Luxton G
University of Southern California School of Medicine, Los Angeles 90033.
Med Phys. 1994 May;21(5):631-41. doi: 10.1118/1.597317.
A model based on the EGS4 Monte Carlo system is developed for calculating dose rate to water from an embedded low-energy brachytherapy source, given measurement data of dose rate to water within a water-substitute solid phantom for a source of given strength. The EGS4-based model is used to calculate point source dose rate distributions per unit source strength for water and for several species of solid phantoms. As a model for the dose rate distribution to an infinitesimal water inhomogeneity embedded in the solid phantom, the Monte Carlo is used to calculate dose rate to a thin spherical shell of water contained within the solid phantom at various distances centered on the source. Comparing this distribution to that calculated for a homogeneous water phantom, one can derive correction factors to determine dose rate to homogeneous water medium from measurements with water dose-calibrated detectors within the solid phantom. These factors are calculated for polymethylmethacrylate (PMMA or acrylic), solid water (WT1), and RW-1, a material optimized for low-energy dosimetry, with photon spectra from Pd-103 and from two commercial models of I-125 seed used as input. For model 6711 I-125 seeds at 1 cm in PMMA and WT1, the calculated ratios of dose rate to water to dose rate to water in the solid phantom are 0.893 and 1.038, respectively. Applying these factors to published, apparently divergent measurements of the specific dose rate constant based on measurements in PMMA and WT1 phantoms, the specific dose rate constant inferred for the model 6711 seed are found to be in excellent agreement: 0.878 cGy h-1U-1 (1.115 cGy- cm2h-1mCi-1) from the mean of the WT1 measurements and 0.879 cGy h-1U-1 from the PMMA measurements. Among the phantoms studied for Pd-103 and I-125, measurements in RW-1 would most closely approximate those for water.
基于EGS4蒙特卡罗系统开发了一个模型，用于根据给定强度源在水替代固体模体中对水的剂量率测量数据，计算嵌入式低能近距离放射治疗源对水的剂量率。基于EGS4的模型用于计算水和几种固体模体每单位源强度的点源剂量率分布。作为对嵌入固体模体中无限小水不均匀性的剂量率分布模型，蒙特卡罗方法用于计算在以源为中心的不同距离处，固体模体内包含的薄水球壳的剂量率。将此分布与均匀水体模计算得到的分布进行比较，可以得出校正因子，以根据固体模体内用水剂量校准探测器的测量结果确定均匀水介质的剂量率。针对聚甲基丙烯酸甲酯（PMMA或丙烯酸）、固体水（WT1）和RW-1（一种为低能剂量测定优化的材料）计算了这些因子，使用Pd-103以及两种商用型号I-125种子的光子能谱作为输入。对于在PMMA和WT1中1 cm处的6711型I-125种子，计算得到的水剂量率与固体模体中水剂量率的比值分别为0.893和1.038。将这些因子应用于基于在PMMA和WT1模体中测量的已发表的、明显不同的比剂量率常数测量结果，发现6711型种子推断的比剂量率常数非常一致：WT1测量平均值为0.878 cGy h-1U-1（1.115 cGy·cm2h-1mCi-1），PMMA测量为0.879 cGy h-1U-1。在针对Pd-103和I-125研究的模体中，RW-1中的测量结果最接近水的测量结果。
