Daily monitoring of linear accelerator beam parameters using an amorphous silicon EPID

An amorphous silicon EPID has been investigated to test its suitability as a daily check device for linac output and to provide daily monitoring of beam profile parameters such as flatness, symmetry, field size and wedge factor. Open and wedged 6 and 8 MV photon beams were collected on a daily basis for a period of just over a year and analysed in software to determine daily values of these parameters. Daily output results gave agreement between EPID measured dose and ion chamber measurements with a standard deviation of 0.65%. Step changes in flatness, symmetry and field size were readily detected by the EPID and could be correlated with adjustments made on service days and QC sessions. The results could also be used to assess the long term beam stability. Recalibration of the EPID required new baseline values of the parameters to be set. Wedge factors measured at one collimator angle proved stable but sensitive to changes in beam steering. The EPID proved to be a useful daily check device for linac output which can simultaneously be used for daily monitoring of beam profiles and field sizes.