[Monte Carlo simulation of a dynamic multileaf collimator:implementation and applications]

A model for the simulation of the accelerator heads of two identical linear accelerators was designed at the University Hospital of Tübingen, using the BEAM program developed at the National Research Council of Canada. Both linear accelerators are equipped with multileaf collimators (MLCs) and backup jaws (y-direction) with curved leaf-ends. The accelerator models were divided into two parts. The first part consisted of target, primary collimator, flattening filter, monitor chamber, and mirror. After the Monte Carlo simulation of these parts, the phase-space characteristics below the mirror were stored in a file and used as source for the second part of the accelerator head (jaw, MLC). The electron source was assumed to deliver a gaussian energy spectrum, with parallel direction to the beam axis. With this electron source, there was good agreement between the measured and simulated depth dose curves in water, with difference < 2%. A new module was created for the BEAM program to simulate backup jaws, while the standard MLCQ module from BEAM was used to simulate a MLC with curved leaf-ends. As a result, MLCs and backup jaws with curved leaf-ends make the shoulder of the y-profile higher than the straight-end MLCs.