In clinical photon beams, independent determination of the photon spectra and the incident electron energy is useful for beam (re)commissioning and for detector response modelling. In this study, an approach is developed for that purpose, and validated on a research linac whose photon spectra and electron beams are directly and independently known. In this approach, an optimized combination of transmission curves is measured using multiple attenuators and detectors to maximize energy differentiation. For validation, transmission measurements are made for 8 beams from 10–30 MV, with bremsstrahlung targets from Be to Pb. A protocol is established to account for many influence quantities including linac drifts (2%), polarity (6%), ion recombination (0.2%), leakage (0.3%), room scatter (0.8%), non‐ideal attenuation (1.5%), attenuator mass thickness (4%), and photonuclear effect (5.6%). The experimental accuracy on the smallest signals is 0.4%. EGSnrc is upgraded to model photonuclear attenuation (without tracking secondary particles), and then used to model the full experiment. For direct transmission comparisons, the agreement is 2%. This allows for an estimate of 0.5% on the upper limit of photon cross section uncertainties, which is much better than the current estimate of 1–2%. The unfolded spectra agree with the benchmark ones within 4.5%. The incident electron energy is accurate within 5%, with 95% confidence. The overall improvement over the commonly used methods is a factor of 3. This transmission study is the first to independently determine the incident electron energy, and to recognize the significant role of the photonuclear effect at higher energies.

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Journal Medical Physics
Ali, E., Mcewen, M., & Rogers, D.W.O. (2012). Sci—Thur PM: YIS — 02: A validated approach for clinical linacs to accurately determine the photon spectra and the incident electron energy. Medical Physics, 39(7). doi:10.1118/1.4740099