放射線治療装置の回転座標系誤差が軸外targetの照射精度に及ぼす影響とTG142のトレランスの評価

【目的】放射線治療装置の回転座標系の誤差が軸外targetの照射精度に及ぼす影響を定量的に評価し，TG142における回転座標系誤差（±1.0°）のトレランスの妥当性を検討する．【方法】Elekta社製放射線治療装置（Elekta, Stockholm, Sweden）とMultiMet-WL QAファントム（Sun Nuclear, Melbourne, FL, USA）を用いて，6個のtargetに対してoff isocenterのWinston–Lutz test（WL test）を実施した．Baselineの測定に加え，意図的にcollimator，gantry，couchに+0.5°, +1.0°回転誤差を加えた6条件で測定を行い，照射野中心とtarget中心のベクトル距離（S値）および各方向（gantry-target: GT, left-right: LR, anterior-posterior: AP）の位置ずれを解析した．【結果】Isocenterからの距離が大きいtargetほど位置ずれが顕著であった．特にcollimator回転誤差の影響が最も大きく，isocenterから7 cm離れたtargetでは0.5°の回転誤差でもS値が最大1.24 mmに達した．次に影響が大きかったのはcouch回転であり，gantry回転はtargetの配置が回転軸に近いものが多く相対的に影響が少なかった．回転座標系の誤差は幾何学的誤差の影響が強く，位置ずれに方向依存性があった．【結語】Collimatorやcouchの影響が大きく，0.5°の誤差でも1 mm以上の位置ずれが生じることがあった．Gantryの影響はtargetの配置依存があり，相対的に小さかった．軸外targetの照射において，TG142の±1.0°のトレランスは放射線治療装置の種類にかかわらず最低限遵守するべき基準であり，targetの配置次第では臨床的に十分なマージンを保証できない可能性が示された．Target配置に応じたより厳格な基準と定期的quality assurance（QA）の重要性が示唆された．

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.