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Calculations for plane‐parallel ion chambers in 60 Co beams using the EGSnrc Monte Carlo code
Author(s) -
MainegraHing Ernesto,
Kawrakow Iwan,
Rogers D. W. O.
Publication year - 2003
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 - 10.1118/1.1536291
Subject(s) - monte carlo method , physics , imaging phantom , computational physics , ionization , electron , photon , ionization chamber , plane (geometry) , cross section (physics) , dosimetry , atomic physics , ion , nuclear physics , optics , nuclear medicine , mathematics , statistics , geometry , quantum mechanics , medicine
The EGSnrc Monte Carlo simulation system is used to obtain, for 10 plane‐parallel ionization chambers in60 Co beams, the correction factors K compand P wallthat account for the nonequivalence of the chamber wall material to the buildup cap and the phantom material, respectively. A more robust calculation method has been used compared to that used in previous works. A minor conceptual error related to the axial nonuniformity correction factor, K an , has been identified and shown to have an effect of about 0.2%. The assumption that P wallin‐phantom is numerically equal to K compcalculated for a water buildup cap is shown to be accurate to better than 0.06%, thereby justifying the use of K compcalculations which are much more efficient. The effect on the calculated dose to the air in the cavity of the particle production threshold and transport energies used in the simulations is studied. Uncertainties in the calculated correction factors due to uncertainties in the photon and electron cross‐section data are studied. They are 0.14% and 0.24%, respectively (1 standard deviation), for K compfactors. The uncertainties on K wallfactors are 0.03% from photon cross‐section uncertainties and negligible from electron cross‐section uncertainties. A comparison with previous EGS4/PRESTA calculations shows that present results are systematically higher by an average of 0.8%, ranging from 0.4% up to 1.4%. The present results are in better agreement with reported experimental values.