The use of an electronic portal imaging system to measure portal dose and portal dose profiles

The dosimetric characteristics of a scanning liquid-filled ionization chamber (SLIC) electronic portal imaging device have been investigated. To assess the system's response in relation to incident radiation beam intensity, a series of characteristic curves are obtained for various field sizes and nominal energies of 6 and 10 MV photons. The response of the imaging system is dependent on incident radiation intensity and can be described to within 1% accuracy on central axis using a square root function. Portal dose measurements with the SLIC at the plane of the detector, on central axis of the beam using homogeneous attenuating phantom materials show that the imaging system is capable of measuring the portal (transmission) dose to within 3% of the ionization chamber results for homogeneous material. For two-dimensional dosimetry applications, the system is calibrated with a 10 cm Perspex block used as beam flattening material on the detector cassette to correct for variations in individual ion chamber sensitivity and the effect of nonuniform beam profiles produced by the flattening filter. Open and wedged dose profiles measured with the SLIC agreed with ion chamber measured profiles to within 3.5% accuracy.