Cotterill John, Flynn Sam, Thomas Russell, Subiel Anna, Lee Nigel, Shipley David, Palmans Hugo, Lourenço Ana
Medical Radiation Science Group, National Physical Laboratory, Teddington TW11 0LW, UK.
Particle Physics Group, University of Birmingham, Edgbaston B15 2TT, UK.
Phys Imaging Radiat Oncol. 2023 Nov 8;28:100506. doi: 10.1016/j.phro.2023.100506. eCollection 2023 Oct.
Accurate dosimetry in Ultra-High Dose Rate (UHDR) beams is challenging because high levels of ion recombination occur within ionisation chambers used as reference dosimeters. A Small-body Portable Graphite Calorimeter (SPGC) exhibiting a dose-rate independent response was built to offer reduced uncertainty on secondary standard dosimetry in UHDR regimes. The aim of this study was to quantify the effect of the geometry and material properties of the device on the dose measurement.
A detailed model of the SPGC was built in the Monte Carlo code TOPAS (v3.6.1) to derive the impurity and gap correction factors, and . A dose conversion factor, , was also calculated using FLUKA (v2021.2.0). These factors convert the average dose to its graphite core to the dose-to-water for a 249.7 MeV mono-energetic spot-scanned clinical proton beam. The effect of the surrounding Styrofoam on the dose measurement was examined in the simulations by substituting it for graphite.
The and correction factors were 0.9993 ± 0.0002 and 1.0000 ± 0.0001, respectively when the Styrofoam was not substituted, and 1.0037 ± 0.0002 and 0.9999 ± 0.0001, respectively when substituted for graphite. The dose conversion factor was calculated to be 1.0806 ± 0.0001. All uncertainties are Type A.
Impurity and gap correction factors, and the dose conversion factor were calculated for the SPGC in a FLASH proton beam. Separating out the effect of scatter from Styrofoam insulation showed this as the dominating correction factor, amounting to 1.0043 ± 0.0002.
超高剂量率（UHDR）射束的精确剂量测定具有挑战性，因为用作参考剂量计的电离室内会发生高水平的离子复合。构建了一种具有与剂量率无关响应的小型便携式石墨量热计（SPGC），以降低UHDR模式下次级标准剂量测定的不确定性。本研究的目的是量化该装置的几何形状和材料特性对剂量测量的影响。
在蒙特卡罗代码TOPAS（v3.6.1）中构建了SPGC的详细模型，以推导杂质和间隙校正因子， 以及 。还使用FLUKA（v2021.2.0）计算了剂量转换因子， 。这些因子将其石墨芯的平均剂量转换为249.7 MeV单能点扫描临床质子束的水吸收剂量。在模拟中，通过用石墨替代聚苯乙烯泡沫塑料来研究其对剂量测量的影响。
在未替代聚苯乙烯泡沫塑料时， 和 校正因子分别为0.9993±0.0002和1.0000±0.0001，在用石墨替代时，分别为1.0037±0.0002和0.9999±0.0001。计算出的剂量转换因子为1.0806±0.0001。所有不确定度均为A类。
计算了FLASH质子束中SPGC的杂质和间隙校正因子以及剂量转换因子。分离出聚苯乙烯泡沫塑料绝缘散射的影响表明这是主要的校正因子，总计为1.0043±0.0002。
