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Wall effects in plane-parallel ionization chambers

B Nilsson1, A Montelius, P Andreo

  • 1Department of Medical Radiation Physics, Karolinska Institutet, Sweden.

Physics in Medicine and Biology
|April 1, 1996
PubMed
Summary
This summary is machine-generated.

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Dosimetry protocols often neglect wall perturbation factors for electron beams. This study reveals an energy-dependent wall perturbation factor (pwall) in plane-parallel chambers, impacting dose accuracy.

Area of Science:

  • Medical Physics
  • Radiation Dosimetry

Background:

  • Plane-parallel ionization chambers are recommended for electron beam dosimetry below 10-15 MeV.
  • Current dosimetry protocols assume negligible chamber perturbation effects, including wall effects (pwall).

Purpose of the Study:

  • To experimentally determine the wall perturbation factor (pwall) for plane-parallel chambers used in electron dosimetry.
  • To investigate the energy dependence of pwall and its impact on dose determination.

Main Methods:

  • Utilized a large plane-parallel ionization chamber with interchangeable walls to measure perturbation effects.
  • Compared experimental results with EGS4 Monte Carlo (DOSRZ code) calculations.
  • Investigated backscatter differences between phantom materials (Plastic Water, polystyrene) and chamber materials.

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Main Results:

  • An energy-dependent pwall factor was observed in several commercial chambers, primarily due to backscatter differences.
  • Backscatter differences between Plastic Water and polystyrene can reach 2% at low electron energies.
  • A 0.5 mm graphite front wall in the NACP chamber increased response by 0.7% in a PMMA phantom.

Conclusions:

  • The assumption of unity for pwall in some dosimetry protocols (e.g., AAPM TG39) may lead to inaccuracies.
  • Monte Carlo calculations (EGS4/DOSRZ) can accurately predict perturbation factors within 0.5% uncertainty.
  • Accurate determination of pwall is crucial for precise electron beam dosimetry.