  Background: With the advent of Intensity Modulated Radiotherapy (IMRT) and
recently, Volumetric Modulated Arc Therapy (VMAT), treatment planning using
Flattening Filter Free (FFF) beams can meet all of the energy requirements in
radiation therapy clinics. Manufacturers of linear accelerators no longer need
to install a flattening filter (FF) in gantry head. This study aims to provide
evidence of the superiority of FFF to FF through both dosimetric measurements
and clinical treatment plans. Materials and Methods: A 50x50x50cm3 water
phantom was created in the RayStation treatment planning system (TPS) for
dosimetry comparisons. Flat beam profiles were generated using FFF beam through
an optimization process for 10x10 to 30x30cm2 field sizes. Next, a comparison
of treatment plans was made using 21 Head and Neck and 14 Lung/Mediastinum
treatment sites using 6MV and 6MV-FFF beams. Results: Using FFF beams, profiles
with flatness and symmetry identical to or better than those of the flattened
beams were produced. At the very edge of the optimized plans for FFF beams,
horns had the highest gamma index deviation <1.5% of the normalized dose. For
clinical plans evaluated, most of the mean doses to organs_atrisk (OAR) volumes
receiving 5% to 30% of the prescription dose were reduced with FFF beams.
Conclusion: These results indicate the feasibility of delivering flat beams
with FFF quality and producing treatment plans with equal or higher qualities
in PTV coverage while achieving better sparing of OAR which will allow
escalation of target dose if desired. Plus, removing FF will simplify the
gantry head and reduces quality assurance and machine maintenance efforts.
