Clinical Analysis of Rotational-Translational Error Coupling in 3DoF Versus 6DoF CBCT Couch Corrections:A Cross-Sectional Study.

IntroductionThe 3-degree-of-freedom (3DOF) and 6-degree-of-freedom (6DOF) couches can enhance the accuracy of tumor localization in radiotherapy. However, the relationship between rotational and translational error corrections remains unclear. This study aims to evaluate the setup errors of three common tumor types using cone-beam computed tomography (CBCT) and registration analysis, with a focus on elucidating the coupling between rotational and translational errors, to optimize setup correction strategies and provide data support for the clinical application of 3DOF correction.MethodsA total of 46 patients with nasopharyngeal carcinoma (NPC), 48 with esophageal cancer(EC) and 23 with rectal cancer (RC) were enrolled in this cross-sectional study. For each patient, the pre-treatment CBCT images and planning CT images were registered offline using the same region of interest (ROI) and bony registration algorithm. Registration was performed in 3DoF and 6DoF modes, amounting to 1,720 registrations in total. The setup errors of registrations were analysed to investigate the relationship between rotational and translational deviations.ResultsSignificant registration errors were found only in the vertical direction for NPC and EC patients (P < 0.05), but not for RC patients (P > 0.05). The linear vector directions (D3D for 3DoF and D6D for 6DoF) showed strong linear correlations to varying degrees, R2NPC = 0.597, R2EC = 0.798, R2RC = 0.781 (P<0.01). Strong coupling correlations were observed between rotational and translational errors, particularly at the NPC site (pitch vs. ΔVrt: r = - 0.893; yaw vs. ΔLat: r = 0.826), the EC site (pitch vs. ΔLng: r = 0.544) and the RC site (pitch vs. ΔVrt: r = - 0.579).ConclusionA site-specific coupling exists between rotational and translational errors. When rotational errors exceed threshold ranges, performing site-specific rotational adjustments followed by translation is requisite. This strategy significantly enhances radiotherapy precision and patient safety in non-6DoF environments.