Comparison of methodologies for dosimetric characterization of flattening filter-free (FFF) photon beams in radiotherapy

Abstract

The introduction of flattening filter-free (FFF) photon beams in radiotherapy has brought significant benefits, including increased dose rates and reduced treatment delivery times. However, the modification of the dose profile resulting from the removal of the flattening filter limits the direct application of traditional quality control parameters developed for conventional beams. In this scenario, different approaches have been proposed for the dosimetric characterization of FFF beams. In this study, three methodologies described in the literature were evaluated and compared: the Fogliata (FG) method: profile renormalization; the Sharma (SH) method: inflection point; and the Budgell (BDG) method: virtual flattening filter. The analysis was performed for 6 MV FFF and 10 MV FFF photon beams generated by a TrueBeam linear accelerator (Varian Medical Systems, Palo Alto, CA, USA), considering parameters such as symmetry, field size, penumbra, and degree of unflatness. The results demonstrated good agreement among the evaluated methodologies. Differences in penumbra values were below 0.7 mm across all field sizes, and symmetry values remained within 1% for all measurements, complying with recommended tolerances. Field size differences were within ±1.2 mm relative to nominal values, also within established clinical limits. The unflatness parameter showed consistent behavior with literature data, with variations dependent on field size and beam energy. Among the evaluated approaches, the SH method was identified as the most suitable for clinical routine, as it preserves the intrinsic characteristics of FFF beams while providing reliable and consistent dosimetric parameters.