2026 Volume 82 Issue 2 Article ID: 26-1566
Purpose: The aim of this study was to quantitatively evaluate the impact of gantry, collimator, and couch rotational errors in a linear accelerator on the irradiation accuracy of off-isocenter targets, and to assess the validity of the rotational error tolerance (±1.0°) specified in American Association of Physicists in Medicine TG142. Methods: Using an Elekta linear accelerator (Elekta, Stockholm, Sweden) and the MultiMet-WL QA phantom (Sun Nuclear, Melbourne, FL, USA), an off-isocenter Winston–Lutz test was performed on six targets. In addition to baseline measurements, six conditions were evaluated by intentionally introducing rotational errors of +0.5° and +1.0° in the collimator, gantry, and couch. The vector distance (S value) between the field center and the target center, as well as positional deviations in each direction (gantry-target: GT, left-right: LR, anterior-posterior: AP), were analyzed. Results: Targets located farther from the isocenter exhibited more significant positional deviations. The collimator rotation had the greatest impact; at 7 cm from the isocenter, even a 0.5° error resulted in a maximum S value of 1.24 mm. Couch rotation had the next largest effect, while gantry rotation had relatively smaller effects, likely because most targets were located near the gantry’s rotational axis. The rotational errors mainly caused geometric deviations with direction-dependent positional shifts. Conclusion: 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.
