title: A novel position-sensitive mega-size dosimeter for photoneutrons in high-energy X-ray medical accelerators.
abstract: OBJECTIVE
  A novel position-sensitive mega-size polycarbonate (MSPC) dosimeter is introduced. It provides photoneutron (PN) dose equivalent matrix of positions in and out of a beam of a high energy X-ray medical accelerator under a single exposure.
  
  
  METHODS
  A novel position-sensitive MSPC dosimeter was developed and applied. It has an effective etched area of 50×50cm(2), as used in this study, processed in a mega-size electrochemical etching chamber to amplify PN-induced-recoil tracks to a point viewed by the unaided eyes. Using such dosimeters, PN dose equivalents, dose equivalent profiles and isodose equivalent distribution of positions in and out of beams for different X-ray doses and field sizes were determined in a Siemens ONCOR Linac.
  
  
  RESULTS
  The PN dose equivalent at each position versus X-ray dose was linear up to 20Gy studied. As the field size increased, the PN dose equivalent in the beam was also increased but it remained constant at positions out of the beam up to 20cm away from the beam edge. The jaws and MLCs due to material differences and locations relative to the target produce different PN contributions.
  
  
  CONCLUSIONS
  The MSPC dosimeter introduced in this study is a perfect candidate for PN dosimetry with unique characteristics such as simplicity, efficiency, dose equivalent response, large size, flexibility to be bent, resembling the patient's skin, highly position-sensitive with high spatial resolution, highly insensitive to X-rays, continuity in measurements and need to a single dosimeter to obtain PN dose equivalent matrix data under a single X-ray exposure.
authors.affiliation: Health Physics and Dosimetry Research Laboratory, Department of Energy Engineering and Physics, Amirkabir University of Technology, Tehran, Iran. Electronic address: dr_msohrabi@yahoo.com.
authors.forename: Mehdi
authors.initials: M
authors.lastname: Sohrabi
authors.affiliation: Health Physics and Dosimetry Research Laboratory, Department of Energy Engineering and Physics, Amirkabir University of Technology, Tehran, Iran.
authors.forename: Amir
authors.initials: A
authors.lastname: Hakimi
authors.affiliation: Department of Medical Physics, Faculty of Medicine, Iran University of Medical Sciences, Tehran, Iran.
authors.forename: Seyed Rabi
authors.initials: SR
authors.lastname: Mahdavi
mesh_terms: D009502:Neutrons; D010315:Particle Accelerators / Q000295:instrumentation*; D017785:Photons; D011076:Polycarboxylate Cement; D011829:Radiation Dosage; D011874:Radiometry / Q000295:instrumentation*; D014965:X-Rays
chemical_list: D011076:Polycarboxylate Cement; C005506:polycarbonate
keywords: High-energy X-rays; Mega-size; Photoneutron dosimeter; Polycarbonate; Position-sensitive; Radiotherapy