The Correcting Value Of Mega Voltage Computed Tomography For The Setup Error And Boundary Expansion Of Tomotherapy For Nasopharyngeal Carcinoma

Purpose:In this study,the daily Mega voltage computed tomography before helical tomography radiotherapy was used to record the setup error of patients with nasoph-aryngeal carcinoma before radiotherapy.Aaccording to the calculation formula proposed by VANHERK(MPTV = 2.5∑ + 0.7σ)to calculate the expanding margins of planning target volume(MPTV)of the overall lesion,the head region and the neck region,finding the target volume boundary suitable for TOMO technology and provideing a clinical basis for improving the accuracy of radiotherapy in our hosptial.Materials and methods:A total of 37 patients with nasopharyngeal carcinoma treated with TOMO in Jiangxi Provincial Cancer Hospital from July 2021 to December 31,2021 were collected.After enhanced positioning scan,the target area and normal tissues and organs was delineated in the Pinnacle treatment planning system.Daily MVCT scans were performed before radiotherapy,and bony markers were registered offline with the scan images and radiotherapy localization,supplemented by manual fine-tuning if necessary.During radiotherapy,the daily fractional error and the setup error of the overall lesion,head and neck were recorded.According to the planning target volume(PTV)boundary extension formula,the PTV extension range of the whole lesion,head and neck was calculatedand and analyze whether the radiotherapy time has any relationship with the setup error.Results:(1)The errors in the lateral,long,vert and roll directions of the patients’ overall lesions were 0.73±1.35 mm,0.93±2.86 mm,0.33±1.77 mm and 0.27± 1.23°;The errors in the lateral,long,vert and roll directions of the head are 0.87±0.72 mm,0.95±0.83 mm,0.45±0.84 mm and 0.33±1.16°,respectively;and the errors in the lateral,long,veet and roll directions of the neck are 1.65±1.12 mm,1.75±1.21 mm,0.49±1.15 mm and 0.45±1.34°,respectively(2)In the overall structure,the outward expansion range of MPTV in the lateral,long and vert directions is 2.25 mm,2.76 mm and 1.09 mm,respectively.Taking the bony structure as the matching point,the outward expansion ranges of MPTV in the lateral,long and vert directions of the head and neck were 2.39 mm,2.89 mm and 1.49 mm and 3.35 mm,4.91 mm,and 2.47 mm respectively.(3)During the treatment period,from the 5th week,there were statistical differences in the setup errors in different directions compared with the first treatment,indicating that the patients’ setup errors had changed since the 5th week of radiotherapy,and the plan should be revised if necessary,and the difference was statistically significant(P< 0.05).Conclusion:In the overall structure,the MPTV’s outward expansion ranges in the lateral,long and vert directions are 2.25 mm,2.76 mm and 1.09 mm,respectively.The head’s setup error is smaller than the neck’s setup error.And from the fifth week after radiotherapy,there was a significant difference in the setup error compared with the first week(P<0.05).