AbstractID: 7657 Title: Spectral Comparison of High Energy Photon Beams Examinations of beam spectra of high energy photon beams are typically not a part of the routine quality assurance program for clinical accelerators. However, a novel method for reconstructing beam spectra from measured transmission data through varying thicknesses of a beam attenuator has been presented in a previous publication. This method, which can be readily performed on an accelerator in a clinical environment, uses a non-linear fit of the transmission data with three physically significant parameters – a normalization parameter, the maximum energy of the beam, and the effective thickness of the flattening filter and other attenuators. In this work, the spectra of several different 6MV beams were reconstructed using this method to investigate the variation in spectrum in beams of similar energy. The beams studied include a single energy linac with a vertically mounted accelerator guide and several dual-energy linacs from different manufacturers, each equipped with horizontal accelerator guides and bending magnets. Discussion includes the presentation of the reconstructed spectra, comparison of the fit parameters to the physical characteristics and design of the accelerators, and analysis of the variations between the spectra of the beams.