE-named by Philips-subtraction of dynamic-enhanced T1 high-resolution isotropic volume excitation)were delineated,respectively,and then manually fused to CT,respectively.The clips,seroma in combination with surgical scars were applied to guide the CT-based contours for clinical target volume(CTV-CT).For MRI-based delineations,the CTV-MRIs were obtained respectively by extrapolating 1.0 cm of the gross target volume(GTV-MRIs).Various planning target volume(PTVs)were defined as unified margins of 15 mm expanded based on the corresponding CTVs.Differences were measured in terms of consistence index(CI),dice coefficient(DC),geographical miss index(GMI)and normal tissue index(NTI).Results: The CTV and PTV volumes in T1 W,T2W,T2W-SPAIR,DWI,dyn-eTHRIVE and sdyn-eTHRIVE,were all smaller than those in CT.The differences of delineation volumes between all sequences(CT and MRI)were significant,both in the CTVs(P=0.035)and PTVs(P<0.001).In the meanwhile,the value of the conformity parameters,CI and DC,for sdyneTHRIVE fusion to CT were the largest among all MRI sequence fusion to CT,and the nonconformity parameters,GMI and NTI,were the smallest,which indicated that the overlap between the two sequences is the most.Conclusions: The CTV and PTV generated from preoperative MRI were smaller than those from CT mostly,further verifying the inaccuracy of volume delineation based on postoperative CT alone.Specifically,the sdyn-eTHRIVE sequence of pre-MRIs,due to its apparently different volume but the largest overlap with CT,which clearly outlines the tumor lump,did help improve the delineation of CT-based CTV and PTV in comparison with other MRI sequences.The outcome obtained from the study demonstrates that preoperative MRI could optimize the CT-based target delineation to some extent,as well could be used for target volume delineation of neoadjuvant radiotherapy of breast cancer.