A Comparative Study On Target Volumes Of Partial Breast Irradiation Based On Preoperative MRI And Postoperative Stimulation CT In Different Positions

Objective:To investigate the difference of target volumes and dosimetric parameters between supine and prone positions and correlation between the target volumes defined using preoperative prone diagnostic magnetic resonance（MR）image and postoperative prone computed tomography（CT）simulation image based on deformable image registration（DIR）for the patients after breast-conserving surgery（BCS）.Methods:1.Thirty breast cancer patients with T1N0M0 stage who underwent three-dimensional conformal radiation therapy（3D-CRT）EB-PBI after BCS were enrolled from July 2016 to April2017.Supine and prone scan sets were acquired during free breathing for all patients.Target volumes and organs at risk（OARs）including heart,ipsilateral lung and bilateral breast were contoured by the same radiation oncologist.The tumor bed（TB）was determined based on surgical clips.The clinical target volume（CTV）consisted of the TB plus 1.0 cm margin and the planning target volume（PTV）was the CTV plus 0.5 cm.Dose metrics for target volumes and OARs were compared between supine and prone positions.2.Eighteen breast cancer patients suitable for external-beam partial breast irradiation（EB-PBI）after BCS were enrolled.Preoperative prone diagnostic MR and postoperative prone CT scan sets were acquired during free breathing for all patients.The gross tumor volume（GTV）delineated on the preoperative diagnostic MR images was defined as GTV_preMR,the clinical target volumes(CTV_preMR+10reMR+10 and CTV_preMR+20)were defined as 10 mm and 20 mm margins around the GTV_preMR,respectively,the planning target volume(PTV_preMR+10 and PTV_preMR+20)were defined as 15 mm and 25mm margins around the GTV_preMR,respectively.Tumor bed（TB）delineated on the postoperative prone CT simulation images acquired during free breathing was defined as GTV_postCT,CTV_postCT and PTV_postCT were defined as 10 mm and 15 mm margins around the GTV_postCT,respectively.The target volume of the whole breast contoured on the MR and CT images were defined as CTV_Breast-MRI and CTV_Breast-CT,respectively.The MR and CT images were registered deformably in MIM software system.Results:1.The median volumes of theTB,the CTV and the PTV in supine were 14.10 cm³,57.35cm³,108.85 cm³ and 14.40 cm³,62.60 cm³,113.7 cm³ for prone positions,respectively.The CTV and PTV in prone position were significantly greater than those in supine position,respectively（Z=-3.01,-2.87,P<0.05）,but the TBs were not statistically different（Z=-1.40,P=0.162）.The median homogeneity indexs（HI）in supine position was less than that in prone position（Z=-3.137,P<0.05）,and the mean conformal indexs（CI）was increasing with prone positioning（t=9.034,P<0.05）.The mean dose（Dmean）to the heart in the supine position was significantly lower than prone（Z=-4.12,P<0.05）,it was 0.34 Gy vs 1.19 Gy for supine and prone positions,respectively.The Dmean to the ipsilateral lung in the prone position was significantly lower than that in supine（Z=-2.18,P<0.05）,it was 1.59 Gy vs 1.72 Gy,respectively.There was not significantly different for the Dmean to the ipsilateral breast,it was10.01 Gy vs 10.40 Gy,respectively（Z=-0.70,P=0.480）.2.The GTV _postCT,CTV _postCT and PTV _postCTostCT were significantly greater than GTV_preMR,GTV_preMR+10reMR+10 and PTV_preMR+10,respectively（Z=-3.593,-3.593,-2.983,all P<0.05）.Meanwhile,the CTV_postCT and PTV_postCTostCT were significantly less than the CTV_preMR+20 and PTV_preMR+20,respectively（Z=-2.722,-2.853,all P<0.05）.When based center-coincidence of the compared targets,the conformal index（CI）and degree of inclusion（DI）of GTV_postCT-GTVpreMR,GTVpostCT-CTVpreMR+10,CTVpostCT-GTVpreMR and CTVpostCT-CTVpreMR+10 were better than those based on DIR of the thorax（Z=-3.724、-3.724、-2.591、-3.593,all P<0.05;Z=-3.724、-3.724、-3.201、-3.724,all P<0.05）.Conclusions:1.It was feasible to carry out 3DCRT EB-PBI in prone position in free breathing for Chinese breast cancer patient,its primary advantages were the better CI and the significantly lower radiation dose to the ipsilateral lung.2.For the patients enrolled for prone EB-PBI,when defining the target on the preoperative prone MR images,the target volumes were significantly smaller compared to that on postoperative prone CT images,but the a statistically significant positive correlation was found between the MR and CT target volumes.Although based on DIR,there were relatively poor spatial overlap whether the whole breast or the targets between the preoperative prone diagnostic MR images and the postoperative prone simulation CT images.Therefore,it is unreasonable to use the preoperative prone diagnostic MR images to guide postoperative target delineation of EB-PBI.