Stopping-power ratios are a function of field 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, [L̄/ρ)(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 (L̄/ρ)(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 (L̄/ρ)(air)/(water) for broad mono- energetic beams will give the correct answer for the ROF measurement within 0.4% compared to using (L̄/ρ)(air)/(water) data for which the field-size effect is considered for realistic electron beams.

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Journal Medical Physics
Zhang, G.G., Rogers, D.W.O, Cygler, J.E., & Mackie, T.R. (1998). Effects of changes in stopping-power ratios with field size on electron beam relative output factors. Medical Physics, 25(9), 1711–1716. doi:10.1118/1.598351