Department of Radiation Oncology, Massachusetts General Hospital, Boston, MA 02114, USA.
Med Phys. 2012 Apr;39(4):2100-7. doi: 10.1118/1.3694098.
In vivo range verification is one of the most important parts of proton therapy to fully utilize its benefits delivering high radiation dose to tumor, while sparing the normal tissue with the so-called Bragg peak. Currently, however, range verification method is not used in clinics. The purpose of the present study is to optimize and evaluate the configuration of an array-type prompt gamma measurement system on determining distal dose edge for in vivo range verification of proton therapy.
To effectively measure the prompt gammas against the background gammas, the Monte Carlo simulations with the MCNPX code were employed in optimizing the configuration of the measurement system, and the Monte Carlo method was also used to understand the effect of the background gammas, mainly neutron capture gammas, in the measured gamma distribution. To reduce the effect of the background gammas, the optimized energy window of 4-10 MeV in measuring the prompt gammas was employed. A parameterized source was used to maximize computation speed in the optimization study. A simplified test measurement system, using only one detector moving from one measurement location to the next, was constructed and applied to therapeutic proton beams of 80-220 MeV. For accurate determination of the distal dose edge, the sigmoidal curve-fitting method was applied to the measured distributions of the prompt gammas, and then, the location of the half-value between the maximum and minimum value in the curve-fitting was determined as the distal dose edge and compared with the beam range assessed by the proton dose distribution.
The parameterized source term employed in optimization process improved the calculation speed by up to ∼300 times. The optimization study indicates that an array-type measurement system with 3, 2, 2, and 150 mm for scintillator thickness, slit width, septal thickness, and slit length, respectively, can effectively measure the prompt gamma distributions minimizing the contribution of background gammas. The present results show that a few hundred counts of prompt gammas can be easily obtained by measuring 10 s at each measurement location for proton beams of ∼4 nA. The distal dose edges determined by the prompt gamma distribution are 5.45, 14.73, and 27.74 cm for proton beams of 5.17 (80 MeV), 14.99 (150 MeV), and 27.38 (220 MeV) cm, respectively.
The results show that the array-type measurement system can measure prompt gamma distributions from a therapeutic proton beam within a short measurement time, and that the distal dose edge can be determined within a few millimeters of error without using any sophisticated analysis.
在体射程验证是质子治疗充分利用其优势将高剂量辐射递送至肿瘤，同时利用所谓的布拉格峰保护正常组织的最重要部分之一。然而，目前该验证方法并未在临床上使用。本研究旨在优化和评估阵列型瞬发伽马测量系统的配置，以确定质子治疗的在体射程验证的远端剂量边缘。
为了有效测量相对于本底伽马的瞬发伽马，采用蒙特卡罗模拟 MCNPX 代码优化测量系统的配置，并采用蒙特卡罗方法了解本底伽马（主要是中子俘获伽马）在测量伽马分布中的影响。为了降低本底伽马的影响，在测量瞬发伽马时采用了 4-10 MeV 的优化能窗。使用参数化源在优化研究中提高计算速度。构建并应用简化的测试测量系统，仅使用一个探测器从一个测量位置移动到另一个位置，用于 80-220 MeV 的治疗质子束。为了准确确定远端剂量边缘，应用指数曲线拟合方法对测量的瞬发伽马分布进行拟合，然后确定曲线拟合中最大值和最小值之间半值的位置作为远端剂量边缘，并与质子剂量分布评估的束射程进行比较。
优化过程中采用的参数化源项将计算速度提高了约 300 倍。优化研究表明，具有 3、2、2 和 150mm 的闪烁体厚度、狭缝宽度、隔板厚度和狭缝长度的阵列型测量系统可以有效测量瞬发伽马分布，最大限度地减少本底伽马的贡献。本研究结果表明，对于约 4 nA 的质子束，在每个测量位置测量 10 秒，很容易获得几百个瞬发伽马计数。由瞬发伽马分布确定的远端剂量边缘分别为 5.17（80 MeV）、14.99（150 MeV）和 27.38（220 MeV）质子束的 5.45、14.73 和 27.74cm。
结果表明，该阵列型测量系统可以在短测量时间内测量治疗质子束的瞬发伽马分布，并且可以在几毫米的误差范围内确定远端剂量边缘，而无需使用任何复杂的分析。
