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Effects of changes in stopping‐power ratios with field size on electron beam relative output factors
Author(s) -
Zhang G. G.,
Rogers D. W. O.,
Cygler J. E.,
Mackie T. R.
Publication year - 1998
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.598351
Subject(s) - stopping power , field size , cathode ray , beam (structure) , physics , electron , atomic physics , nuclear physics , optics , computational physics , detector
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,( L ̄ / ρ ) air wateris 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 ̄ / ρ ) air waterwith d maxis 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 ̄ / ρ ) air waterdata for broad mono‐energetic beams will give the correct answer for the ROF measurement within 0.4% compared to using( L ̄ / ρ ) air waterdata for which the field‐size effect is considered for realistic electron beams.