Monte Carlo simulation of a clinical linear accelerator

The effects of the physical parameters of an electron beam from a Siemens P RIMUS clinical linear accelerator (linac) on the dose distribution in water were investigated by Monte Carlo simulation. The EGS4 user code, OMEGA/BEA M, was used in this study. Various incident electron beams, for example. wi th different energies, spot sizes and distances from the point source, were simulated using the detailed linac head structure in the 6 MV photon mode. Approximately 10 million particles were collected in the scored plane, whi ch was set under the reticle to form the so-called phase space file. The ph ase space file served as a source for simulating the dose distribution in w ater using DOSXYZ. Dose profiles at D-max (1.5 cm) and PDD curves were calc ulated following simulating about I billion histories for dose profiles and 500 million histories for percent depth dose (PDD) curves in a 30 x 30 x 3 0 cm(3) water phantom. The simulation results were compared with the data m easured by a CEA film and an ion chamber. The results show that the dose pr ofiles are influenced by the energy and the spot size, while PDD curves are primarily influenced by the energy of the incident beam. The effect of the distance from the point source on the dose profile is not significant and is recommended to be set at infinity. We also recommend adjusting the beam energy by using PDD curves and, then, adjusting the spot size by using the dose profile to maintain the consistency of the Monte Carlo results and mea sured data. (C) 2001 Elsevier Science Ltd. All rights reserved.