Wang L, Jette D
Department of Radiation Oncology, University of Pennsylvania, Philadelphia 19104, USA.
Med Phys. 1999 Aug;26(8):1454-65. doi: 10.1118/1.598677.
The transport of the secondary electrons resulting from high-energy photon interactions is essential to energy redistribution and deposition. In order to develop an accurate dose-calculation algorithm for high-energy photons, which can predict the dose distribution in inhomogeneous media and at the beam edges, we have investigated the feasibility of applying electron transport theory [Jette, Med. Phys. 15, 123 (1988)] to photon dose calculation. In particular, the transport of and energy deposition by Compton electron and electrons and positrons resulting from pair production were studied. The primary photons are treated as the source of the secondary electrons and positrons, which are transported through the irradiated medium using Gaussian multiple-scattering theory [Jette, Med. Phys. 15, 123 (1988)]. The initial angular and kinetic energy distribution(s) of the secondary electrons (and positrons) emanating from the photon interactions are incorporated into the transport. Due to different mechanisms of creation and cross-section functions, the transport of and the energy deposition by the electrons released in these two processes are studied and modeled separately based on first principles. In this article, we focus on determining the dose distribution for an individual interaction site. We define the Compton dose deposition kernel (CDK) or the pair-production dose deposition kernel (PDK) as the dose distribution relative to the point of interaction, per unit interaction density, for a monoenergetic photon beam in an infinite homogeneous medium of unit density. The validity of this analytic modeling of dose deposition was evaluated through EGS4 Monte Carlo simulation. Quantitative agreement between these two calculations of the dose distribution and the average energy deposited per interaction was achieved. Our results demonstrate the applicability of the electron dose-calculation method to photon dose calculation.
高能光子相互作用产生的二次电子的输运对于能量重新分布和沉积至关重要。为了开发一种准确的高能光子剂量计算算法，该算法能够预测非均匀介质中以及射束边缘处的剂量分布，我们研究了将电子输运理论[杰特，《医学物理》15，123（1988）]应用于光子剂量计算的可行性。特别地，研究了康普顿电子以及由电子对产生的电子和正电子的输运和能量沉积。初级光子被视为二次电子和正电子的源，使用高斯多重散射理论[杰特，《医学物理》15，123（1988）]将它们输运通过被照射介质。光子相互作用产生的二次电子（和正电子）的初始角分布和动能分布被纳入输运过程。由于产生机制和截面函数不同，基于第一原理分别研究并模拟了这两个过程中释放的电子的输运和能量沉积。在本文中，我们专注于确定单个相互作用位点的剂量分布。我们将康普顿剂量沉积核（CDK）或电子对产生剂量沉积核（PDK）定义为在单位密度的无限均匀介质中，对于单能光子束，相对于相互作用点的剂量分布，每单位相互作用密度。通过EGS4蒙特卡罗模拟评估了这种剂量沉积解析模型的有效性。在剂量分布的这两种计算以及每次相互作用沉积的平均能量之间实现了定量一致性。我们的结果证明了电子剂量计算方法在光子剂量计算中的适用性。
