Wall correction factors, P wall , for thimble ionization chambers

The EGSnrc Monte Carlo user‐code CSnrc is used to calculate wall correction factors, P wall , for thimble ionization chambers in photon and electron beams. CSnrc calculated values of P wall give closer agreement with previous experimental results than do the values from the standard formalism used in current dosimetry protocols. A set of P wall values, computed at the reference depth in water, is presented for several commonly used thimble chambers. These values differ from the commonly used values by up to 0.8% for megavoltage photon beams, particularly for nominal beam energies below 6 MV . The sleeve effect, which is not currently taken into account by the TG‐51 dosimetry protocol, is computed to be up to 0.3% and is in some cases larger than the P wall correction itself. In electron beams, where dosimetry protocols assume a wall correction of unity, CSnrc calculations show P wall values of up to 0.6% at the reference depth, depending on the wall material. P wall is shown to be sensitive to the depth of measurement, varying by 2.5% for a graphite‐walled cylindrical Farmer‐like chamber between a depth of 0.5 cm and R 50 in a 6 MeV electron beam.