Spread-out Bragg peak proton FLASH irradiation using a clinical synchrocyclotron: Proof of concept and ion chamber characterization
Spread-out Bragg peak proton FLASH irradiation using a clinical synchrocyclotron: Proof of concept and ion chamber characterization
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DOI：
10.1002/mp.15021
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发表时间：
2021-07-11
期刊：
影响因子：
3.8
通讯作者：
Zhao, Tianyu
中科院分区：
文献类型：
--
作者：
Darafsheh, Arash;Hao, Yao;Zhao, Tianyu
目的：(A)论证使用质子治疗同步回旋加速器在超高剂量率(Flash)下发射扩展布拉格峰(SOBP)质子束的可行性，作为实现临床前研究的实验平台的主要步骤；以及(B)评估四种模型的电离室在这样的辐射场中的响应。方法对临床使用的Mevion HYPERSCAN(R)同步回旋加速器进行调整，以适应超高剂量率的质子输送。标称能量为230MeV的质子以脉冲的形式被发射，时间宽度从12.5mU S到24mU S，跨度从常规剂量率到闪光剂量率。在光束路径中放置了碳化硼吸收体和距离调制器块，用于距离调制和创建SOBP剂量分布。辐射场由直径为11 mm的黄铜口径定义。两个法拉第杯被用来测定不同剂量率下每个脉冲的质子数。评估了两个圆柱形电离室(IBA CC04和CC13)和两个平面平行电离室(IBA PPC05和PTW Advanced Markus(R))的剂量学响应。用平面平行电离室测量剂量率。积分深度剂量(IDD)用PTW Bragg Peak(R)电离室测量。用EBT-XD辐射变色胶片测量横向束流分布。在TOPAS中进行了蒙特卡罗仿真，作为测量的二次校核和进一步优化距离调制器设计的工具。结果法拉第杯测量表明，在S脉宽为24 mU时，每个脉冲的最大质子数为39.9pC。测量和模拟的IDD和横向光束分布之间有很好的一致性。圆柱形电离室显示出非常高的离子复合，被认为不适合在超高剂量率下进行绝对剂量测量。用PPC05型电离室测得原始布拉格峰的平均剂量率为163GY/S，SOBP束深1 cm处的平均剂量率为126GY/S。测量了SOBP的射程为24.4 mm，调制为19 mm。原始的布拉格波束射程为25.6 mm。仿真结果表明，距离调制器的腔径和扫描光斑数目可以分别改善干涉仪的离散度和轮廓平坦度。结论证明了用临床同步回旋加速器以~gt；100Gy/S平均剂量率的SOBP模式传输质子是可行的。通过优化距离调制器和投射光斑数目，可以改善剂量的不均匀性。与本文中使用的其他电离室相比，电极间距较小的平面平行室更适合闪光剂量测量。
Purpose The purpose of this work is to (a) demonstrate the feasibility of delivering a spread-out Bragg peak (SOBP) proton beam in ultra-high dose rate (FLASH) using a proton therapy synchrocyclotron as a major step toward realizing an experimental platform for preclinical studies, and (b) evaluate the response of four models of ionization chambers in such a radiation field. Methods A clinical Mevion HYPERSCAN(R) synchrocyclotron was adjusted for ultra-high dose rate proton delivery. Protons with nominal energy of 230 MeV were delivered in pulses with temporal width ranging from 12.5 mu s to 24 mu s spanning from conventional to FLASH dose rates. A boron carbide absorber and a range modulator block were placed in the beam path for range modulation and creating an SOBP dose profile. The radiation field was defined by a brass aperture with 11 mm diameter. Two Faraday cups were used to determine the number of protons per pulse at various dose rates. The dosimetric response of two cylindrical (IBA CC04 and CC13) and two plane-parallel (IBA PPC05 and PTW Advanced Markus(R)) ionization chambers were evaluated. The dose rate was measured using the plane-parallel ionization chambers. The integral depth dose (IDD) was measured with a PTW Bragg Peak(R) ionization chamber. The lateral beam profile was measured with EBT-XD radiochromic film. Monte Carlo simulation was performed in TOPAS as the secondary check for the measurements and as a tool for further optimization of the range modulators' design. Results Faraday cups measurement showed that the maximum protons per pulse is 39.9 pC at 24 mu s pulse width. A good agreement between the measured and simulated IDD and lateral beam profiles was observed. The cylindrical ionization chambers showed very high ion recombination and deemed not suitable for absolute dosimetry at ultra-high dose rates. The average dose rate measured using the PPC05 ionization chamber was 163 Gy/s at the pristine Bragg peak and 126 Gy/s at 1 cm depth for the SOBP beam. The SOBP beam range and modulation were measured 24.4 mm and 19 mm, respectively. The pristine Bragg peak beam had 25.6 mm range. Simulation results showed that the IDD and profile flatness can be improved by the cavity diameter of the range modulator and the number of scanned spots, respectively. Conclusions Feasibility of delivering protons in an SOBP pattern with >100 Gy/s average dose rate using a clinical synchrocyclotron was demonstrated. The dose heterogeneity can be improved through optimization of the range modulator and number of delivered spots. Plane-parallel chambers with smaller gap between electrodes are more suitable for FLASH dosimetry compared to the other ion chambers used in this work.