Keall P J, Hoban P W
Medical Physics Department, Royal Adelaide Hospital, Australia.
Med Phys. 1996 Apr;23(4):479-85. doi: 10.1118/1.597679.
The superposition/convolution method and the transport of pregenerated Monte Carlo electron track data have been combined into the Super-Monte Carlo (SMC) method, an accurate 3-D x-ray dose calculation algorithm. The primary dose (dose due to electrons ejected by primary photons) is calculated by transporting pregenerated (in water) Monte Carlo electron tracks from each primary photon interaction site, weighted by the terma for that site. The length of each electron step is scaled by the inverse of the density of the medium at the beginning of the step. Because the density scaling of the electron tracks is performed for each individual transport step, the limitations of the macroscopic scaling of kernels (in the superposition algorithm) are overcome. This time-consuming step-by-step transport is only performed for the primary dose calculation, where current superposition methods are most lacking. The scattered dose (dose due to electrons set in motion by scattered photons) is calculated by superposition. In both a water-lung-water phantom and a two lung-block phantom, SMC dose distributions are more consistent with Monte Carlo generated dose distributions than are superposition dose distributions, especially for small fields and high energies-for an 18-MV, 5 X 5-cm(2) beam, the central axis dose discrepancy from Monte Carlo is reduced from 4.5% using superposition to 1.5% using SMC. The computation time for this technique is approximately 2 h (depending on the simulation history), 20 times slower than superposition, but 15 times faster than a full Monte Carlo simulation (on our platform).
叠加/卷积方法与预生成的蒙特卡罗电子轨迹数据传输相结合，形成了超级蒙特卡罗（SMC）方法，这是一种精确的三维X射线剂量计算算法。初始剂量（由初始光子射出的电子产生的剂量）通过从每个初始光子相互作用位点传输预生成的（在水中）蒙特卡罗电子轨迹来计算，并根据该位点的比释动能率进行加权。每个电子步长按该步开始时介质密度的倒数进行缩放。由于对每个单独的传输步长都进行了电子轨迹的密度缩放，因此克服了（叠加算法中）核宏观缩放的局限性。这种耗时的逐步传输仅用于初始剂量计算，而当前叠加方法在这方面最为欠缺。散射剂量（由散射光子激发的电子产生的剂量）通过叠加计算。在水-肺-水模体和双肺遮挡模体中，SMC剂量分布比叠加剂量分布更符合蒙特卡罗生成的剂量分布，特别是对于小射野和高能量——对于18兆伏、5×5平方厘米射野，与蒙特卡罗相比，叠加法计算的中心轴剂量差异为4.5%，而SMC法为1.5%。该技术的计算时间约为2小时（取决于模拟历史），比叠加法慢20倍，但比完整的蒙特卡罗模拟快15倍（在我们的平台上）。
