Master of Science (MS)
Physics and Astronomy
Purpose: The purpose of this work was to use a permanent magnet electron energy spectrometer to measure and compare electron beam energy spectra from matched electron beams on Elekta radiotherapy accelerators at Mary Bird Perkins Cancer Center
Methods: Electron beamcollimation, X-ray background collimation, and irradiation geometry were developed to produce suitable latent images in computed radiography (CR) strips at the exit side of a 0.5-Tesla permanent magnet spectrometer.An analytical technique transformed CR strip images into energy spectra on central axis at 95-cm source-to-collimator distance.Electron energy spectra at 7, 9, 10, 11, 13, 16, and 20 MeV for four accelerators, whose beams had been matched to one of two reference accelerators at commissioning, were measured and compared to measured energy spectra of their reference accelerator.Percent depth-dose (%DD) matching criteria of 0.05 cm for R50 and 0.1 cm for R80-20correlated to 0.12 MeV and 2.0 MeV for the modified incident peak mean energy (PME) and full-width-at-half-maximum (FWHM) of the energy spectra, respectively.
Results: For 19 of the 28 matched beams, PME did not match PME of their reference accelerator to within 0.12 MeV, showing shifts likely due to inadequate beam tuning subsequent to commissioning and energy drifting of the accelerator within quality assurance (QA) limits ± 0.2cm of R50).However, FWHM of all 28 matched beams matched FWHM of their reference accelerator despite having considerably different spectral shapes.By correlating %DD metrics with energy spectra metrics, it was determined that R50 had a linear relationship with PME, whereas R80-20depended primarily on PME and secondarily on FWHM.
Conclusions: First, this study demonstrated that not all MBPCC electron beam energy spectra were matched.Second, electron beams with matched %DD metrics do not necessarily have matched energy spectra, which are more sensitive for comparing beams than %DD curves.Third, the instrument, methods, and analytical analysis developed in this study offer the potential for an inexpensive, lightweight, real-time electron energy spectrometer which could be useful for beam tuning and matching, QA, and determination of X-ray and electron beam energy spectra for radiation transport calculations.
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McLaughlin, David, "Energy spectra comparisons for matched clinical electron beams on Elekta linear accelerators using a permanent magnet spectrometer" (2013). LSU Master's Theses. 103.