Comparison of orthogonal kilovolt X-ray images and cone-beam CT matching results in setup error assessment and correction for EB-PBI during free breathing

Abstract

Objective To compare the differences in setup error (SE) assessment and correction between the orthogonal kilovolt X-ray images and CBCT in EB-PBI patients during free breathing.Methods Nineteen patients after breast conserving surgery EB-PBI were recruited.Interfraction SE was acquired using orthogonal kilovolt X-ray setup images and CBCT,after on-line setup correction,calculate the residual error and compare the SE,residual error and setup margin (SM) quantified for orthogonal kilovolt X-ray images and CBCT.Wilcoxon sign-rank test was used to evaluate the differences.Results The CBCT based SE (systematic error,∑) was smaller than the orthogonal kilovolt X-ray images based ∑ in AP direction (-1.2 mm vs 2.00 mm; P =0.005),and there was no statistically significant differences for three dimensional directions in random error (σ) (P =0.948,0.376,0.314).After on-line setup correction,CBCT decreases setup residual error than the orthogonal kilovolt X-ray imagesin AP direction (∑:-0.20 mm vs 0.50 mm,P =0.008 ; σ:0.45 mm vs 1.34 mm,P =0.002).And also the CBCT based SM was smaller than orthogonal kilovolt X-ray images based SM in AP direction (∑:-1.39 mm vs 5.57 mm,P =0.003 ;σ:0.00 mm vs 3.2 mm,P =0.003).Conclusions Compared with kilovolt X-ray images,CBCT underestimate the setup error in the AP direction,but decreases setup residual error significantly.An image-guided radiotherapy and setup error assessment using kilovolt X-ray images for EB-PBI plans was feasible. Key words: External beam partial breast irradiation; Tomography, X-ray computed, beam cone; Orthogonal kilovolt X-ray images; Setup error; Residual error