Unfolding linac photon spectra and incident electron energies from experimental transmission data, with direct independent validation

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DOIResolve DOI: http://doi.org/10.1118/1.4754301
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TypeArticle
Journal titleMedical Physics
ISSN0094-2405
Volume39
Issue11
Pages65856596; # of pages: 12
Subjectarticle; electron; human; methodology; Monte Carlo method; photon; reproducibility; spectroscopy; statistics; Electrons; Humans; Monte Carlo Method; Photons; Reproducibility of Results; Spectrum Analysis; Statistics as Topic
AbstractPurpose: In a recent computational study, an improved physics-based approach was proposed for unfolding linac photon spectra and incident electron energies from transmission data. In this approach, energy differentiation is improved by simultaneously using transmission data for multiple attenuators and detectors, and the unfolding robustness is improved by using a four-parameter functional form to describe the photon spectrum. The purpose of the current study is to validate this approach experimentally, and to demonstrate its application on a typical clinical linac. Methods: The validation makes use of the recent transmission measurements performed on the Vickers research linac of National Research Council Canada. For this linac, the photon spectra were previously measured using a NaI detector, and the incident electron parameters are independently known. The transmission data are for eight beams in the range 10-30 MV using thick Be, Al and Pb bremsstrahlung targets. To demonstrate the approach on a typical clinical linac, new measurements are performed on an Elekta Precise linac for 6, 10 and 25 MV beams. The different experimental setups are modeled using EGSnrc, with the newly added photonuclear attenuation included. Results: For the validation on the research linac, the 95 confidence bounds of the unfolded spectra fall within the noise of the NaI data. The unfolded spectra agree with the EGSnrc spectra (calculated using independently known electron parameters) with RMS energy fluence deviations of 4.5. The accuracy of unfolding the incident electron energy is shown to be ∼3. A transmission cutoff of only 10 is suitable for accurate unfolding, provided that the other components of the proposed approach are implemented. For the demonstration on a clinical linac, the unfolded incident electron energies and their 68 confidence bounds for the 6, 10 and 25 MV beams are 6.1 ± 0.1, 9.3 ± 0.1, and 19.3 ± 0.2 MeV, respectively. The unfolded spectra for the clinical linac agree with the EGSnrc spectra (calculated using the unfolded electron energies) with RMS energy fluence deviations of 3.7. The corresponding measured and EGSnrc-calculated transmission data agree within 1.5, where the typical transmission measurement uncertainty on the clinical linac is 0.4 (not including the uncertainties on the incident electron parameters). Conclusions: The approach proposed in an earlier study for unfolding photon spectra and incident electron energies from transmission data is accurate and practical for clinical use. © 2012 American Association of Physicists in Medicine.
Publication date
LanguageEnglish
AffiliationNational Research Council Canada (NRC-CNRC); NRC Institute for National Measurement Standards (INMS-IENM)
Peer reviewedYes
NPARC number21269456
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Record identifierf17b9404-7208-44c3-bea2-86ec955eb1d5
Record created2013-12-12
Record modified2016-05-09
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