title: A comparison of phantom scatter from flattened and flattening filter free high-energy photon beams.
abstract: Flattening filter free (FFF) photon beams have different dosimetric properties from those of flattened beams. The aim of this work was to characterize the collimator scatter (Sc) and total scatter (Scp) from 3 FFF beams of differing quality indices and use the resulting mathematical fits to generate phantom scatter (Sp) data. The similarities and differences between Sp of flattened and FFF beams are described. Sc and Scp data were measured for 3 flattened and 3 FFF high-energy photon beams (Varian 6 and 10MV and Elekta 6MV). These data were fitted to logarithmic power law functions with 4 numerical coefficients. The agreement between our experimentally determined flattened beam Sp and published data was within ± 1.2% for all 3 beams investigated and all field sizes from 4 × 4 to 40 × 40cm(2). For the FFF beams, Sp was only within 1% of the same flattened beam published data for field sizes between 6 × 6 and 14 × 14cm(2). Outside this range, the differences were much greater, reaching - 3.2%, - 4.5%, and - 4.3% for the fields of 40 × 40cm(2) for the Varian 6-MV, Varian 10-MV, and Elekta 6-MV FFF beams, respectively. The FFF beam Sp increased more slowly with increasing field size than that of the published and measured flattened beam of a similar reference field size quality index, i.e., there is less Phantom Scatter than that found with flattened beams for a given field size. This difference can be explained when the fluence profiles of the flattened and FFF beams are considered. The FFF beam has greatly reduced fluence off axis, especially as field size increases, compared with the flattened beam profile; hence, less scatter is generated in the phantom reaching the central axis.
authors.affiliation: Department of Medical Physics, The James Cook University Hospital, Middlesbrough, UK. Electronic address: neil.richmond@stees.nhs.uk.
authors.forename: Neil
authors.initials: N
authors.lastname: Richmond
authors.affiliation: The Northern Centre for Cancer Care, The Freeman Hospital, Newcastle upon Tyne, UK.
authors.forename: Vince
authors.initials: V
authors.lastname: Allen
authors.affiliation: Department of Medical Physics, The James Cook University Hospital, Middlesbrough, UK.
authors.forename: Jim
authors.initials: J
authors.lastname: Daniel
authors.affiliation: The Northern Centre for Cancer Care, The Freeman Hospital, Newcastle upon Tyne, UK.
authors.forename: Rob
authors.initials: R
authors.lastname: Dacey
authors.affiliation: Department of Medical Physics, The James Cook University Hospital, Middlesbrough, UK.
authors.forename: Chris
authors.initials: C
authors.lastname: Walker
mesh_terms: D004867:Equipment Design; D019544:Equipment Failure Analysis; D005374:Filtration / Q000295:instrumentation*; D017785:Photons / Q000627:therapeutic use*; D011874:Radiometry / Q000379:methods*; D011879:Radiotherapy Dosage; D011882:Radiotherapy, High-Energy / Q000295:instrumentation*; D015203:Reproducibility of Results; D012542:Scattering, Radiation; D012680:Sensitivity and Specificity
keywords: Dosimetry; Flattening filter free; Phantom scatter; Radiotherapy