高密度金屬物質在直線加速器放射治療之劑量擾動效應的蒙地卡羅模擬

THE MONTE CARLD SIMULATION ON THE DOSE DISTURBANCE EFFECT OF EMBEDDED HIGH-DENSITY METAL MATERIAL

張柏菁(B.J.Chang) ； 李宗其(C.C.Lee) ； 許任玓(R.D.Sheu) ； 葉健一(C.Y.Yeh) ； 陳俊丞(C.C.Chen) ； 徐椿壽(C.S.Chui)

《放射治療與腫瘤學》 10卷2期 (2003/06) Pp. 79-88

https://doi.org/10.6316/TRO/200310(2)79

摘要

目的：以蒙地卡羅法程式模擬高密度金屬物質在放射治療時對劑量擾動的效應。材料與方法：本研究以OMEGA/BEAM 程式為主，進行目前臨床使用Varian 21 EX 機型直線加速器的模擬，首先針對6MV，40 × 40 ㎝2照野，SSD=100㎝設定在均勻水假體中進行模擬，與實驗量測之深度劑量以及劑量剖面等結果驗證比較，並進行參數調整，確認各項模擬參數正確無誤後，再針對6MV，10 × 10 ㎝2照野，SAD=100㎝設定進行高密度金屬材質對劑量擾動效應之模擬。
結果與討論：經以實驗量測的深度劑量與劑量剖面驗證求得之加速器參數( 6 MV 照野大子為40 × 40 ㎝2、10 × 10 ㎝2)，其模擬結果均與量測值的誤差在2%以內，證實各項推測之參數正確無誤，顯示已建立Varian 21 EX 機型加速器模擬之運跑能力。以假牙金屬樣品為例，進行6MV照野大小為10 × 10 ㎝2 ，SAD=100 ㎝ ，樣品位於水假體內 5 ㎝ 深之模擬，結果顯示劑量擾動的範圍，在深度劑量方面前後約為 1 ㎝ ，樣品前的電子回散射效應造成約40%的劑量增加，樣品後的遮蔽效應約在0.2 至 0.6 MeV 之間。
結論：本研究中所採用的金屬樣品體積雖小但密度甚高，擾動效應相當顯著，但由放擾動的範圍較小，因此將在實驗量測上顯得較為困難。在本研究中透過蒙地卡羅模擬提供深度劑量、劑量剖面、以及相關位置的光子能譜分布影響之資訊，可作為實驗量測設計之參考，以進一步研究探討在臨床上此類材質所造成的實際影響。

關鍵字

蒙地卡羅方法,直線加速器

並列摘要

Purpose: This study performed the simulation of the dose distribution around the embedded high-den-sity metal material.
Materials and Mathods: The simulation is focused on the Varian 21EX LINAC machine and imple-mented by the OMEGA/BEAM code system. For modeling the linear accelerator, the simulation para-meters are adjusted by comparing the results of the depth doses and the setup of 6 MV, 40 × 40 cm2, SAD= 100 cm in a homogeneous water phantom. Then, the dose disturbance has been investigated for the setup of 6 MV, 10 × 10 cm2 SAD= 100 cm with a denture sample.
Results and Discussion: The adjusted LINAC parameters have been proved by the verifications of the depth doses and dose profiles calculated by DOSXYZ with the measurement setup of 6 MV, 40 × 40 cm2, SSD = 100 cm. It shows that the capability of simulating the Varian 21EX LINAC has been established. For the Degubond 4 denture sample at a depth of 5 cm with a setup of 6 MV, 10 × 10 cm2, SAD = 100 cm, the dose disturbance range in depth dose is around 2 cm. The back scattering electrons cause a 40% dose increment in front of the sample and the shielding effect decreases the dose of about 20% behind the sample. Maximum range disturbs the dose profile is about 1.2 cm. The energy range of the back scattering photons is between 0.2 and 0.6 MeV.
Conclusion: The size of the denture sample used in the study is small, but the density is quite high, hence the perturbation effect is still significant. Besides, the spatial range of the perturbation is relative-ly small; it is difficult to be measured. In this study, it shows that the detailed information includes depth dose, dose profile at several depth, and the photon/electron spectrums could be obtained by Monte Carlo simulation, and could provide for the further study.

並列關鍵字

Monte Carlo method,LINAC