表層組織之深度百分率劑量曲線的蒙地卡羅模擬

Surface Percent Depth Dose Curve Simulation BY Monte Carlo Method

林松彥(Sung-Yen Lin) ； 林招膨(Jao-Perng Lin) ； 劉幕台(Mu-Tai Liu) ； 朱鐵吉(Tieh-Chi Chu)

《放射治療與腫瘤學》 7卷4期 (2000/12) Pp. 275-281

https://doi.org/10.6316/TRO/200007(4)275

摘要

目的：以蒙地卡羅方法模擬表層組織的深度百分劑量(PDD)曲線，作為往後準確皮膚劑量評估之參考。
材料與方法：本研究以OMEGA/BEAM為主要模擬軟體，模式化調整後之Siemens PRIMUS 以BEAM模擬並於SSD為100cm 處收集產生的粒子群，該粒子群作為半徑30cm，厚 10 cm 之水假體表面劑量之射源。本研究係對6與15 MV ， 10 × 10 、 20 × 20 與 30 × 30 cm2等照野進行模擬，分別放水假體圓心選取半徑1.0、2.0與3.0 cm的圓形區域為 10 × 10 、 20 × 20 與 30 × 30 cm2 照野中劑量計算區域，由水表面開始共分割厚 0.05 mm × 50μm)的體積元20個，並在Dmax處設置半徑與上述體積元面積一致但厚度為0.1cm 的體積元，模擬計算後各點與Dmax處之劑量比值即為PDD分佈圖。
結果與討論：各點模擬之百分誤差大都控制在1%以內，少部份 30 × 30 cm2的誤差在2%以內。模擬所得之各水表面附近PDD曲線，經一元四次多項式擬合，得知在水表面處( X=0) 的 6 MV 之 10 × 10 、20 × 20 與30 × 30 cm2的PDD值分別為 13.67 ± 0.17% 、23.07 ±0.35% 與29.75± 0.52% ， 15MV之結果則為8.71 ± 0.06 % 、 18.77 ± 0.14% 與 27.06±0.32%。擬合的x2值皆大於0.997。
結論：模擬所得之水表面處PDD值可以作為治療計畫系統模式化時調整擬合曲線表面之值 ( X=0)的參考。本研究僅對6MV與15MV開放式的照野作模擬，未來亦將嘗試對固態楔型濾器(physical wedge) 與動態楔型濾器(dynamic wedge)作水表層PDD分佈之模擬，以增加其臨床的運用。

關鍵字

蒙地卡羅方法 ； 皮膚劑量

並列摘要

Purpose: This study performed the Percent Depth Dose (PDD) curve simulation at surface region by Monte Carlo method for the accurately evaluating skin dose in the future.
Materials and Methods: Simulation in this study is carried out by the OMEGA/BEAM code. After modeling the Siemens PRIMUS linear accelerator, phase space data is collected at SSD= 100 cm for following dose distribution simulation in a water phantom. Dose distribution simulation is carried out for 6 MV and 15 MV, 10 × 10、20 × 20 and 30 ×30 cm2 fields in a water phantom. The phantom size in 30 cm in radius and 10 cm in thickness. The areas selected for dose calculation of 10 × 10、20 × 20 and 30 ×30 cm2 fields are 1.0, 2.0 and 3.0 cm in radius, respectively. Total twenty voxels of thickness 0.05mm (50μm) from water surface and 1 voxel of thickness 0.1cm at Dmax were selected for dose calculation. The dose ratios of voxels at surface region to voxel at Dmax form the PDD curve at surface region.
Result and Discussion: The relative error of all voxels is within 1% with the exception of some voxels within 2% in 30 × 30 cm2 field. The PDD curves at surface region were fit with 4th order polynomial function. The x2 value of each fitting is better than 0.997. The PDD value at water surface ( X=0) for 6 MV, 10 × 10、20 × 20 and 30 × 30 cm2 fields are 13.67 ± 0.17%、23.07 ± 0.35% and 29.75 ± 0.52%, respectively. For 15 MV, the values are 8.71± 0.06%、18.77 ± 0.14% and 27.06 ± 0.32%, respectively.
Conclusion: The PDD value at water surface can be taken as reference for modeling of treatment planning system. This study only carried out the simulation for 6 and 15 MV open fields. More efforts on surface PDD of physical wedge and dynamic wedge fields are needed to widen the clinical application.

並列關鍵字

Monte Carlo method ； PDD ； Skin dose