COMPARISON OF IAEA-1987 AND AAPM-1983 PROTOCOLS FOR DOSIMETRY CALIBRATION OF RADIOTHERAPY BEAMS

The IAEA 1987 protocol is an international protocol which has made a number of improvements over the AAPM 1983 protocol for calibration of high-energy photon and electron beams. We present a detailed numerical comparison between the two protocols by calculating (i) N(gas) and N(D) for PTW (PMMA wall), Capintec (air-equivalent plastic wall) and NEL (graphite wall) Farmer type ionization chambers for Co-60 gamma rays; (ii) dose-to-water with chamber in water irradiated by 4- or 25-MV x rays; (iii) dose-to-water with chamber in water, PMMA, and polystyrene phantoms irradiated by 5- and 10-MeV electrons; and (iv) dose-to-water with chamber in water irradiated by 20-MeV electrons. For photons, the IAEA protocol gives results which are in good agreement with the AAPM protocol; on average the IAEA results are 0.6% smaller than the AAPM results while discrepancies between the two are in the range of - 0.4% to - 1.2%. For 10-MeV electrons also, the IAEA protocol gives results which are in excellent agreement with the AAPM protocol; on average the IAEA results are 0.3% smaller than the AAPM results while discrepancies between the two are in the range of - 1.0% to + 0.5%. In contrast to the above, for 5-MeV electrons, the IAEA protocols give results smaller than the AAPM protocol by 2.0% on average with discrepancies between protocols ranging from - 4.1% to - 0.7% depending upon the ionization chamber and phantom material used. For 5-MeV electrons, the discrepancies are particularly large for polystyrene phantom; the average discrepancies being - 1.4%, - 1.1%, and - 3.6% for water, PMMA, and polystyrene, respectively. If data for 5-MeV electrons with polystyrene phantom are excluded, then the overall agreement between the two protocols for photons and electrons is within the range of - 1.9% to + 0.5%. Principal reasons for the observed discrepancies are (i) IAEA uses the correct expression for N(D) resulting in up to + 0.8% correction; (ii) IAEA uses the most recent stopping power ratio for graphite-to-air resulting in up to + 0.5% correction; (iii) IAEA uses a correction of up to + 0.8% for the central electrode which AAPM ignores; (iv) the present estimates of the percent depth doses which arise from the differences in measurement depths in the two protocols; and (v) IAEA uses measured values of the fluence correction factor while AAPM uses a theoretical estimate resulting in corrections of up to - 2.2%.