Interobserver variability study for daily cone beam computed tomography registration of prostate volumetric modulated arc therapy

Purpose. - This work evaluated the interobserver variability in cone beam computed tomography (CBCT) registration for prostate cancers treated with intensity-modulated radiotherapy. Material and methods. - Twelve technologists realized 286 CBCT/CT registrations (bone registration followed by prostate to prostate registration). The registration results were compared to those obtained by two radiation oncologists (reference). Each technologist reported the shifts calculated by the software in all three axes. A statistical analysis allowed us to calculate the minimum threshold under which 95% of the observers found similar values. A variance analysis followed by the post hoc test were used to find differences in interobserver registration variability and determine whether any individual users performed registrations which differed significantly from those of the other users. Results. - The registration differences compared to the reference in the three directions in terms of 95th percentile are: 2.1 mm left-right, 3.5 mm target-gun, 7.3 mm anterior-posterior. In the posterior direction, 4% of the observers have found differences superior to 8 mm, margin used in routine without the use of a daily CBCT. The variance test revealed a P-value