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Comparison of IAEA 1987 and AAPM 1983 protocols for dosimetry calibration of radiotherapy beams
Author(s) -
Huq M. Saiful,
Nath Ravinder
Publication year - 1991
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.596720
Subject(s) - ionization chamber , dosimetry , electron , imaging phantom , calibration , nuclear physics , range (aeronautics) , photon , materials science , physics , ionization , nuclear medicine , optics , medicine , ion , quantum mechanics , composite material
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 60 Co γ 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%.