醫用迴旋加速器(MINItraceTM)誘發產生之中子能譜與人員劑量評估

Evaluation of neutron spectra and personal doses induced by the MINItraceTM medical cyclotron

郭明杰(Ming-Jay Kuo)

指導教授 ： 許靖涵

國立清華大學/原子科學院/生醫工程與環境科學系/博士(2015年)

https://doi.org/10.6843/NTHU.2015.00605

摘要

新光醫院迴旋加速器中心主要是生產18F-FDG之放射性藥物，提供新光醫院正子照影(Positron Emission Tomography, PET)中心做診斷相關檢查。本研究主要評估迴旋加速器中心之工作人員在生產、分裝與運送18F -FDG(2-fluoro-2-[18F]-fluoro-D-glucose)藥物過程中工作人員所接受到的輻射劑量曝露。此外，醫用迴旋加速器(GE MINI-trace)在生產放射性同位素18F過程中，會使用鈮靶(Niobium water target)與銀靶(Silver water target)系統，在不同的射束電流下，探討不同的靶極系統所誘發的中子能譜變化。
本實驗利用雙熱發光劑量計方法(TLD-600/700)評估迴旋加速器中心之工作人員在生產、分裝與運送18F-FDG藥物過程中，工作人員所接受到全身Hp(10)以及手指部Hp(0.07)輻射劑量曝露。此外，利用金箔搭載鎘片(113Cd)之中子活化分析的方法測量熱中子通量，以及採用波納式球能譜儀(Bonner Sphere Spectrometer, BSS)搭載6LiI(Eu)閃爍偵檢器進行中子能譜測量，探討醫用迴旋加速器在銀靶系統(Target 1與Target 4)與鈮靶系統(Target 4)下，不同射束電流所產生的中子能譜的變化。
根據實驗結果得知，工作人員在分裝過程中，全身Hp(10)及手指部Hp(0.07)的總曝露劑量分別為5.30 mSv y-1及252.35 mSv y-1，在運送過程中分別是1.04 mSv y-1及25.56 mSv y-1；在生產過程中，全身Hp(10)光子與熱中子的劑量分別為309.06 μSv y-1及22.48 μSv y-1。當迴旋加速器使用Target 2運轉過程中，在屏蔽外側獲得的熱中子劑量率(15.93 nGy min-1)與熱中子通量率(19.7 n cm-2s-1)是Target 1約2.5倍；且Target 2所獲得的中子能量通量率是Target 1約2倍，主要是因為靶體積的差異造成影響。此外，鈮靶系統(Target 4)的平均18F飽和產率(93.3 mCi μA-1)比Taregt 1多10.1 mCi μA-1，但Target 4獲得的中子能量通量率比Target 1增加2.7倍，主要是因為靶系統間之靶體積、靶體的材質、靶體的幾何形狀與靶的組成之間的差異所造成中子能譜的影響。

總之，迴旋加速器中心之三位工作人員平均每人的全身Hp(10)及手指部Hp(0.07)的劑量分別為2.18及92.64 mSv y-1，最大的劑量貢獻是在分裝過程所接受的曝露。當迴旋加速器在使用不同的靶系統與射束電流下，會產生不同的18F飽和產率及誘發不同強度的中子能譜，且此一變化量是不可忽略。

關鍵字

鈮靶 ； 波納式球能譜儀 ； 中子能譜 ； 醫用迴旋加速器

並列摘要

The Cyclotron Center in Shin Kong Wu Ho-Su Memorial Hospital (SK Cyclotron Center) produced the 18F-FDG compound and provided it to the Positron Emission Tomography (PET) center for diagnosis services. In this study, the exposure doses of workers during the procedures of production of 18F, dispensation, and transportation of the compound (18F–FDG) (2-fluoro-2-[18F]-fluoro-D-glucose) are estimated. Besides, this study discussed the variations of neutrons spectra induced from the niobium and silver water targets during the production of fluoride (18F) using the medical cyclotron (GE MINI-trace). Then,
To estimate the exposure for the staffs working in the SK Cyclotron Center, the dual-TLD chips (TLD-600/700) method was used to measure the doses contributed from photons and neutrons during the operation of the cyclotron, and the doses contributed from photons during the dispensations and transportations. The neutron activation methods of gold foils with and without 113Cd covered were used to correct the doses and to measure flux of thermal neutrons. Furthermore, the method of 6LiI(Eu) detector with Bonner Sphere Spectrometer (BSS) was used to measure neutron spectra. The variations of neutron spectra as the medical cyclotron was performed in different target systems (Target 1, Target 2 and Target 4) and beam currents are discussed.
According to results of this research, for the staffs working in SK Cyclotron Center, Hp(10) of 309.06 and 22.48 μSv y-1 for photons and thermal neutrons due to operating cyclotron, Hp(10) of 5.30 mSv y-1 and Hp(0.07) of the finger of 252.35 mSv y-1 for photons due to dispensation, and Hp(10) of 1.04 mSv y-1 and Hp(0.07) of the finger of 25.56 mSv y-1 for photons due to transportations, respectively. The contributions of thermal neutron dose rate (15.93 nGy min-1) and thermal neutron flux (19.7 n cm-2s-1) for using Target 2 at the outside of self-shielding were 2.5 times as larger as Target 1. The energy fluence rates in Target 2 were 2 times as larger as Target 1. The volumes of targets affected the induced neutron spectra and doses. In addition, the mean saturation yield for using niobium water target (Target 4) (93.3 mCi μA-1) was 10.1 mCi μA-1 higher than Target 1. However, the energy fluence rates in Target 4 were 2.7 times highter than Target 1. Volumes, materials, geometric and components of targets impacted the induced neutron spectra.
In conclusions, the mean Hp(10) and Hp(0.07) of the finger for a worker in the SK cyclotron center were 2.18 mSv y-1 and 92.64 mSv y-1, respectively, moreover, the maximum dose contribution appeared in dispensations. Besides, the cyclotron was operated in different targets system and beam currents that induced different saturation yield and neutron spectra. These variations were no ignoring.

並列關鍵字

Niobium water target ； Bonner Sphere Spectrometer ； Neutron spectrum ； Medical cyclotron

參考文獻

40. 林震榮，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，國立清華大學，碩士論文，民國一百年

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