Department of Radiation Oncology, Washington University School of Medicine, St. Louis, MO, USA.
Mevion Medical Systems, Littleton, MA, USA.
Med Phys. 2021 Aug;48(8):4472-4484. doi: 10.1002/mp.15021. Epub 2021 Jul 11.
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.
A clinical Mevion HYPERSCAN 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 μs to 24 μs 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 ) 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 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.
Faraday cups measurement showed that the maximum protons per pulse is 39.9 pC at 24 μs 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.
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.
本研究旨在（a）展示使用质子治疗同步回旋加速器以超高峰值剂量率（FLASH）递送扩展布拉格峰（SOBP）质子束的可行性，这是实现用于临床前研究的实验平台的重要一步，以及（b）评估四种电离室模型在这种辐射场中的响应。
临床 Mevion HYPERSCAN 同步回旋加速器经过调整，可用于超高峰值剂量率质子输送。标称能量为 230 MeV 的质子以 12.5 μs 至 24 μs 的时间宽度脉冲输送，涵盖常规至 FLASH 剂量率。碳化硼吸收体和射程调制块放置在射束路径中，用于射程调制和创建 SOBP 剂量分布。辐射场由直径为 11 mm 的黄铜孔径定义。两个法拉第杯用于在不同剂量率下确定每个脉冲的质子数。评估了两个圆柱形（IBA CC04 和 CC13）和两个平面平行型（IBA PPC05 和 PTW Advanced Markus）电离室的剂量学响应。使用平面平行电离室测量剂量率。使用 PTW Bragg Peak 电离室测量积分深度剂量（IDD）。使用 EBT-XD 放射色胶片测量横向射束轮廓。在 TOPAS 中进行了蒙特卡罗模拟，作为对测量的二次检查，并作为进一步优化射程调制器设计的工具。
法拉第杯测量显示，在 24 μs 脉冲宽度时，每个脉冲的最大质子数为 39.9 pC。观察到测量的 IDD 和横向射束轮廓与模拟结果非常吻合。圆柱形电离室表现出非常高的离子复合，不适合在超高剂量率下进行绝对剂量测量。使用 PPC05 电离室测量的平均剂量率在原始布拉格峰处为 163 Gy/s，在 SOBP 射束 1 cm 深度处为 126 Gy/s。SOBP 射束的射程和调制分别测量为 24.4 mm 和 19 mm。原始布拉格峰射束的射程为 25.6 mm。模拟结果表明，通过调整射程调制器的腔径和扫描点的数量，可以分别改善 IDD 和轮廓平整度。
使用临床同步回旋加速器以超过 100 Gy/s 的平均剂量率递送 SOBP 模式质子束的可行性得到了证明。通过优化射程调制器和输送点的数量，可以改善剂量异质性。与本工作中使用的其他离子室相比，电极之间间隙较小的平面平行室更适合 FLASH 剂量测量。
