RESUMEN
Objective To analyze the setup error during tumor radiotherapy by linear accelerator cone beam CT(CBCT),and to determine the presence of correlation between subcutaneous fat thickness and setup error. Methods From May 2016 to June 2017 59 patients with pelvic tumors underwent CT scan after body positioning with thermoplastic model,CBCT images were obtained before treatment and registered automatically and manually along with localizable CT images. The displacement data at X(left-right), Y(head-foot) and Z(front-back) directions as well as pelvic subcutaneous fat thickness were recorded,and SPSS 18.0 was used for data statistics.Results The 59 patients had the mean setup errors at X,Y and Z directions being(1.07±2.18),(-0.07±5.00)and(1.15±2.38)mm respectively,and the subcutaneous fat thickness ranging from 0.3 to 2.9 cm with a mean value of 1.6 cm.Spearman analysis showed the correlations between the absolute value of the errors and the subcutaneous fat thickness being 0.15, 0.11 and 0.20 respectively at X, Y and Z directions, and there were no significant differences between the correlations(P>0.05).Conclusion The setup method verifies its feasibility by restraining its errors at X, Y and Z directions between -5 and +5 mm, and there is a weak correlation between the setup error and subcutaneous fat thickness.[Chinese Medical Equipment Journal,2018,39(5):64-67]