2010
Monte Carlo calculations of k Q, the beam quality conversion factor
Publication
Publication
Medical Physics , Volume 37  Issue 11 p. 5939 5950
Purpose: To use EGSnrc Monte Carlo simulations to directly calculate beam quality conversion factors, k Q, for 32 cylindrical ionization chambers over a range of beam qualities and to quantify the effect of systematic uncertainties on Monte Carlo calculations of k Q. These factors are required to use the TG51 or TRS398 clinical dosimetry protocols for calibrating external radiotherapy beams. Methods: Ionization chambers are modeled either from blueprints or manufacturers' user's manuals. The dosetoair in the chamber is calculated using the EGSnrc usercode egschamber using 11 different tabulated clinical photon spectra for the incident beams. The dose to a small volume of water is also calculated in the absence of the chamber at the midpoint of the chamber on its central axis. Using a simple equation, k Q is calculated from these quantities under the assumption that W/e is constant with energy and compared to TG51 protocol and measured values. Results: Polynomial fits to the Monte Carlo calculated k Q factors as a function of beam quality expressed as % dd (10) x and TPR 10 20 are given for each ionization chamber. Differences are explained between Monte Carlo calculated values and values from the TG51 protocol or calculated using the computer program used for TG51 calculations. Systematic uncertainties in calculated k Q values are analyzed and amount to a maximum of one standard deviation uncertainty of 0.99% if one assumes that photon crosssection uncertainties are uncorrelated and 0.63% if they are assumed correlated. The largest components of the uncertainty are the constancy of W/e and the uncertainty in the crosssection for photons in water. Conclusions: It is now possible to calculate k Q directly using Monte Carlo simulations. Monte Carlo calculations for most ionization chambers give results which are comparable to TG51 values. Discrepancies can be explained using individual Monte Carlo calculations of various correction factors which are more accurate than previously used values. For small ionization chambers with central electrodes composed of highZ materials, the effect of the central electrode is much larger than that for the aluminum electrodes in Farmer chambers.
Additional Metadata  

beam quality conversion factor, dosimetry protocols, EGSnrc, k Q, Monte Carlo, photon beams  
dx.doi.org/10.1118/1.3495537  
Medical Physics  
Organisation  Department of Physics 
Muir, B.R., & Rogers, D.W.O. (2010). Monte Carlo calculations of k Q, the beam quality conversion factor. Medical Physics, 37(11), 5939–5950. doi:10.1118/1.3495537
