title: Beam quality and dose perturbation of 6 MV flattening-filter-free linac.
abstract: The aim of this study is twofold: (a) determination of the spectral differences for flattening-filter-free (FFF) versus standard (STD) linac under various clinical conditions, (b) based on an extensive list of clinically important beam configurations, identification of clinical scenarios that lead to higher macroscopic dose perturbations due to the presence of high-Z material. The focus is on dose enhancement due to contrast agents including high-Z elements such as gold or gadolinium. EGSnrc was used to simulate clinical beams under various irradiation conditions: open/IMRT/spit-IMRT fields, in/out-off-field areas, different depths and field sizes. Spectra were calculated and analyzed for about 80 beams and for a total of 480 regions. Quantitative differential effects in beam quality were characterized using energy-dependent and cumulative dose perturbation metrics. Analysis of the spectral database showed that even though the general trends for both linacs (FFF/STD) were the same, there were crucial differences. In general, the relative changes between different conditions were smaller for FFF spectra. This was because of the higher component of low-energy photons of the FFF linac, which already lead to higher dose enhancement than for the STD linac (photon energies were more "uniformly" distributed for FFF spectra and henceforth their perturbation resulted in lesser relative changes). For out-of-field FFF spectra and split-IMRT fields the strongest enhancement were observed (∼25 and ∼5 respectively). Different spectral scenarios lead to different dose enhancements, however, they scale with the higher effective-Z of the materials and were directly related to the lower range of the spectra (<200 keV).
authors.affiliation: Brigham and Women Hospital and Harvard Medical School, Boston, MA 02115, USA; Medical Physics Department, Medical School, University of Thessaly, Larisa 4100, Greece. Electronic address: ptsiamas@lroc.harvard.edu.
authors.forename: P
authors.initials: P
authors.lastname: Tsiamas
authors.affiliation: University of Lowell Massachusetts, Department of Physics and Applied Physics, Medical Physics Program, Lowell, MA 01854, USA.
authors.forename: E
authors.initials: E
authors.lastname: Sajo
authors.affiliation: University of Lowell Massachusetts, Department of Physics and Applied Physics, Medical Physics Program, Lowell, MA 01854, USA.
authors.forename: F
authors.initials: F
authors.lastname: Cifter
authors.affiliation: Medical Physics Department, Medical School, University of Thessaly, Larisa 4100, Greece.
authors.forename: K
authors.initials: K
authors.lastname: Theodorou
authors.affiliation: Medical Physics Department, Medical School, University of Thessaly, Larisa 4100, Greece.
authors.forename: C
authors.initials: C
authors.lastname: Kappas
authors.affiliation: Brigham and Women Hospital and Harvard Medical School, Boston, MA 02115, USA.
authors.forename: M
authors.initials: M
authors.lastname: Makrigiorgos
authors.affiliation: Brigham and Women Hospital and Harvard Medical School, Boston, MA 02115, USA. Electronic address: kmarcus@lroc.harvard.edu.
authors.forename: K
authors.initials: K
authors.lastname: Marcus
authors.affiliation: Brigham and Women Hospital and Harvard Medical School, Boston, MA 02115, USA. Electronic address: pzygmanski@lroc.harvard.edu.
authors.forename: P
authors.initials: P
authors.lastname: Zygmanski
mesh_terms: D009010:Monte Carlo Method; D010315:Particle Accelerators; D017785:Photons / Q000627:therapeutic use; D011829:Radiation Dosage; D011879:Radiotherapy Dosage; D050397:Radiotherapy, Intensity-Modulated
keywords: Dose enhanced radiation therapy; Flattening-filter-free; Monte Carlo; Photon fluence