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Novel approach for the Monte Carlo calculation of free-air chamber correction factors
Tijdschriftbijdrage - Tijdschriftartikel
A self-consistent approach for the Monte Carlo calculation of free-air chamber (FAC) correction factors needed to convert the chamber reading into the quantity air-kerma at the point of measurement is introduced, and its implementation in the new EGSnrc user code egs_fac is discussed. To validate the method, comparisons between computed and measured FAC correction factors for attenuation A(x), scatter (A(scat)), and electron loss (A(eloss)) are performed in the medium energy range where the experimental determination is believed to be accurate. The Monte Carlo calculations utilize a full simulation of the x-ray tube with BEAMnrc and a detailed model of the parallel-plate FAC. Excellent agreement between the computed A(scat) and A(eloss) and the measured values for these correction factors currently used in the National Research Council (NRC) of Canada primary FAC standard is observed. Our simulations also agree with previous Monte Carlo results for A(scat) and A(eloss) for the 135 and 250 kVp Consultative Committee for Ionizing Radiation reference beam qualities. The computed attenuation correction agrees with the measured A(att) within the stated uncertainties, although the authors' simulations demonstrate that the evacuated-tube technique employed at NRC to measure the attenuation correction slightly overestimates A(att) in the medium energy range. The newly introduced corrections for backscatter, beam geometry, and lack of charged particle equilibrium along the beam axis are found to be negligible. On the other hand, the correction for photons leaking through the FAC aperture, currently ignored in the NRC standard, is shown to be significant.
Tijdschrift: MEDICAL PHYSICS
Pagina's: 3650 - 3660
Jaar van publicatie:2008