RESUMEN

Objective To analyze the individualized CTV-to-PTV margin dose and positioning errors in radiotherapy for esophageal cancer for improving the treatment accuracy while meeting dose requirements.Methods Fifty-four esophageal cancer patients admitted to Sun Yat-sen University Cancer Center at Huangpu District from June 2021 to June 2022 were enrolled.All of the patients underwent CBCT scans in each fraction,and a total of 1283 CBCT images were collected.The image registration between CBCT image before radiotherapy and planning CT image was carried out to obtain errors in vertical(VRT),longitudinal(LNG),lateral(LAT),Roll,Pitch,and YAW directions.The mean values of six-dimensional positioning errors in the first 5 fractions were calculated,and the results were compared with the total fractional errors using the single sample t-test method for determining the differences.The CTV-to-PTV margin was calculated with the formula(margin=2.5∑+0.7δ),and the calculated margins were divided into 5 groups:Group A(5 mm expansion in all directions),Group B(7.9 mm expansion in LAT direction,and 5 mm expansion in other directions),Group C(11.03 mm expansion in LNG direction,and 5 mm expansion in the other directions),Group D(6.36 mm expansion in VRT direction,and 5 mm expansion in the other directions),and Group E(7.9 mm expansion in LAT direction,11.03 mm expansion in LNG direction,and 6.36 mm expansion in VRT direction).Simulation planning was conducted for 10 patients.Results The proportions of differences between the mean values of six-dimensional errors in the first 5 fractions and the total fractional errors in 54 patients were analyzed.There was no significant difference in 192 out of the 324 directions in 54 patients,accounting for 59.26%(P>0.05).Among them,the LAT,LNG,VRT,Pitch,Roll and YAW directions accounted for 64.81%,57.41%,51.85%,64.81%,57.41%and 59.26%of the total cases.The calculated CTV-to-PTV margin was 7.90,11.03 and 6.36 mm in LAT,LNG and VRT directions.The statistical analysis showed that the differences in the coverage rates of organs-at-risk and target areas among the 5 groups of CTV-to-PTV margins were trivial(P>0.05).Conclusion Using the positioning errors in the first 5 fractions of radiotherapy for esophageal cancer to predict subsequent positioning errors is feasible.The reasonable individualized margin in radiotherapy for esophageal cancer can reduce the inter-fractional off-target rate without increasing the dose delivered to organs-at-risk.The study provides a reference for the target volume margin of esophageal cancer and an important basis for precision treatment.