Comparative Evaluation of Setup Errors Using Planar Imaging and Cone Beam Computed Tomography: Feasibility of Asymmetric Planning Margins.

Background Accurate patient positioning in radiotherapy is critical to ensure adequate dose delivery to tumors and minimize radiation dose to normal tissues. Two image-guided radiotherapy (IGRT) techniques, planar and cone beam computed tomography (CBCT), are commonly used to measure patient setup deviations. Objective This study aims to compare the mean setup errors between planar and CBCT procedures in various irradiated areas, across the three orthogonal axes and at different times of day (morning-afternoon), and to evaluate whether the use of asymmetric planning margins between CTV and PTV is statistically justified. Methods A total of 178 patients, irradiated in four anatomical sites, were enrolled in the study. They were irradiated for prostate (n = 51, 28.6%), head and neck (n = 48, 27.0%), lung (n = 45, 25.3%), and metastatic brain tumors (n = 34, 19.1%). Each patient underwent both planar and CBCT imaging for the first three treatment days and every five days thereafter. Bone anatomy matching was carried out with both imaging techniques. Paired-samples t-tests, independent samples t-tests, and multivariate analysis of variance (MANOVA) were used. The van Herk formula was employed to determine symmetric and asymmetric planning margins based on error distributions. Statistical significance was defined at α = 0.05 (5%). Results CBCT demonstrated statistically higher deviations than planar imaging in specific X-Y-Z axes and anatomical sites, namely, along the X-axis in the prostate, lung, and cranial regions and along Z-axis in the head and neck, lung, and cranial regions. Time of treatment did not affect our results. Statistically significant differences between positive and negative errors in some regions might support the use of asymmetric van Herk margins. Conclusion According to our study, CBCT is more sensitive in detecting setup deviations/errors compared to planar imaging in certain anatomical regions and dimensions. Asymmetric planning margins may increase treatment precision and reduce the irradiated volume of normal tissues, particularly in lung and cranial irradiation.