Monte Carlo simulation of the iView MV portal imager dosimetry

Full text: Electronic Portal Imager Devices (EPID) are increasingly used in external beam radiation therapy to verify the geometric agreement between irradiation field and patient position in order to guarantee the complete irradiation of tumors, protecting as much as possible the neighboring tissues. In this work a simulation based on Monte Carlo (MC) algorithms has been developed and validated to obtain portal images. The MCNP5 code has been used to accurately tally the energy deposition at the flat-panel light phosphor layer by means of a new feature that this version displays, an array of point detectors for planar radiograph flux image (FIR tally). The study is focused in calibrating EPID dosimetry, which ensures that all image pixels ill respond correctly to a given irradiation. To that, real dose images acquired using the electronic portal imaging device (iViewGT) fitted on a linac Elekta Precise are compared to previous dose images simulated by Monte Carlo methods with identical irradiation model conditions. A detailed description of the EPID amorphous silicon flat-panel has been implemented in the MC model. EPID dosimetric characteristics have been analysed demonstrating, in the presence of an attenuation material placed in the beam (a solid water slab), the linear relation between deposited dose at the imager and the pixel gray-scale intensity of the resultant image. The simulation has been validated by comparing the EPID image pixels intensity for field sizes between 5 cm x 5 cm and 26 cm x 26 cm at the isocenter. All measurements were performed with a 6 MV linear accelerator photon beam. A dark-field irradiation and a flood-field one are necessaries to correct pixel values reported by simulation of the electronic portal images. The Monte Carlo model succeeded in predicting the EPID response at the center of the fields of various sizes to within 5% of the measurements