Center for Proton Therapy, Paul Scherrer Institute , Villigen , Switzerland.
Department of Radiation Therapy, Rijksuniversiteit Groningen , Groningen , the Netherlands.
Acta Oncol. 2019 Oct;58(10):1463-1469. doi: 10.1080/0284186X.2019.1627416. Epub 2019 Jun 26.
This study aimed at evaluating spatially varying instantaneous dose rates for different intensity-modulated proton therapy (IMPT) planning strategies and delivery scenarios, and comparing these with FLASH dose rates (>40 Gy/s). In order to quantify dose rates in three-dimensions, we proposed the 'dose-averaged dose rate' (DADR) metric, defined for each voxel as the dose-weighted mean of the instantaneous dose rates of all spots (i.e., pencil beams). This concept was applied to four head-and-neck cases, each planned with clinical (4 fields) and various spot-reduced IMPT techniques: 'standard' (4 fields), 'arc' (120 fields) and 'arc-shoot-through' (120 fields; 229 MeV only). For all plans, different delivery scenarios were simulated: constant beam intensity, variable beam intensity for a clinical Varian ProBeam system, varied per energy layer or per spot, and theoretical spot-wise variable beam intensity (i.e., no monitor/safety limitations). DADR distributions were calculated assuming 2-Gy or 6-Gy fractions. Spot-reduced plans contained 17-52 times fewer spots than clinical plans, with no deterioration of plan quality. For the clinical plans, the mean DADR in normal tissue for 2-Gy fractionation was 1.7 Gy/s (median over all patients) at maximum, whereas in standard spot-reduced plans it was 0.7, 4.4, 7.1, and 12.1 Gy/s, for the constant, energy-layer-wise, spot-wise, and theoretical spot-wise delivery scenarios, respectively. Similar values were observed for arc plans. Arc-shoot-through planning resulted in DADR values of 3.0, 6.0, 14.1, and 24.4 Gy/s, for the abovementioned scenarios. Hypofractionation (3×) generally resulted in higher dose rates, up to 73.2 Gy/s for arc-shoot-through plans. The DADR was inhomogeneously distributed with highest values at beam entrance and at the Bragg peak. FLASH dose rates were not achieved for conventional planning and clinical spot-scanning machines. As such, increased spot-wise beam intensities, spot-reduced planning, hypofractionation and arc-shoot-through plans were required to achieve FLASH compatible dose rates.
本研究旨在评估不同强度调制质子治疗（IMPT）计划策略和递送方案的空间变化瞬时剂量率，并将其与 FLASH 剂量率（>40Gy/s）进行比较。为了在三维空间中量化剂量率，我们提出了“剂量平均剂量率”（DADR）度量，该度量针对每个体素定义为所有点（即铅笔束）的瞬时剂量率的剂量加权平均值。该概念应用于四个头颈部病例，每个病例均采用临床（4 野）和各种点减少的 IMPT 技术进行计划：“标准”（4 野）、“弧形”（120 野）和“弧形贯穿”（120 野；仅 229MeV）。对于所有计划，模拟了不同的递送方案：恒定束强度、临床瓦里安 ProBeam 系统的可变束强度、按能量层或按点变化，以及理论上的点状可变束强度（即无监测/安全限制）。假设 2Gy 或 6Gy 分次剂量，计算 DADR 分布。点减少的计划比临床计划包含少 17-52 倍的点，但计划质量没有恶化。对于临床计划，2Gy 分次的正常组织的平均 DADR 在最大值为 1.7Gy/s（所有患者的中位数），而在标准点减少的计划中，在恒定、能量层、点和理论点的递送方案中，其值分别为 0.7、4.4、7.1 和 12.1Gy/s。弧形计划也观察到类似的值。弧形贯穿计划导致的 DADR 值分别为 3.0、6.0、14.1 和 24.4Gy/s，适用于上述情况。低分次（3×）通常会导致更高的剂量率，对于弧形贯穿计划，最高可达 73.2Gy/s。DADR 呈不均匀分布，在束入口和布拉格峰处具有最高值。对于常规计划和临床点扫描机，未达到 FLASH 剂量率。因此，需要增加点状束强度、点减少计划、低分次和弧形贯穿计划，以达到与 FLASH 兼容的剂量率。
