Monte Carlo calculation of output factors for circular, rectangular, and square fields of electron accelerators (6-20 MeV)

Monte Carlo (MC) techniques can be used to build a simulation model of an e lectron accelerator to calculate output factors for electron fields. This c an be useful during commissioning of electron beams from a linac and in cli nical practice where irregular fields are also encountered. The Monte Carlo code BEAM/EGS4 was used to model electron beams (6-30 MeV) from a Varian 2 100C linear accelerator. After optimization of the Monte Carlo simulation m odel, agreement within 1% to 2% was obtained between calculated and measure d (with a Si diode) lateral and depth dose distributions or within 1 mm in the penumbral regions. Output factors for square, rectangular, and circular fields were measured using two different plane-parallel ion chambers (Mark us and NACP) and compared to MC simulations. The agreement was usually with in 1% to 2%. This study was not primarily concerned with minimizing the sim ulation time required to obtain output factors but some considerations with respect to this are presented. It would be particularly useful if the MC m odel could also be used to calculate output factors for other, similar lina cs, To see if this was possible, the primary electron energies in the MC mo del were retuned to model a recently commissioned similar linac. Good agree ment between calculated and measured output factors was obtained for most f ield sizes for this second accelerator. (C) 2001 American Association of Ph ysicists in Medicine.