MCNP5 Monte Carlo simulation of amorphous silicon EPID dosimetry from MLC radiation therapy treatment beams

The present work is focused on a MCNP Monte Carlo (MC) simulation of a multi-leaf collimator (MLC) radiation therapy treatment unit including its corresponding Electronic Portal Imaging Device (EPID). We have developed a methodology to perform a spatial calibration of the EPID signal to obtain dose distribution using MC simulations. This calibration is based on several images acquisition and simulation considering different thicknesses of solid water slabs, using a 6 MeV photon beam and a square field size of 20 cm x 20 cm. The resulting relationship between the EPID response and the MC simulated dose is markedly linear. This signal to dose EPID calibration was used as a dosimetric tool to perform the validation of the MLC linear accelerator MCNP model. Simulation results and measurements agreed within 2% of dose difference. The methodology described in this paper potentially offers an optimal verification of dose received by patients under complex multi-field conformal or intensity-modulated radiation therapy (IMRT).