| Objective: At present,the domestic and foreign standards on the selection of CT slice thickness for radiotherapy of nasopharyngeal carcinoma are not uniform.This study analyzes and studies the volume and dosimetric characteristics of target area and organ at risk of different CT reconstruction slice thickness in nasopharyngeal carcinoma VMAT,and provides more data support and reference for the optimization of CT slice thickness and the design of precise radiotherapy plan for nasopharyngeal carcinoma VMAT.Methods: Thirty-five patients with nasopharyngeal carcinoma diagnosed in the radiotherapy center of the Third Affiliated Hospital of Kunming Medical University from May 2020 to August 2022 were selected.The minimum reconstruction thickness(1.5mm)of the existing positioning CT was selected as the thinnest benchmark.Considering the loss of 1.5mm scanning layer thickness to the machine,3mm is selected as the original scanning layer thickness.After the scanning is completed,four sets of CT slices with 1.5mm,2mm,4mm and 5mm layer thickness are reconstructed on the large aperture CT simulator system.The target delineation and VMAT radiotherapy plan were completed on the 1.5mm positioning CT images of35 patients according to the unified delineation standard,and then copied to the 2mm～ 5mm layer thickness using the Monaco system.The VMAT plans with 5 slice thicknesses were normalized to 95 % PGTVnx(that is,the prescription dose of PGTVnx in the VMAT plan with 5 slice thicknesses included 95 % target volume).Finally,the volume and dosimetric differences of the target area and organs at risk on different CT slice thicknesses were analyzed and compared based on the VMAT plan on the 1.5 mm slice thickness positioning CT.Results : 1.Volume : When the volume of 0 ～ 5cm3 target is 5mm,the error is the largest,which can reach 8.33 % ± 5.20 %.When the volume of 300 ～ 500cm3 target is 5mm,the error is only 1.25 % ± 1.11 %.With the increase of volume,the influence of layer thickness on volume decreases gradually.The error of the cross-section at 2mm thickness is 14.36 % ± 11.61 %,and the error at 5mm thickness is 30.48 % ± 24.86 %.When the volume is constant,the increase of layer thickness increases the volume error between layer thicknesses.2.Dose :(1)PGTVnx : The homogeneity index(HI)of PGTVnx increased gradually with the increase of layer thickness.The value of D98 % decreased slightly with the increase of layer thickness.The values of D2 % and D50 % also increased with the increase of layer thickness.When HI,D98 %,D2 % and D50 % were compared between 2mm ～ 5mm and1.5mm,the differences were statistically significant(P < 0.05).(2)PGTVnd : Like PGTVnx,HI value increases with the increase of slice thickness.The values of HI,D98 %,D2 % and D50 % were significantly different between 2mm ～ 5mm and1.5mm(P < 0.05).(3)PCTV1 : The conformity index(CI)was the largest at1.5mm and the smallest at 5mm,and there was a significant decreasing trend.The differences of CI,HI and D98 % between 3mm ～ 5mm and 1.5mm were statistically significant(P < 0.05).The values of D2 % and D50 % were significantly different between 2mm ～ 5mm and 1.5mm thickness(P < 0.05).(4)PCTV2 : The change of CI value with slice thickness was small.There was no significant difference in CI between 2mm ～ 5mm and 1.5mm.There were significant differences in HI,D98 %and D50 % between 3 mm ～ 5 mm thickness and 1.5 mm thickness(P < 0.05).D2 %was statistically significant between 2mm-5mm layer thickness and 1.5mm layer thickness(P < 0.05).(5)Organs at risk: The difference between the Dmean and D2%values of the pituitary and left optic nerve at 2mm and 1.5mm was statistically significant(P<0.05);The difference between the D2% values of the eyes at 2mm,5mm,and 1.5mm was statistically significant(P<0.05),while the difference between the Dmean values of the eyes and chiasma was statistically significant(P<0.05);The difference in time difference between the left cochlear Dmean values of 2mm,3mm,and 1.5mm was statistically significant(P<0.05),the difference in time difference between the left cochlear D2% values of 2mm,5mm,and 1.5mm was statistically significant(P<0.05),the difference in time difference between the right cochlear Dmean values of 2mm,3mm,5mm,and 1.5mm was statistically significant(P<0.05),and the difference in time difference between the right cochlear D2% values and1.5mm in other layer thicknesses was statistically significant(P<0.05);When the D2% value of the right brachial plexus was 3mm,4mm,5mm,and 1.5mm,the difference was statistically significant(P<0.05);The difference in time difference between the Dmean values of the brain stem at 2mm and 1.5mm was statistically significant(P<0.05),and the difference in D2% values of the brain stem at 2mm,3mm,5mm and 1.5mm was statistically significant(P<0.05);The Dmean value of the spinal cord was statistically significant when compared to 1.5mm in other layers(P<0.05),and the D2% value of the spinal cord was statistically significant when compared to 3mm～5mm,and 1.5mm(P<0.05);When the D2%,V35,V40,and Dmean values of the oral cavity were 2mm and 1.5mm,the differences were statistically significant(P<0.05);The D2%,V30,and Dmean values of the trachea were significantly different between 5mm and 1.5mm(P<0.05);The D2%,V30,and Dmean values of the esophagus were significantly different between 5mm and 1.5mm(P<0.05).Conclusion :1.In terms of volume,layer thickness affects the volume of target area and organs at risk.The layer thickness has a great influence on the target area and organs at risk with a volume of 0 ～ 5cm3.The larger the layer thickness,the larger the volume deviation.The thickness has little effect on the volume of 50 ～ 300cm3 target area and organs at risk.The larger the volume,the smaller the effect of the thickness on the volume.2.In terms of target dose,layer thickness affects the target dose of each segmented volume.The thicker the target layer,the greater the dose of D2 % and D50 %,the smaller the dose of D98 %,and the worse the conformity and uniformity of the target area.3.In terms of dose to organs at risk,slice thickness has a greater impact on the dose of cochlea,spinal cord,temporomandibular joint and esophagus,which should be paid attention to when formulating radiotherapy plans;4.In the radiotherapy of nasopharyngeal carcinoma,the selection of CT layer thickness affects the dose indexes of nasopharyngeal carcinoma target area and organs at risk,although the thinner the layer thickness,the more accurate the plan,but the thin layer thickness brings a large workload to the target area delineation.Based on the results of this study and clinical considerations,in general,the positioning CT layer thickness of nasopharyngeal carcinoma radiotherapy can be considered to select2 mm or 3mm to meet clinical needs.If you need to pay attention to the protection of cochlea,eye,spinal cord,brainstem,temporomandibular joint and esophagus,you can choose to reconstruct a 1.5 mm slice thickness CT image for target delineation and planning design. |
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