Monte Carlo simulation of electron beams from an accelerator head using PENELOPE

The Monte Carlo code PENELOPE has been used to simulate electron beams from a Siemens Mevatron KDS linac with nominal energies of 6, 12 and 18 MeV, Ow ing to its accuracy, which stems from that of the underlying physical inter action models, PENELOPE is suitable for simulating problems of interest to the medical physics community. It includes a geometry package that allows t he definition of complex quadric geometries, such as those of irradiation i nstruments, in a straightforward manner. Dose distributions in water simula ted with PENELOPE agree well with experimental measurements using a silicon detector and a monitoring ionization chamber. Insertion of a lead slab in the incident beam at the surface of the water phantom produces sharp variat ions in the dose distributions, which are correctly reproduced by the simul ation code. Results from PENELOPE are also compared with those of equivalen t simulations with the EGS4-based user codes BEAM and DOSXYZ. Angular and e nergy distributions of electrons and photons in the phase-space plane (at t he downstream end of the applicator) obtained from both simulation codes ar e similar, although significant differences do appear in some cases. These differences, however, are shown to have a negligible effect on the calculat ed dose distributions. Various practical aspects of the simulations, such a s the calculation of statistical uncertainties and the effect of the 'laten t' variance in the phase-space file, are discussed in detail.