Department of Human Oncology, University of Wisconsin-Madison, Madison, Wisconsin, USA.
Medscint, Québec, Canada.
Med Phys. 2024 Sep;51(9):6475-6484. doi: 10.1002/mp.17192. Epub 2024 Jun 6.
MRI-guided radiation therapy (MRgRT) requires unique quality assurance equipment to address MR-compatibility needs, minimize electron return effect, handle complex dose distributions, and evaluate real-time dosimetry for gating. Plastic scintillation detectors (PSDs) are an attractive option to address these needs.
To perform a comprehensive characterization of a multi-probe PSD system in a low-field 0.35 T MR-linac, including detector response assessment and gating performance.
A four-channel PSD system (HYPERSCINT RP-200) was assembled. A single channel was used to evaluate repeatability, percent depth dose (PDD), detector response as a function of orientation with respect to the magnetic field, and intersession variability. All four channels were used to evaluate repeatability, linearity, and output factors. The four PSDs were integrated into an MR-compatible motion phantom at isocenter and in gradient regions. Experiments were conducted to evaluate gating performance and tracking efficacy.
For repeatability, the maximum standard deviation of repeated measurements was 0.13% (single PSD). Comparing the PSD to reference data, PDD had a maximum difference of 1.12% (10 cm depth, 6.64 × 6.64 cm). Percent differences for rotated detector setups were negligible (< 0.3%). All four PSDs demonstrated linear response over 10-1000 MU delivered and the maximum percent difference between the baseline and measured output factors was 0.78% (2.49 × 2.49 cm). Gating experiments had 400 cGy delivered to isocenter with < 0.8 cGy variation for central axis measures and < 0.7 cGy for the gradient sampled region. Real-time dosimetry measurements captured spurious beam-on incidents that correlated to tracking algorithm inaccuracies and highlighted gating parameter impact on delivery efficiency.
Characterization of the multi-point PSD dosimetry system in a 0.35 T MR-linac demonstrated reliability in a low-field MR-Linac setting, with high repeatability, linearity, small intersession variability, and similarity to baseline data for PDD and output factors. Time-resolved, multi-point dosimetry also showed considerable promise for gated MR-Linac applications.
磁共振引导放射治疗（MRgRT）需要独特的质量保证设备来满足磁共振兼容性需求，最大限度地减少电子返回效应，处理复杂的剂量分布，并评估实时剂量计用于门控。塑料闪烁探测器（PSD）是满足这些需求的一个有吸引力的选择。
在低场 0.35T 磁共振直线加速器中，对多探头 PSD 系统进行全面的特性描述，包括探测器响应评估和门控性能。
组装了一个四通道 PSD 系统（HYPERSCINT RP-200）。使用单个通道评估重复性、百分深度剂量（PDD）、探测器相对于磁场的方向的响应以及会话间的可变性。所有四个通道都用于评估重复性、线性度和输出因子。将四个 PSD 集成到位于等中心和梯度区域的磁共振兼容运动体模中。进行实验以评估门控性能和跟踪效果。
对于重复性，重复测量的最大标准偏差为 0.13%（单个 PSD）。将 PSD 与参考数据进行比较，PDD 的最大差异为 1.12%（10cm 深度，6.64×6.64cm）。旋转探测器设置的百分比差异可忽略不计（<0.3%）。所有四个 PSD 在 10-1000MU 输送过程中均表现出线性响应，基线和测量输出因子之间的最大百分比差异为 0.78%（2.49×2.49cm）。门控实验向等中心输送了 400cGy，中心轴测量的变化<0.8cGy，梯度采样区域的变化<0.7cGy。实时剂量测量捕捉到了与跟踪算法不准确相关的虚假束流开启事件，并强调了门控参数对输送效率的影响。
在 0.35T 磁共振直线加速器中对多点 PSD 剂量测量系统进行的特性描述表明，在低场磁共振直线加速器环境中具有可靠性，具有高重复性、线性度、会话间可变性小，与 PDD 和输出因子的基线数据相似。时间分辨、多点剂量测量也为门控磁共振直线加速器应用提供了很大的希望。
