旋繞疊加演算法對空腔部位的劑量計算準確度評估

使用蒙地卡羅模擬（Monte Carlo simulation）及旋繞疊加（convolution/superposition）劑量演算系統來計算模擬不同空腔尺寸壓克力假體的中心軸百分深度劑量（percent depth dose, PDD）曲線分佈，以研究縱向及側向電子非平衡現象，並驗證其結果。空腔尺寸分別為20×20×1.0，20×20×2.0，20×20×3.0，20×20×4.0，和20×20×4.95 cm3以研究縱向的電子非平衡現象，以及空腔尺寸為3.6×3.6×4.95，4.5×4.5×4.95，5.4×5.4×4.95和20×20×4.95cm3以研究側向電子非平衡現象，蒙地卡羅模擬是使用OMEGA/BEAM和DOSXYZ程式，而旋繞疊加演算法的計算則是使用商業化的ADAC治療計畫系統。最後並使用雷鉻底片（radiochromic film）量測以驗證蒙地卡羅模擬結果。由蒙地卡羅模擬與雷鉻底片量測所得到空腔中的劑量下降程度會比旋繞疊加劑量演算法所得的結果嚴重，這表示旋繞疊加演算法在空腔中所使用的劑量核心，無法正確的描述側向及縱向電子非平衡空腔劑量的高估，也因此再增建（re-buildup）區的劑量便會受到影響，影響的區域則是從壓克力假體表面至0.5公分的深度。其計算劑量的差異大小與空腔大小、形狀、射束照野大小、能量與入射方向有關。當使用旋繞疊加演算系統的比例密度修正法來計算非均勻材質時，應該要考慮空腔內劑量核心的擴散造成的電子非平衡影響。而旋繞疊加演算法中用於處理非均的電子非平衡影響。而旋繞疊加演算法中用於處理非均質介質劑量分佈問題時使用有比例密度法必須加以改進，加入考慮劑量核心在電子不平衡空腔中的過度發散性。

關鍵字

蒙地卡羅模擬；旋繞疊加演算法；百分深度劑量曲線； OMEGA/BEAM程式； DOSXYZ程式；電子非平衡

並列摘要

Using Monte Carlo simulation and the convolution/superposition algorithm to examine percent depth dose curves of the central axis in an acrylic phantom with variously-sized air cavities to evaluate longitudinal and lateral electron disequilibrium. The sizes of air cavities are 20×20×1.0, 20×20×2.0, 20×20×3.0, 20×20×4.0 and 20×20×4.95 cm3 for study of longitudinal electron disequilibrium and 3.6×3.6×4.95, 4.5×4.5×4.95, 5.4×5.4×4.95 and 20×20×4.95 cm3 for study of lateral electron disequilibrium. The Monte Carlo simulation is performed by using OMEGA/BEAM and DOSXYZ codes, and the convolution/superposition calculation relies on an ADAC commercial treatment planning system. Radiochromic film is also used to verify the Monte Carlo results. The dose drop-off is more severe than that expected from the convolution/superposition algorithm, as revealed by the Monte Carlo simulation and the measuring of radiochromic film. Therefore, the dose kernel in the air cavity, used in convolution/superposition algorithm, cannot precisely represent the phenomenon of lateral and longitudinal electron disequilibrium, but underestimates the expansion of the dose kernel in air. Conseequently, dose in rebuild-up region is influenced. The influenced region is on the acrylic phantom surface to a depth of about 0.5 cm. The difference between the dose in air and in rebuild-up region, simulated by Monte Carlo and those obtained in air by convolution/superposition algorithm depend on the cavities’ shapes as well as beam size, energy and even orientation. The density scaling method of the convolution/superposition algorithm, applied to heterogeneous media, should be enhanced to account for the over-expansion of the dose kernel in the cavity of electron disequilibrium.

並列關鍵字

Monte Carlo simulation ； convolution/superposition algorithm ； percent depth dose curve ； OMEGA/BEAM ； DOSXYZ ； electron disequilibrium

國際替代計量

旋繞疊加演算法對空腔部位的劑量計算準確度評估

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