運用實驗最佳化分析醫用直線加速器光子百分深度劑量之最佳調整參數

Optimization of the Percentage Depth Dose for Linac Photon Beam Using Taguchi Methodology

林佳平(Chia-Ping Lin) ； 傅筱如(Hsiao-Ju Fu) ； 李沛霖(Pei-Ling Li) ； 林佳輝(Chia-Hui Lin) ； 謝玲鈴(Ling-Ling Hsieh)

《台灣應用輻射與同位素雜誌》 10卷2期 (2014/06) Pp. 837-843

摘要

醫用直線加速器輸出光子射束（photon beam）對於深部腫瘤可給予較高劑量而殺死腫瘤，且對於皮膚表面的劑量極低，臨床治療效果極佳。加速器光子能量是利用微波推動電子接近光速；撞擊靶產生制動輻射，因此加速器光子能量調整參數對於光子的百分深度劑量（Percentage Depth Dose, PDD）分佈具有極大影響度。一般而言，調整PDD僅憑經驗法則符合物理師臨床光子深度點劑量2%內，常造成時間浪費與昂貴機器損耗，或常於更換相關零件後，對其參數影響光子能量的不了解，而花費時間去驗證或疏忽其重要性造成臨床誤差。本研究嘗試應用實驗設計法以最少的實驗組數評量光子能量條件所需最適參數條件，找尋PDD標準的最小誤差參數組合，可節省實驗時間，進階探討各參數間對光子能量影響程度，達到最佳的PDD分佈與提升機器效能運用目的。研究結果顯示最佳參數組合為電子槍電流（Gun I ctrl）：7.45，微波頻率（LP ph.ctrl）：16，偏轉系統（Bending C）：40，脈衝電壓（Chargerate）：27.5。以本研究所獲得最佳參數組合可使PDD誤差僅為0.05%。另外，由變異分析（ANOVA）結果顯示偏轉系統因子貢獻度最大，電子槍電流貢獻度次之，而微波頻率與脈衝電壓為次要影響因子。因此從本研究結果可獲得量化的光子能量調整參數觀念，有助於日後醫用直線加速器操作及設定，以減少放射治療給予劑量之誤差。

關鍵字

醫用直線加速器 ； 百分深度劑量 ； 實驗設計法

並列摘要

Photon beam output from the medical linear accelerator (Linac) gives the enriched dose to tumor in deeper area and kills the tumor, and has the benefit of arising the skin sparing effect. In clinical, photon therapy has the excellent result in radiotherapy. Photon beam energy from the Linac used microwave to gets electron kinetic approaching the speed of light, the electron strikes the target to generate bremsstrahlung radiation. The parameters of photon energy are directly related to PDD. Generally, PDD rely on experience to adjust for the requirements of the physicist within 2% in clinical, resulting in wasted time and loss machine performance, after replaced the relevant parts, but do not understand the parameters of photon energy to take the time to verify machine or neglect the importance to caused clinical error. This study using taguchi method to optimize the parameters of photon energy with the least number of experimental conditions to reach minimum error of PDD, advanced to explore the influence of each parameter of photon energy to reach PDD optimum distribution and improve machine performance. The optimized parameters setting were electron gun current (Gun I ctrl):7.45, microwave frequency (LP ph.ctrl):16, bending system (Bending C):40, pulsing voltage (Chargerate):27.5 from this study result. This study got the best parameters combination of PDD and error is only 0.05%. In addition, the greatest contribution is bending system factor, the secondary contribution is electron gun current factor, the microwave frequency and pulsing voltage are secondary factor is based on the analysis of variance (ANOVA). Therefore, the results of study obtained to quantify the parameters of photon energy to help future medical linear accelerator operation and settings to reduce the dose error in radiation therapy.

並列關鍵字

Linac ； PDD ； Taguchi Method