高能(15 MV)醫用直線加速器誘發產生之中子能譜與劑量研究

Spectrum and dose assessments of neutrons induced by the 15 MV medical linear accelerator

林震榮(Chen-Jung Lin)

指導教授 ： 許靖涵 ； 許芳裕

國立清華大學/原子科學院/生醫工程與環境科學系/碩士(2011年)

https://doi.org/10.6843/NTHU.2011.00276

摘要

放射線治療利用醫用直線加速器產生高能光子射束殺死腫瘤細胞，同時高能光子也與機頭高原子序物質經由巨偶極共振(giant dipole resonance)產生光中子反應(,n) (photoneutron reaction)，所產生的中子能量分佈從熱中子至快中子能量均有；高能光子射束伴隨中子汙染，使病人接受不必要的劑量。
利用TLD600與TLD700之雙TLD方法搭配鎘差法可以在光子、中子混合場中，分辨光子劑量與熱中子劑量，因此一般常將雙TLD放置於Rando假體以評估醫用直線加速器產生之熱中子劑量。實際上醫用直線加速器所產生之中子為熱中子至快中子皆有分佈，在假體內因空間有限無法放置鎘片，使雙TLD方法測得中子劑量有一部份來源為快中子貢獻而造成誤差。
本研究利用多球體搭配金箔測量醫用直線加速器照射不同假體深度下之中子能譜，利用此中子能譜以蒙地卡羅軟體計算出快中子劑量與熱中子劑量佔所有中子劑量之比例，將此修正因子用於實際量測值可以發現，在假體表面、4公分、10公分、15公分、20公分之誤差分別為99%、45%、21%、12%、12%，從結果可以發現在假體表面之中子能譜主要以快中子能譜為主，故此時以雙TLD方法(不搭配鎘差法)評估之熱中子劑量誤差最大，而在假體較深處，隨著中子進入假體產生作用使中子緩速，中子能量越來越接近熱中子，誤差也越來越小。

關鍵字

光中子 ； 光核反應 ； 中子汙染 ； 放射治療

並列摘要

In radiation therapy, the high energy photon beams generated by the medical linear accelerators (Linac) were usually used to kill the tumors. Thorough the giant dipole resonances, (,n) photoneutron reactions occurred by the high energy photons interacted with materials of high atomic number.. The energy distribution of induced photoneutrons is from thermal to fast energy. The neutrons induced by high energy photons increase the doses of patients.
Dual-TLD method coupling with Cadmium-difference method is usually used to distinguish the doses of thermal neutrons form photons in a mixed radiation field. TLD-600 and TLD-700 were easily used to put inside a Rando phantom to investigate the dose contributed by the thermal neutrons induced by a high energy photon beam. Due to the hole space inside the Rando phantom is small to fill a set of dual-TLD with Cd cover, in this case, the estimated neutrons would include neutrons with energies of thermal and above thermal, so the estimated thermal neutron doses would include a contribution for some fast neutrons.
In this study, a system of Bonner sphere spectrometer with gold foils was used to measure the neutron spectra (at different depth of phantom) induced by a Linac (15 MV photon beam). By means of the measured neutron spectra and the Monte Carlo method, the dose ratios contributed by thermal and fast neutrons were assessed and relative correction factors were calculated. In the results, the dose errors of the thermal neutrons due to not subtracting the doses contributed by the higher energy neutrons were 99 %, 45 %, 21 %, 12 % and 12 % for 0, 4, 10, 15 and 20 cm depth in phantom, respectively. On the surface of the phantom, the fast neutrons dominated in the neutron spectrum, therefore, dose of thermal neutrons estimated by dual-TLD method (without using cadmium-difference method) had a maximum dose error. In deeper depths, due to large amount of the fast neutrons were slowing down, estimated dose errors of thermal neutrons decreased.

參考文獻

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