Determinations of Ngas and Prepl factors from commercially available parallel-plate chambers: AAPM Task Group 21 protocol.

Using a Farmer chamber as a reference dosimeter, we have measured the Ngas (cavity-gas calibration factor) and Prepl (replacement correction factor) values for four parallel-plate chambers: a Holt chamber, a Capintec chamber, a Markus chamber, and an SHM chamber. Ngas determinations were done primarily using 60Co photons and nominal 18-MeV electrons for Ngas determinations. For Prepl determinations, we used nominal 18-, 15-, 12-, 9-, 6-, and 4-MeV electrons.