The energy-dependent electron loss model for pencil beam dosekernels

Abstract

The `monoenergetic' electron loss model was derived in a previous work toaccount for pathlength straggling in the Fermi-Eyges pencil beam problem.In this paper, we extend this model to account for energy-loss stragglingand secondary knock-on electron transport in order to adequately predict adepth dose curve. To model energy-loss straggling, we use a weightedsuperposition of a discrete number of monoenergetic pencil beams withdifferent initial energies where electrons travel along the depth-energycharacteristics in the continuous slowing down approximation (CSDA). Theenergy straggling spectrum at depth determines the weighting assigned toeach monoenergetic pencil beam. Supplemented by a simple transport modelfor the secondary knock-on electrons, the `energy-dependent' electron lossmodel predicts both lateral and depth dose distributions from the electronpencil beams in good agreement with Monte Carlo calculations andmeasurements. The calculation of dose distribution from a pencil beam takes0.2 s on a Pentium III 500 MHz computer. Being computationally fast, the`energy-dependent' electron loss model can be used for the calculation of3D energy deposition kernels in dose optimization schemes without usingprecalculated or measured data.