A comparative study between flattening filter-free beams and flattening filter beams in radiotherapy treatment

Flattening filter-free (FFF) beams generated by medical linear particle accelerators (linacs) have recently been used in radiotherapy clinical practice. FFF beams have fundamental physical parameter differenceswith respect to standard flattening filter (FF) beams, such that the generally used dosimetric parameters and definitions are not always viable. This study investigates dosimetric parameters for use in the qualityassurance of FFF beams generated by medical linacs in radiotherapy. The main characteristics of the photon beams are analyzed using specific data generated by a Varian TrueBeam linac having both FFFand FF beams of 6 and 10 MV (megavolt) energy, respectively. Definitions for dose profile parameters are suggested, starting from the renormalization of the FFF with respect to the corresponding FF beam. Fromthis point, the flatness concept is translated into one of “un-flatness”, and other definitions are proposed, maintaining a strict parallelism between FFF and FF parameter concepts. The quality controls used inestablishing a quality assurance program when introducing FFF beams into the clinical environment are given, maintaining similarity to those used for standard FF beams, and recommendations for the introduction of FFF beams into clinical radiotherapy application for breast cancer patients are provided as an example for comparison between FFF and FF for dose distribution and coverage for a target volume. Although there aremany advantages of using a FFF beam, especially for advanced radiotherapy techniques, there are a few limitations (e.g., using a relatively higher energy photon beam for stereotactic radiotherapy (SRT), limitedspeed of current multileaf collimators (MLCs), and off-axis distance-dependent modulation in intensitymodulated radiation therapy (IMRT)) as well as challenges (e.g., criteria for beam quality evaluation andpenumbra, establishment of dosimetry methods, and consequences of photon target burn-up) that need to be addressed for establishing the FFF beam as a viable alternative to the FF beam.