THE CALIBRATION AND USE OF PLANE-PARALLEL IONIZATION CHAMBERS FOR DOSIMETRY OF ELECTRON-BEAMS

The AAPM TG 39 protocol has proposed three different methods of calibr ating plane-parallel ionization chambers, i.e., in-phantom irradiation with a high-energy electron beam and in-phantom and in-air Co-60 irra diation. To verify the consistency of the three methods, we have measu red N-gas(pp) values using each of these techniques for the five most commonly used plane-parallel chambers considered by the protocol. Our results demonstrate that the measured N-gas(pp) values for the three d ifferent methods for any of the chambers agree to within +/-0.6%. Once N-gas(pp) was measured, the determination of absorbed dose for electr on beams with different energies for an AECL Therac 20 and Philips SL2 5 was carried out according to the AAPM TG 39 protocol. The results sh ow that the determinations of the absorbed dose outputs for any of the five chambers agree to within +/-0.7% for electron-beam energies of 4 -20 MeV if all five chambers had N-gas(pp) values determined by the el ectron-beam method. The uncertainties are well. within the expected er ror for these approaches.