Impact Of CT Slice Thickness On Clinical Target Volume And Dose For Chest Cancer And The Research Of Starting Layer Selection

Purpose: To evaluate the impact of different computed tomography(CT)slice thickness by means of two reconstruction methods on the target volume and dose in chest cancer radiotherapy.Materials and Methods: Patients with chest cancer were included in the study.The CT images with 2 mm slice thickness(2-CT)were transferred to the treatment planning system(TPS)and then the images with different slice thickness(4,6,8 and 10 mm)were reconstructed by two methods(Method#1: based on the biggest target layer;Method#2: based on the metal marker layer)in TPS.The target and organ at risk(OAR)were delineated on 2-CT images and then were copied slice by slice on CT images with other slice thicknesses.Similarly,the intensity modulated radiotherapy(IMRT)plans were developed on the 2-CT images and then copied to on the other CT images.The dose was computed independently for each image.The results in 2-CT images were chosen as a reference standard.Results: The volume change in two reconstruction methods was gradually increasing with the increase of slice thickness,particularly for 10-CT images in which the volume deviation for targets of 1.3 cm3 was up to 84.62%;the deviation in 8-CT images was 38.46%(Method#1),84.62%(Method#2);for 30–100 cm3,the volume changes in Method#1 were significantly smaller than correspondingly that in Method#2;for >100 cm3,the deviation was very small.The changes in radiation conformal index(CI)were gradually larger as the thickness increased,but the change trends in homogeneity indexes(HI)were slightly smaller.The changes in CI of small targets(<30 cm3)were particularly significant in 8-CT(Method#1: 6.56%;Method#2: 13.18%)and 10-CT images(Method#1: 13.6%;Method#2: 11.18%)and that for big targets with 30–100 cm3 were 1.29%–3.03%(Method#1)and 1.34%–5.42%(Method#2).The variation tendency of CI in Method#1 was more steadier than that in Method#2;nevertheless,the difference of HI between two methods was not obvious.The changes of HI were large on 8-CT(12.95%;16.15%)and 10-CT(17.42%;15.43%)images.Two types of restructuring of IMRT plan,endanger the injectivity of organs have also changed.In method 1,the Dmin of spinal cord in 2-CT vs.4-CT at P=0.027,2-CT vs.6-CT at P=0.000,the Dmax of spinal cord in 2-CT vs.6-CT at P=0.043;In the Dmean of spinal cord all layers thickness compareed with 2-CT,P values <0.05.In method 2,the Dmin of spinal cord in 2-CT vs.4-CT at P = 0.070,2-CT vs.6-CT at P=0.000,the Dmax of spinal cord in 2-CT vs.8-CT at P=0.023;Similarly,the Dmean layers thick than 2-CT,P values< 0.05.Two methods of pulmonary V5 in 4-CT,8-CT,10-CT were P<0.05;Method 1 V30 in 4-CT,CT,6-CT,8-CT,10-CT were P<0.05,however the method 2 of pulmonary V30 in 6-CT,8-CT,10-CT were P<0.05.the heart V30 and V40 in Method 1 on 6-CT at P=0.034,0.020;Method 2 heart V30 in 6-CT,10-CT P = 0.039,0.004,heart V40 on 6-CT,10-CT P =0.011,0.005.Conclusions: To more accurately evaluate the volume and dose and also preferably the OAR sparing,the reconstruction method based on the biggest target layer was recommended.The 4-CT or less slice thickness should be adopted for small targets(for example ≤ 1 cm3)and targets of large volume were not sensitive to the requirement of CT slice thickness.