How accurately can EGS4/PRESTA calculate ion-chamber response?

Ion-chamber responses are calculated for graphite, PMMA, and aluminum-walled ion chambers free-in-air in 60Co beams and 200-keV beams for a graphite chamber. The EGS4 Monte Carlo system is used with various electron step-size algorithms, in particular the PRESTA algorithm, the much simpler ESTEPE constraint on the energy loss per step, and in combination. Contrary to previous reports, it is found that there are variations in the calculated ion chamber response of up to 3% in 60Co beams and up to 8% in the 200-keV beam. It is recommended that all ion chamber calculations be done with the PRESTA algorithm plus an additional constraint on the energy losses per step of 1% (or less for higher-Z materials). The systematic uncertainty in the calculations for 60Co beams is found to be 1% (1 sigma) and somewhat higher in 200-keV beams because of sensitivity to transport parameters such as AE and ECUT.