CERN, 1211, Geneva 23, Switzerland.
Physics Institute 3B, RWTH Aachen University, 52074, Aachen, Germany.
Med Phys. 2020 Jun;47(6):2516-2525. doi: 10.1002/mp.14119. Epub 2020 Mar 21.
Commercially available systems for ion beam reference dosimetry in water are mainly based on ionization chambers. In those systems, a large number of small detectors are typically arranged in a two-dimensional (2D) array or matrix to achieve high spatial resolution (order of several millimeters) and large field coverage at the same time. The goal of this work was to investigate the reliability of a detector of superior spatial resolution to perform three-dimensional (3D) ionization measurements in carbon ion pencil beams.
The GEMPix is a small gaseous detector with a highly pixelated readout, consisting of a drift region (with 2.8 cm × 2.8 cm × 0.3 cm volume), three gas electron multipliers (GEMs) for signal amplification and four Timepix ASICs with 55 µm pixel pitch and a total of 262,144 pixels. An integrated system was designed and built, which consists of a commercial water phantom with a three-axis motorized arm, a reference large-area ionization chamber for signal normalization to the beam output and the GEMPix itself. Measurements at different depths in water have been performed at the Italian National Centre for Oncological Hadrontherapy (CNAO) with three carbon ion beam energies. Lateral beam profiles measured with the GEMPix at the shallowest depth were compared to those measured with radiochromic EBT3 films in air in the position of the reference ionization chamber. The Timepix readout was calibrated in energy by using one independent depth scan with carbon ions of 150 mm range. Bragg peak curves were also simulated using the Monte Carlo FLUKA code as a reference.
Beam profiles measured with the GEMPix were smooth and showed similar shape and full width at half maximum when compared to those measured with radiochromic EBT3 films. Smooth, reproducible Bragg curves were obtained with statistical uncertainties of about 2%, matching FLUKA simulations of the Bragg curves within 15% for most data points. This difference is partially explained for the measurement with carbon ions of 150 mm range by a saturation effect in the GEMs. The high granularity of the readout allowed to produce 2D images of the deposited dose at different depths, as well as 3D data distributions.
This paper demonstrates the capability of the GEMPix detector to measure the 3D dose distribution of carbon ions in water for a clinical pencil beam reliably. In the future, the detector area will be increased to cover fields of scanned beams. Measurements at higher beam intensities and with protons are planned.
市售的水基离子束参考剂量测量系统主要基于电离室。在这些系统中，通常会将大量的小探测器排列成二维（2D）阵列或矩阵，以实现高空间分辨率（几毫米量级）和大视场覆盖。本工作的目的是研究一种具有优越空间分辨率的探测器在碳离子笔形束中进行三维（3D）离子化测量的可靠性。
GEMPix 是一种具有高度像素化读出的小型气体探测器，由一个漂移区（2.8 cm×2.8 cm×0.3 cm 体积）、三个气体电子倍增器（GEM）用于信号放大和四个 Timepix ASIC 组成，具有 55 µm 像素间距和总共 262144 个像素。设计并构建了一个集成系统，该系统由一个带有三轴电动臂的商业水模体、一个用于将信号归一化为束输出的参考大面积电离室和 GEMPix 本身组成。在意大利国家肿瘤强子治疗中心（CNAO）用三种碳离子束能量在水中的不同深度进行了测量。在最浅深度用 GEMPix 测量的横向束轮廓与在参考电离室位置用空气的放射色 EBT3 胶片测量的束轮廓进行了比较。通过用 150 mm 射程的碳离子进行一次独立的深度扫描，对 Timepix 读出进行了能量校准。也使用 Monte Carlo FLUKA 代码作为参考来模拟布喇格峰曲线。
与用放射色 EBT3 胶片测量的结果相比，用 GEMPix 测量的束轮廓是平滑的，并且具有相似的形状和半高全宽。用统计学不确定性约为 2%的平滑、可重复的布喇格曲线得到了，与大多数数据点的 15%以内的 FLUKA 模拟布喇格曲线吻合较好。对于用 150 mm 射程的碳离子进行的测量，GEM 中的饱和效应部分解释了这种差异。读出的高粒度允许在不同深度产生沉积剂量的 2D 图像，以及 3D 数据分布。
本文证明了 GEMPix 探测器可靠地测量水临床笔形束中碳离子 3D 剂量分布的能力。将来，探测器面积将扩大以覆盖扫描束的区域。计划进行更高束强度和质子的测量。
