title: Parameterization of photon beam dosimetry for a linear accelerator.
abstract: OBJECTIVE
  In radiation therapy, accurate data acquisition of photon beam dosimetric quantities is important for (1) beam modeling data input into a treatment planning system (TPS), (2) comparing measured and TPS modeled data, (3) the quality assurance process of a linear accelerator's (Linac) beam characteristics, (4) the establishment of a standard data set for comparison with other data, etcetera. Parameterization of the photon beam dosimetry creates a data set that is portable and easy to implement for different applications such as those previously mentioned. The aim of this study is to develop methods to parameterize photon beam dosimetric quantities, including percentage depth doses (PDDs), profiles, and total scatter output factors (S(cp)).
  
  
  METHODS
  S(cp), PDDs, and profiles for different field sizes, depths, and energies were measured for a Linac using a cylindrical 3D water scanning system. All data were smoothed for the analysis and profile data were also centered, symmetrized, and geometrically scaled. The S(cp) data were analyzed using an exponential function. The inverse square factor was removed from the PDD data before modeling and the data were subsequently analyzed using exponential functions. For profile modeling, one halfside of the profile was divided into three regions described by exponential, sigmoid, and Gaussian equations. All of the analytical functions are field size, energy, depth, and, in the case of profiles, scan direction specific. The model's parameters were determined using the minimal amount of measured data necessary. The model's accuracy was evaluated via the calculation of absolute differences between the measured (processed) and calculated data in low gradient regions and distance-to-agreement analysis in high gradient regions. Finally, the results of dosimetric quantities obtained by the fitted models for a different machine were also assessed.
  
  
  RESULTS
  All of the differences in the PDDs' buildup and the profiles' penumbra regions were less than 2 and 0.5 mm, respectively. The differences in the low gradient regions were 0.20% ± 0.20% (<1% for all) and 0.50% ± 0.35% (<1% for all) for PDDs and profiles, respectively. For S(cp) data, all of the absolute differences were less than 0.5%.
  
  
  CONCLUSIONS
  This novel analytical model with minimum measurement requirements was proved to accurately calculate PDDs, profiles, and S(cp) for different field sizes, depths, and energies.
authors: Department of Radiation Oncology, University of Florida College of Medicine, Gainesville, Florida 32610-0385 and J. Crayton Pruitt Family Department of Biomedical Engineering, University of Florida, Gainesville, Florida 32611.
authors: Sharon
authors: S
authors: Lebron
authors: Department of Radiation Oncology, University of Florida College of Medicine, Gainesville, Florida 32610-0385.
authors: Bo
authors: B
authors: Lu
authors: Department of Radiation Oncology, University of Florida College of Medicine, Gainesville, Florida 32610-0385.
authors: Guanghua
authors: G
authors: Yan
authors: Department of Radiation Oncology, University of Florida College of Medicine, Gainesville, Florida 32610-0385.
authors: Darren
authors: D
authors: Kahler
authors: Department of Radiation Oncology, University of Florida College of Medicine, Gainesville, Florida 32610-0385.
authors: Jonathan G
authors: JG
authors: Li
authors: Department of Radiation Oncology, University of Florida College of Medicine, Gainesville, Florida 32610-0385 and J. Crayton Pruitt Family Department of Biomedical Engineering, University of Florida, Gainesville, Florida 32611.
authors: Brendan
authors: B
authors: Barraclough
authors: Department of Radiation Oncology, University of Florida College of Medicine, Gainesville, Florida 32610-0385.
authors: Chihray
authors: C
authors: Liu
mesh_terms: D010315:Particle Accelerators; D017785:Photons / Q000627:therapeutic use*; D011874:Radiometry / Q000295:instrumentation*