硼中子捕獲治療為新穎的癌症治療技術，其結合(超)熱中子照射與含硼-10標靶藥物兩大要素，相較於光子與質子治療，其除可有效殺死癌細胞，也可大幅降低相關併發症發生的機率。由於硼-10與熱中子反應時會產生0.478 MeV的瞬發加馬射線，因此原理上可藉由量測0.478 MeV瞬發加馬射線用以分辨硼-10於中子束照野中的空間分佈以確保治療成效。本研究之目標即是發展基於準直儀之瞬發加馬射線偵檢系統以應用於硼中子捕獲治療中量測瞬發加馬射線之變化並推估硼-10的空間分佈。
本研究首先透過蒙地卡羅模擬分析中子場與偵檢系統特性，包括中子與硼反應所誘發之瞬發加馬射線的產量與空間分佈以及偵檢系統之空間解析能力。模擬帶入清華大學開放水池式反應爐(Tsing Hua Open-pool Reactor, THOR)之超熱中子能譜為中子源，以體積為5 × 5 × 6立方公分之壓克力假體以概括模擬腦部，並於壓克力假體深度5毫米處模擬2 × 3 × 2立方公分之純硼酸代表腫瘤。模擬結果顯示將純硼酸置於壓克力假體深度5毫米處時，每個中子誘發的瞬發加馬光子平均數量為0.21。另於模擬中假體的側向設定一個含1.5英吋碘化鈉閃爍偵檢器與36公分長、2毫米寬之鉛準直儀的偵檢系統，並用以解析0.478 MeV瞬發加馬射線隨深度的強度變化。模擬結果顯示此偵檢系統之空間解析度可近於2毫米，且紀錄之瞬發加馬射線隨深度的強度變化之訊雜比為626.3。
本研究也利用THOR反應器的BNCT中子射束照射含純硼靶材之壓克力假體來驗證如模擬所建立的準直儀偵檢系統量測0.478 MeV加馬射線空間變化之成效。當使用反應爐功率為100千瓦之中子束照射假體時，偵檢系統量測之瞬發加馬射線強度隨深度之變化的訊雜比為1.18，在此訊雜比大幅下降的原因為環境中存在大量的互毀加馬射線而影響0.478 MeV瞬發加馬射線的分析。為降低量測中互毀加馬射線的影響以提升偵檢系統對0.478 MeV瞬發加馬射線隨深度的變化之訊雜比，研究也測試主動式準直儀瞬發加馬射線偵檢系統以藉由時序反同符技術濾除互毀加馬射線訊號。初步實驗結果顯示，當使用兩個2.4 × 3.6 × 3 cm3 鍺酸鉍閃爍偵檢器搭配長36公分之鉛所組成的2毫米狹縫準直儀時，以反應爐功率15千瓦之中子束照射假體，主動式準直儀瞬發加馬射線偵檢系統可濾除11.74 % 之互毀加馬射線訊號，後續研究可增加鍺酸密閃爍偵檢器之數量來測試濾除互毀加馬射線之能力。本研究已成功發展基於準直儀之瞬發加馬射線偵檢系統，並掌握偵檢系統與中子場特性，作為後續進一步改良與發展的基礎。

Boron Neutron Capture Therapy (BNCT) is an innovative form of radiotherapy that combines thermal neutron beam irradiation with Boron-10 (B-10) labeled drugs. Compared to photon and proton therapy, BNCT not only effectively damages tumor cells but also significantly reduces post-treatment side effects. Since the capture reaction between B-10 and thermal neutrons produces 0.478-MeV prompt gamma rays (PGs), it is possible to estimate the spatial distribution of B-10 in the neutron radiation field by measuring the associated emission of these PGs, thereby ensuring the effectiveness of the treatment. Therefore, this study focuses on developing a collimator-based prompt gamma-ray detection system capable of identifying variations in 0.478-MeV PGs and estimating the spatial distribution of B-10 in BNCT.
Monte Carlo simulations were conducted to investigate the properties of the neutron field and the detection system, including the yield and the spatial distribution of 0.478-MeV PGs emitted from the target due to the B-10 neutron capture reaction. In each simulation, the neutron source was assigned with the epithermal neutron energy spectrum experimentally measured for the BNCT port of the Tsing Hua Open-pool Reactor (THOR). A 5 × 5 × 6 cm³ polymethyl methacrylate (PMMA) phantom was defined to model a brain, with a 2 × 3 × 2 cm³ target of pure boric acid inside the PMMA phantom to represent the tumor. Simulation results showed that the average yield of 0.478-MeV PGs is 0.21 per neutron when the boric acid target was positioned at a depth of 5 mm. In addition, a detection system comprising a 1.5-inch NaI(Tl) scintillator and a 36-cm long lead collimator of a slit width of 2 mm was modelled to transversely record the intensity distribution of 0.478-MeV PGs at various depths inside the phantom. The results indicated that the spatial resolution of this collimator–detector system is approximately 2 mm, and the signal-to-noise ratio (SNR) for the recorded intensity distribution of 0.478-MeV PGs is 626.3.
The above-mentioned collimator-detector system and PMMA phantom with a pure boric acid target were experimentally implemented to measure the spatial distribution of 0.478-MeV PGs inside the phantom when irradiated by the epithermal neutron beam from the THOR. At a reactor power of 100 kW, the SNR of the measured intensity distribution of PGs along the target depth is 1.18, which is significantly lower than the simulated value due to the presence of environmental annihilation gamma-rays at 0.511 MeV. To reduce the counts from annihilation gamma-rays in measurements, an active collimator scheme based on anti-coincidence for removing annihilation gamma-ray signals is proposed in this study to improve the SNR for identifying the intensity variation of 0.478-MeV PGs at various depths in the phantom. As a preliminary test, two 2.4 × 3.6 × 3 cm³ BGO scintillators were placed in front of a 36-cm long, 2-mm wide lead slit collimator to form an active collimator for measuring PGs. When the PMMA phantom was irradiated by a neutron beam with a reactor power of 15 kW, the active collimator showed an 11.74% reduction in the annihilation gamma-ray signals compared to the results measured with the default collimator. Increasing the number of BGO scintillators in the active collimator will facilitate the filtering out of annihilation gamma-ray signals in future work. This study serves as a foundation for developing a collimator-based PGs detection system to identify the B-10 distribution in BNCT.

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