The relationship between the pixel value and exit dose was investigated for a new commercially available amorphous silicon electronic portal imaging device. The pixel to dose mapping function was established to be linear for detector distances between 116.5 cm to 150 cm from the source, radiation field sizes from 5 x 5 cm2 to 20 x 20 cm2 and beam energies of 6 to 18 MV. Coefficients in the mapping function were found to be dependent on beam energy and field size. Open and wedged field profiles measured with the device showed agreement to a maximum of 5% and 8%, respectively, as compared to film. A comparison of relative transmission measurements between the EPID and ion chamber indicate a maximum deviation of 6% and 2% at 6 and 18 MV, respectively, for an attenuator thickness of 21 cm and SDD > or = 130 cm. It was found that accuracies of better than 1% could be obtained if detector position and field size specific fitting parameters were used to generate unique mapping functions for each configuration.

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An analytical approximation for the scatter to primary dose ratio (SPR) on the central axis was validated against Monte Carlo results and experimental measurements for homogeneous and inhomogeneous phantoms. The analytical approximation only included first-order Compton scatter. The contribution to the total SPR from first-order Compton scatter, multiply scattered photons and electron scatter was investigated using Monte Carlo simulation for homogeneous phantoms (up to 30 cm thick for 6 and 18 MV beams; source to detector distances from 150 to 230 cm) as well as for a neck, thorax and pelvis phantom. SPRs were measured on the central axis with an ionization chamber for water phantoms (up to 20 cm thick at 4 MV, 30 cm for 6 MV and 10 MV and 40 cm for 18 MV; source to detector distances of 185 and 200 cm) and for phantoms representing the neck, thorax and pelvis (for air gaps of 50 cm and larger). The mean difference between the experimental and analytical SPRs on the central axis for source to detector distances of 170 cm or greater was within: -0.003 (neck); -0.012 (thorax); -0.028 (pelvis, 10 MV) and 0.008 (pelvis, 18 MV) respectively.