Technical Note: Flattening filter free beam from Halcyon linac: evaluation of the profile parameters for quality assurance.

INTRODUCTION The use of flattening filter free (FFF) beams generated by standard linear accelerators is increasing in the clinical practice. The radiation intensity peaked toward the beam central axis is properly managed in the optimization process of treatment planning through intensity modulation. Specific FFF parameters for profile analysis, as unflatness and slope for FFF beams, based on the renormalization factor concept has been introduced for quality assurance purposes. Recently, Halcyon, an O-ring based linear accelerator equipped with a 6 MV FFF beam only has been introduced by Varian. METHODS Renormalization factors and related fit parameters according to Fogliata et al. ["Definition of parameters for quality assurance of flattening filter free (FFF) photon beams in radiation therapy," Med. Phys. 39, 6455-6464 (2012)] have been evaluated for the 6 MV FFF beam generated by Halcyon units. The Halcyon representative beam data provided by Varian were used. Dose fall-off at the field edges was matched with an unflattened beam generated by a 6 MV from a TrueBeam linac. Consistency of the results was evaluated against measurements on a clinical Halcyon unit, as well as a TrueBeam 6 MV FFF for comparison. RESULTS The five parameters in the analytical equation for estimating the renormalization factor were determined with an R2 of 0.997. The comparison of the unflatness parameters between the Halcyon representative and hospital beam data was consistent within a range of 0.6%. Halcyon profiles, relative to the TrueBeam data, resulted slighted less peaked, consistently with the computed parameters. CONCLUSION Renormalization factors and related fit parameters from the 6 MV FFF beam generated by the Varian Halcyon unit are provided.