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 accelerators. To investigate how these variations affect relative output factor measurements made using ion chambers for electron beams, especially for small fields, ((L) over bar/rho)(air)(water) is calculated using the Monte Carlo technique for different field sizes, beam energies, and accelerators and is compared to the data in TG-21 or TG-25, which are 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 overestimate of relative output factors by up to 3%. Ignoring the field-size effect on stopping-power ratio adds an additional overestimate of up to one-half percent, and using mono-energetic stopping-power ratio data instead of realistic beam data gives another error, but in the opposite direction, of up to 0.7%. Due to the cancellation of these latter 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 ROF measurement within 0.4% compared to using ((L) over bar/rho)(air)(water) data for which the field-size effect is considered for realistic electron beams. (C) 1998 American Association of Physicists in Medicine.