Effects of changes in stopping-power ratios with field size on electron beam relative output factors

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 di...

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Veröffentlicht in:Medical physics (Lancaster) 1998-09, Vol.25 (9), p.1711-1716
Hauptverfasser: Zhang, G. G., Rogers, D. W. O., Cygler, J. E., Mackie, T. R.
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Sprache:eng
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Zusammenfassung: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 data 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.
ISSN:0094-2405
2473-4209
DOI:10.1118/1.598351