A Prototype Scintillator Real-Time Beam Monitor for Ultra-high Dose Rate Radiotherapy
A Prototype Scintillator Real-Time Beam Monitor for Ultra-high Dose Rate Radiotherapy
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DOI：
10.1002/mp.17018
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发表时间：
2023-05
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通讯作者：
D. Levin;P. Friedman;C. Ferretti;Nicholas Ristow;M. Tecchio;D. Litzenberg;V. Bashkirov;R. Schulte
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作者：
D. Levin;P. Friedman;C. Ferretti;Nicholas Ristow;M. Tecchio;D. Litzenberg;V. Bashkirov;R. Schulte
背景 闪光放射治疗 (RT) 是一种新兴的癌症放射治疗方式，其整个治疗剂量的剂量率比传统 RT 高 1000 倍以上。为了安全地进行临床试验，需要能够产生超出公差的光束中断的精确、快速的光束监视器。本文描述了总体概念，并提供了用于质子和电子束 FLASH 应用的基于闪烁体的超快束流监测器原型的结果。目的 正在开发一种采用新型专有闪烁体材料的闪光束闪烁体监视器（FBSM）。 FBSM具有传统RT探测器技术无法同时提供的功能：1）大面积覆盖； 2) 低质量轮廓； 3）在宽动态范围内的线性响应； 4）辐射硬度； 5) 实时分析，基于真正的二维光束成像、辐射剂量测量和出色的空间分辨率，提供符合 IEC 标准的快速光束中断信号。方法 FBSM 使用专有的低质量、小于 0.5 毫米水当量、不吸湿、耐辐射的闪烁体材料（称为 HM：混合材料），可通过高帧率 CMOS 相机进行观察。使用镜子的折叠光学器件可实现约 10 厘米的薄显示器轮廓。现场可编程门阵列 (FPGA) 数据采集系统 (DAQ) 在适合 FLASH RT 束模态的时间尺度上生成实时分析：脉冲电子为 100–1000 Hz，准连续扫描质子笔形束为 10–20 kHz。离子束监测器作为这项工作的初始开发平台，并在低能重离子束（86Kr+26 和质子）中进行了测试。制作了 FBSM 原型，然后在各种辐射束中进行测试，其中包括每脉冲电子束的 FLASH 级剂量，以及医院放射治疗诊所的电子束。结果 本报告中提出的结果包括图像质量、响应线性、辐射硬度、空间分辨率和实时数据处理。人们发现 HM 闪烁体具有很强的抗辐射损伤能力。与 216 kGy 累积剂量相比，在 237 Gy/s 的 FLASH 兼容剂量率下连续暴露 15 分钟后，它表现出 0.025%/kGy 信号小幅下降。信号幅度与光束注量的测量结果表明 FBSM 在 FLASH 兼容剂量率 > 40 Gy/s 时的线性响应。与商用 Gafchromic 薄膜的比较表明，FBSM 产生高分辨率的 2D 光束图像，并且可以再现几乎相同的光束轮廓，包括主光束尾部。空间分辨率测量为 35-40 μm。固件 beta 版本的测试表明，可以在 20,000 Hz 帧速率或 50 μs/帧下成功运行，其中光束参数的实时分析在不到 1 μs 的时间内实现。结论 FBSM 旨在在大活动区域提供实时光束轮廓监测，而不会显着降低光束质量。使用连续离子束和脉冲电子束，原型设备已在单粒子电流高达 FLASH 级剂量率的粒子束中上演。使用新型闪烁体，光束分析已被证明适用于从单粒子到 10 nA 电流的电流。辐射损伤很小，即使在 FLASH 条件下，单个点也需要 ≥ 50 kGy 的累积暴露，才能导致信号输出减少 1%。光束成像与放射变色胶片相当，无需数小时的处理即可提供即时图像。实时数据处理时间不到 50 μs（数据传输和分析时间之和），已在固件中实现，适用于连续质子束的 20 kHz 帧速率。
Background FLASH Radiotherapy (RT) is an emergent cancer radiotherapy modality where an entire therapeutic dose is delivered at more than 1000 times higher dose rate than conventional RT. For clinical trials to be conducted safely, a precise and fast beam monitor that can generate out-of-tolerance beam interrupts is required. This paper describes the overall concept and provides results from a prototype ultra-fast, scintillator-based beam monitor for both proton and electron beam FLASH applications. Purpose A FLASH Beam Scintillator Monitor (FBSM) is being developed that employs a novel proprietary scintillator material. The FBSM has capabilities that conventional RT detector technologies are unable to simultaneously provide: 1) large area coverage; 2) a low mass profile; 3) a linear response over a broad dynamic range; 4) radiation hardness; 5) real-time analysis to provide an IEC-compliant fast beam-interrupt signal based on true two-dimensional beam imaging, radiation do-simetry and excellent spatial resolution. Methods The FBSM uses a proprietary low mass, less than 0.5 mm water equivalent, non-hygroscopic, radiation tolerant scintillator material (designated HM: hybrid material) that is viewed by high frame rate CMOS cameras. Folded optics using mirrors enable a thin monitor profile of ~10 cm. A field programmable gate array (FPGA) data acquisition system (DAQ) generates real-time analysis on a time scale appropriate to the FLASH RT beam modality: 100–1000 Hz for pulsed electrons and 10–20 kHz for quasi-continuous scanning proton pencil beams. An ion beam monitor served as the initial development platform for this work and was tested in low energy heavy-ion beams (86Kr+26 and protons). A prototype FBSM was fabricated and then tested in various radiation beams that included FLASH level dose per pulse electron beams, and a hospital radiotherapy clinic with electron beams. Results Results presented in this report include image quality, response linearity, radiation hardness, spatial resolution, and real-time data processing. The HM scintillator was found to be highly radiation damage resistant. It exhibited a small 0.025%/kGy signal decrease from a 216 kGy cumulative dose resulting from continuous exposure for 15 minutes at a FLASH compatible dose rate of 237 Gy/s. Measurements of the signal amplitude vs beam fluence demonstrate linear response of the FBSM at FLASH compatible dose rates of > 40 Gy/s. Comparison with commercial Gafchromic film indicates that the FBSM produces a high resolution 2D beam image and can reproduce a nearly identical beam profile, including primary beam tails. The spatial resolution was measured at 35–40 μm. Tests of the firmware beta version show successful operation at 20,000 Hz frame rate or 50 μs/frame, where the real-time analysis of the beam parameters is achieved in less than 1 μs. Conclusions The FBSM is designed to provide real-time beam profile monitoring over a large active area without significantly degrading the beam quality. A prototype device has been staged in particle beams at currents of single particles up to FLASH level dose rates, using both continuous ion beams and pulsed electron beams. Using a novel scintillator, beam profiling has been demonstrated for currents extending from single particles to 10 nA currents. Radiation damage is minimal and even under FLASH conditions would require ≥ 50 kGy of accumulated exposure in a single spot to result in a 1% decrease in signal output. Beam imaging is comparable to radiochromic films, and provides immediate images without hours of processing. Real-time data processing, taking less than 50 μs (combined data transfer and analysis times), has been implemented in firmware for 20 kHz frame rates for continuous proton beams.