EFFECTS OF CHANGES IN STOPPING-POWER RATIOS WITH FIELD SIZE ON ELECTRON-BEAM RELATIVE OUTPUT FACTORS

Stopping-power ratios are a function of held size and vary with accele rators. To investigate how these variations affect relative output fac tor measurements made using ion chambers for electron beams, especiall y for small fields, ((L) over bar/rho)(air)(water) is calculated using the Monte Carlo technique for different field sizes, beam energies, a nd accelerators and is compared to the data in TG-21 or TG-25, which a re for mono-energetic broad beams. For very small field sizes defined by cutouts, if the change in ((L) over bar/rho)(air)(water) with d(max ) is ignored (i.e., TG-25 is not carefully followed), there is an over estimate of relative output factors by up to 3%. Ignoring the field-si ze effect on stopping-power ratio adds an additional overestimate of u p to one-half percent, and using mono-energetic stopping-power ratio d ata instead of realistic beam data gives another error, but in the opp osite direction, of up to 0.7%. Due to the cancellation of these latte r two errors, following TG-25 with ((L) over bar/rho)(air)(water) data for broad mono-energetic beams will give the correct answer for the R OF measurement within 0.4% compared to using ((L) over bar/rho)(air)(w ater) data for which the field-size effect is considered for realistic electron beams. (C) 1998 American Association of Physicists in Medici ne.