Measurement of the replacement correction factor for parallel-plate chambers in electron fields.

When parallel-plate chambers are used for dosimetry in electron fields, the AAPM dosimetry protocol recommends a value of 1.0 for the replacement correction factor, P(repl),pp,E, until further data become available. Here, P(repl),pp,E for five commercially available parallel-plate chambers was measured as a function of electron energy from a nominal value of 5.5 to 22 MeV by comparison with a cylindrical chamber whose P(repl),cyl,E was obtained from data in the protocol. Since this method is based on the concept of a constant value for Ngas,pp, the energy and modality dependence of Ngap,pp is also investigated for these chambers for Co-60, 4-, 6-, 24-MV photons and for 22-MeV electrons. It is found that for three of the chambers P(repl),pp,E is independent of energy, consistent with unity within one or two standard deviations (s.d.). For the fourth chamber P(repl),pp,E is similarly consistent with one above 10 MeV, but decreases at lower energies, while for the fifth one it shows a systematic drop with decreasing energy.