The impact of respiratory motion for target dose during external-beam partial breast irradiation / 中华放射医学与防护杂志

ABSTRACT

Objective To evaluate the impact of respiratory motion for dose of target and organ at risk during external-beam partial breast irradiation (EB-PBI).Methods 4D-CT scan sets were acquired for 20 patients who underwent EB-PBI.The volume of the tumour bed (TB) was determined based on seroma or surgical clips on the ten sets of 4D-CT images.For each patient a conventional 3D conformal plan (3D-CRT) was generated based on the 4D-CT end inhalation phase images,then copied and applied to the other phases.The following parameters were calculated to analysemean dose (D),homogeneity index (HI),conformal index (CI),and the volumes that received ≥ x Gy (Vx).Results During free breathing,the TB centroid motion was 0.90,0.75 and 0.80 mm in the lateral,anteroposterior and superior-inferior directions,respectively.The medium spatial motion vector was 0.95 mm.In the superiorinferior direction,TB motion significantly correlated with D HI,and CI of PTV (r =-0.458,-0.451 and 0.462,P ＜ 0.05),as well as D V20 and V30 received by the ipsilateral normal breast (r=0.527,0.488 and0.526,P ＜0.05).And in the motion vector,the D V5,V10,V20 of the ipsilateral lung all correlated with TB motion (r =0.416,0.503,0.522 and 0.498,P ＜ 0.05).A correlation also existed between dose and percent volume of heart and volume variation of heart (Dmean,V5 and V10) (r =0.727,0.704 and 0.695,P ＜ 0.05).Conclusions Small TB motion caused by respiratory motion during free breathing result in dosimetric variation of the target and potential dosimetric off-target or suboptimal dose coverage for EB-PBI.The doses of lung during free breathing were relatively sensitive to TB motion and thorax expansion,while heart doses were not influenced notably.