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 N gas,pp , the energy and modality dependence of N gap,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.