A new approach for the pixel map sensitivity (PMS) evaluation of an electronic portal imaging device (EPID)

When using an electronic portal imaging device (EPID) for dosimetric verifications, the calibration of the sensitive area is of paramount importance. Two calibration methods are generally adopted: one, empirical, based on an external reference dosimeter or on multiple narrow beam irradiations, and one based on the EPID response simulation. In this paper we present an alternative approach based on an intercalibration procedure, independent from external dosimeters and from simulations, and is quick and easy to perform. Each element of a detector matrix is characterized by a different gain; the aim of the calibration procedure is to relate the gain of each element to a reference one. The method that we used to compute the relative gains is based on recursive acquisitions with the EPID placed in different positions, assuming a constant fluence of the beam for subsequent deliveries. By applying an established procedure and analysis algorithm, the EPID calibration was repeated in several working conditions. Data show that both the photons energy and the presence of a medium between the source and the detector affect the calibration coefficients less than 1%. The calibration coefficients were then applied to the acquired images, comparing the EPID dose images with films. Measurements were performed with open field, placing the film at the level of the EPID. The standard deviation of the distribution of the point-to-point difference is 0.6%. An approach of this type for the EPID calibration has many advantages with respect to the standard methods - it does not need an external dosimeter, it is not related to the irradiation techniques, and it is easy to implement in the clinical practice. Moreover, it can be applied in case of transit or nontransit dosimetry, solving the problem of the EPID calibration independently from the dose reconstruction method.