Electronic Portal Imaging Systems (EPIDs) are used in Radiotherapy treatment as part of the patient positioning verification check and for portal dosimetry purposes. The quality control of the imaging performance of an EPID is performed with dedicated phantoms. In this work, an examination through Monte Carlo (MC) simulation is presented in order to determine an appropriate step wedge phantom configuration for measuring low contrast differences in EPIDs. The PENELOPE based MC software package PenEasy was used. A simple geometry of a narrow cone beam with a cross section of 0.00053 cm² at 100 cm distance was assumed. A 2 MeV beam was considered to impinge on a 4 cm water equivalent phantom in conjunction with a metal sheet of Pb, Al, Fe or W positioned at 80 cm distance. At 100 cm distance a Gd2O2S:Tb scintillator, as part of an EPID responsible for detecting X-rays was assumed. The Gd2O2S:Tb thicknesses considered were 0.02cm and 0.03 cm. All the metal thicknesses were allowed to range from 0.1 cm to 1.5 cm per 0.1 cm step. The optical photons escaping to the Gd2O2S:Tb output were calculated by an analytical formula for each metal thickness. Hence, if a wedge metallic pattern from 0.1 cm to 1.5 cm is assumed to be constructed, then the optical photon output originating from each step, as well as the signal contrast between two steps would be known. It was found that a combination of Pb, Fe and W materials can be used for a step wedge phantom design.

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