ACCURATE CHARACTERIZATION OF MONTE-CARLO CALCULATED ELECTRON-BEAMS FOR RADIOTHERAPY

Monte Carlo studies of dose distributions in patients treated with rad iotherapy electron beams would benefit from generalized models of clin ical beams if such models introduce little error into the dose calcula tions. Methodology is presented for the design of beam models, includi ng their evaluation in terms of how well they preserve the character o f the clinical beam, and the effect of the beam models on the accuracy of dose distributions calculated with Monte Carlo. This methodology h as been used to design beam models for electron beams from two linear accelerators, with either a scanned beam or a scattered beam. Monte Ca rlo simulations of the accelerator heads are done in which a record is kept of the particle phase-space, including the charge, energy, direc tion, and position of every particle that emerges from the treatment h ead, along with a tag regarding the details of the particle history. T he character of the simulated beams are studied in detail and used to design various beam models from a simple point source to a sophisticat ed multiple-source model which treats particles from different parts o f a linear accelerator as from different sub-sources. Dose distributio ns calculated using both the phase-space data and the multiple-source model agree within 2%, demonstrating that the model is adequate for th e purpose of Monte Carlo treatment planning for the beams studied. Ben efits of the beam models over phase-space data for dose calculation ar e shown to include shorter computation time in the treatment head simu lation and a smaller disk space requirement, both of which impact on t he clinical utility of Monte Carlo treatment planning. (C) 1997 Americ an Association of Physicists in Medicine.