Dosimetric verification of a Monte Carlo electron beam model for an add-on eMLC.

Dosimetric verification of a new Monte Carlo beam model for multi-leaf collimated electrons was performed using experimental data from an add-on electron multi-leaf collimator (eMLC) prototype. The measurements were compared against calculations using an electron phase space sampled from a parameterized electron beam model and the voxel Monte Carlo++ (VMC++) code for in-phantom energy deposition. Verification of the calculations was performed in a water phantom with the developed eMLC attached to a Varian 2100 C/D radiotherapy accelerator with nominal energies 6 MeV, 9 MeV, 12 MeV, 16 MeV and 20 MeV. The eMLC prototype consisting of 2 cm thick and 5 mm wide steel leaves is fixed under the 20 x 20 cm(2) electron applicator with a source-to-leaf distance 97.2 cm. The eMLC prototype has non-motorized leaves with straight leaf edges and a maximum field size of 20 x 20 cm(2) at SSD 100 cm. The beam model is a coupled multi-source model with parameters derived from detailed beam characterization measurements and a kernel model for the indirect leaf-scattered electrons. Typical calculation times with a 2% mean statistical uncertainty was under 5 min. In extensive set of in-water measurements 88% of the voxels were within 2% /2 mm acceptance criterion. Although at SSD 100 cm the dose near the phantom surface is slightly pronounced due to the short collimator-to-surface distance, the new beam model was suitable for dose calculation of the add-on type eMLC.