Comparison between manual and automatic image registration in image-guided radiation therapy using megavoltage cone-beam computed tomography with an imaging beam line for prostate cancer

This study aimed to compare and assess the compatibility of the bone-structure-based manual and maximization of mutual information (MMI)-algorithm-based automatic image registration using megavoltage cone-beam computed tomography (MV-CBCT) images acquired with an imaging beam line. A total of 1163 MV-CBCT images from 30 prostate cancer patients were retrospectively analyzed. The differences between setup errors in three directions (left-right, LR; superior-inferior, SI; anterior-posterior, AP) of both registration methods were investigated. Pearson's correlation coefficients (r) and Bland-Altman agreements were evaluated. Agreements were defined by a bias close to zero and 95% limits of agreement (LoA) less than ± 3 mm. The cumulative frequencies of the absolute differences between the two registration methods were calculated to assess the distributions of the setup error differences. There were significant differences (p < 0.001) in the setup errors between both registration methods. There were moderate (SI, r = 0.45) and strong positive correlation coefficients (LR, r = 0.74; AP, r = 0.72), whereas the 95% LoA (bias ± 1.96 × standard deviation of the setup error differences) were - 1.61 ± 4.29 mm (LR), - 0.41 ± 5.45 mm (SI), and 0.67 ± 4.29 mm (AP), revealing no agreements in all directions. The cumulative frequencies (%) of the cases with absolute setup error differences within 3 mm in each direction were 80.83% (LR), 81.86% (SI), and 90.71% (AP), with all directions having large proportions of > 3-mm differences. The MMI-algorithm-based automatic registration is not compatible with the bone-structure-based manual registration and should not be used alone for prostate cancer.