Wall correction factors, Pwall, for parallel-plate ionization chambers

The EGSnrc Monte Carlo user-code CSnrc is used to calculate wall correction factors, P-wall, for parallel-plate ionization chambers in photon and electron beams. A set Of P-wall 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 P-wall 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 d(ref) to a depth of R-50, In photon beams, where there is limited information available regarding P-wall for parallel-plate chambers, CSnrc calculations show P-wall values of up to 2.4% at the reference depth over a range of photon energies. The P-wall 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. (C) 2006 American Association of Physicists in Medicine.