In this study, a quantitative estimate is derived for the uncertainty in the XCOM photon mass attenuation coefficients in the energy range of interest to external beam radiation therapy - i.e. 100keV (orthovoltage) to 25MeV - using direct comparisons of experimental data against Monte Carlo models and theoretical XCOM data. Two independent datasets are used. The first dataset is from our recent transmission measurements and the corresponding EGSnrc calculations (Ali et al 2012 Med. Phys. 39 5990-6003) for 10-30MV photon beams from the research linac at the National Research Council Canada. The attenuators are graphite and lead, with a total of 140 data points and an experimental uncertainty of∼0.5% (k=1). An optimum energy-independent cross section scaling factor that minimizes the discrepancies between measurements and calculations is used to deduce cross sectionuncertainty. The second dataset is from the aggregate of cross sectionmeasurements in the literature for graphite and lead (49 experiments, 288 data points). The dataset is compared to the sum of the XCOM data plus the IAEA photonuclear data. Again, an optimum energy-independent cross sectionscaling factor is used to deduce the cross sectionuncertainty. Using the average result from the two datasets, the energy-independent cross sectionuncertainty estimate is 0.5% (68% confidence) and 0.7% (95% confidence). The potential for energy-dependent errors is discussed. Photon cross sectionuncertainty is shown to be smaller than the current qualitative 'envelope of uncertainty' of the order of 1-2%, as given by Hubbell (1999 Phys. Med. Biol 44 R1-22).

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Keywords cross section measurements, EGSnrc, photon cross section uncertainty, transmission, XCOM
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Journal Physics in Medicine and Biology
Ali, E.S.M., Spencer, B., McEwen, M.R., & Rogers, D.W.O. (2015). Towards a quantitative, measurement-based estimate of the uncertainty in photon mass attenuation coefficients at radiation therapy energies. Physics in Medicine and Biology, 60(4), 1641–1654. doi:10.1088/0031-9155/60/4/1641