Williamson J F
Radiation Oncology Center, Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, MO 63110, USA.
Int J Radiat Oncol Biol Phys. 1996 Dec 1;36(5):1239-50. doi: 10.1016/s0360-3016(96)00417-8.
The goal of this study is to assess the accuracy of the Sievert integral dose calculation model for medium and low-energy brachytherapy sources (photon energy: 25-500 keV). A simple modification of the basic model, the isotropic scattering correction, is proposed that significantly improves its accuracy.
Both the classical model and revised Sievert algorithms were tested against 2D dose distributions derived from Monte Carlo photon transport (MCPT) calculations for the following sources: a 169Yb interstitial source, pulsed and high dose rate 192Ir sources and the model 6702 125I source. The Sievert model was implemented as a 3D numerical integral over the radioactivity distribution and included photon attenuation and scattering by the surrounding medium. The Sievert filtration coefficients were approximated by linear energy absorption coefficients, parameters of best fit, and curve fits to simulated open-air transmission measurements. The revised model consists of using the Sievert integral only to calculate the primary dose distribution using contact absorber filtration coefficients. The dose component due to photon scattering in the medium is assumed to be isotropically distributed and is modeled by point-source scatter-to-primary dose ratios.
The classical Sievert integral produces maximum and RMS average dose-calculation errors ranging from -53 to -20% and 3 to 19%, respectively. In contrast, the revised model reproduces the MCPT dose distribution with maximum and RMS mean errors ranging from 5 to 13% and 1 to 6%, respectively.
The classical Sievert model fails to accurately describe brachytherapy dose distributions around heavily filtered sources emitting photons with average energies of 28 to 400 keV. The revised Sievert model accurately models single-source dose distributions for a wide range of sources, using well-defined filtration coefficients and scatter ratios that can be measured or calculated without knowledge of the final dose distribution. The model is potentially useful as a single-source dose-array generator for clinical treatment planning in the low energy domain.
本研究的目的是评估用于中低能近距离放射治疗源（光子能量：25 - 500 keV）的西弗积分剂量计算模型的准确性。提出了对基本模型的一种简单修正，即各向同性散射校正，这显著提高了其准确性。
经典模型和修正后的西弗算法均针对以下源通过蒙特卡罗光子输运（MCPT）计算得出的二维剂量分布进行了测试：一个169Yb组织间源、脉冲和高剂量率192Ir源以及6702型125I源。西弗模型被实现为对放射性分布的三维数值积分，并包括光子在周围介质中的衰减和散射。西弗过滤系数通过线能量吸收系数、最佳拟合参数以及对模拟露天传输测量的曲线拟合来近似。修正后的模型包括仅使用西弗积分，利用接触吸收体过滤系数来计算初始剂量分布。假设介质中光子散射引起的剂量分量是各向同性分布的，并通过点源散射与初始剂量比进行建模。
经典西弗积分产生的最大剂量计算误差和均方根平均剂量计算误差分别在 - 53%至 - 20%和3%至19%之间。相比之下，修正后的模型再现MCPT剂量分布时，最大误差和均方根平均误差分别在5%至13%和1%至6%之间。
经典西弗模型无法准确描述平均能量为28至400 keV的强过滤源周围的近距离放射治疗剂量分布。修正后的西弗模型使用明确定义的过滤系数和散射比，能够准确模拟广泛范围内源的单源剂量分布，这些系数和散射比可以在不知道最终剂量分布的情况下进行测量或计算。该模型作为低能量领域临床治疗计划的单源剂量阵列生成器可能具有实用价值。
