Elekta Synergy直線加速器之周邊劑量特性探討

隨著放射治療技術的持續進步，照野內的劑量分布已可達高度順形化，然而照野外之周邊劑量（Peripheral Dose, PD）仍存在潛在風險，尤其對輻射敏感器官、兒童或孕婦等特殊族群更需關注。現行治療計畫系統（Treatment Planning System, TPS）對PD的模擬準確性有限，臨床上易低估實際劑量，進而影響風險評估與劑量控制。為此，本研究針對Elekta Synergy直線加速器於平坦濾片模式（Flattening Filter, FF）下，實測6MV與10MV光子能量在不同照射條件（角度、照野大小、深度、距離、準直儀結構）下所產生之周邊劑量，並與Pinnacle TPS模擬結果進行比較。本研究採用固態水假體與游離腔Exradin A18進行量測，涵蓋多種照野、深度與距離照野邊緣位置，並進一步分析準直儀角度及備用準直儀（diaphragm）之影響。研究結果顯示：研究結果顯示，周邊劑量（PD）隨照野邊緣距離增加而顯著下降，且照野越大、能量越高，其周邊劑量相對較高。此外，當準直儀角度由0°變更為90°時周邊劑量呈現略微上升的趨勢。使用單一diaphragm相較於與MLC搭配使用，所產生的周邊劑量有顯著增加，特別是在距離照野邊緣3公分處，兩者之間的差異可達9至10%。另一方面，Pinnacle治療計畫系統對周邊劑量的模擬結果普遍存在高估或低估的情形，雖在近照野邊緣距離處預測誤差仍屬可接受範圍，但在30至40公分範圍內的模擬準確性顯著下降，部分情況下差異甚至超過50%綜上所述，本研究所獲得之實測資料可作為臨床周邊劑量評估、治療計畫劑量補正與特殊病患放射防護之重要依據，亦能提供TPS模擬準確性驗證的實證參考。

With the continuous advancement of radiotherapy techniques, the dose distribution within the treatment field has become highly conformal. However, peripheral dose (PD) outside the treatment field remains a potential risk, particularly for radiosensitive organs, pediatric patients, and pregnant women. Current treatment planning systems (TPS) have limited accuracy in modeling PD, often underestimating actual doses, which may compromise risk assessment and dose management. This study aimed to measure PD under various beam conditions—beam angle, field size, depth, distance from the field edge, and collimator structure—using an Elekta Synergy linear accelerator operating in the flattening filter (FF) mode with 6 MV and 10 MV photon beams. The results were compared with those generated by the Pinnacle TPS.Measurements were conducted using a solid water phantom and the Exradin A18 ionization chamber under various field sizes, depths, and distances from the field edge, with additional evaluation of collimator angle and the use of backup diaphragms. The results demonstrated that PD significantly decreased with increasing distance from the field edge and increased with larger field sizes and higher beam energies. Furthermore, changing the collimator angle from 0° to 90° led to an observable increase in PD. The use of a diaphragm alone, compared to its combination with a multileaf collimator (MLC), resulted in a substantial increase in PD, with differences of up to 9–10% observed at 3 cm from the field edge. On the other hand, Pinnacle TPS showed general tendencies to either overestimate or underestimate PD. While its predictions near the field edge remained acceptable, the accuracy significantly declined at distances of 30–40 cm, with discrepancies exceeding 50% under certain conditions.In conclusion, the measurement data obtained in this study provide a valuable reference for clinical PD evaluation, TPS dose correction, and radiation protection strategies for sensitive patient groups. These results also offer empirical evidence for verifying the modeling performance of TPS in out-of-field dose estimation.