Impact of Gantry, Collimator, and Couch Rotational Errors in Radiation Therapy Devices on the Irradiation Accuracy of Off-isocenter Targets and Evaluation of TG142 Tolerance

The effects of the collimator and couch were substantial, with positional deviations exceeding 1 mm even for a 0.5° rotation error. The influence of the gantry was relatively small and dependent on the target configuration. For irradiation of off-axis targets, the TG142 tolerance of ±1.0° should be regarded as a minimum standard that must be strictly observed regardless of the type of linear accelerator. However, depending on the target arrangement, clinically adequate margins may not be ensured. These findings suggest the necessity of applying stricter criteria according to target configuration and emphasize the importance of regular quality assurance.