The EGSnrc Monte Carlo user-code CSnrc is used to calculate wall correction factors, Pwall, for parallel-plate ionization chambers in photon and electron beams. A set of Pwall values, computed at the reference depth in water, is presented for several commonly used parallel-plate chambers. These values differ from the standard assumption of unity used by dosimetry protocols by up to 1.7% for clinical electron beams. Calculations also show that Pwall is strongly dependent on the depth of measurement and can vary by as much as 6% for a 6 MeV beam in moving from a depth of dref to a depth of R50. In photon beams, where there is limited information available regarding Pwall for parallel-plate chambers, CSnrc calculations show Pwall values of up to 2.4% at the reference depth over a range of photon energies. The Pwall values for photon beams are in good agreement with previous estimates of the wall correction but have much lower statistical uncertainties and cover a wider range of photon beam energies.

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Medical Physics
Department of Physics

Buckley, L.A. (Lesley A.), & Rogers, D.W.O. (2006). Wall correction factors, Pwall, for parallel-plate ionization chambers. Medical Physics, 33(6), 1788–1796. doi:10.1118/1.2199988