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Field size dependence of the output factor in passively scattered proton therapy: Influence of range, modulation, air gap, and machine settings
Author(s) -
Daartz J.,
Engelsman M.,
Paganetti Harald,
Bussière M. R.
Publication year - 2009
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 - 10.1118/1.3152111
Subject(s) - proton therapy , imaging phantom , monitor unit , field size , field (mathematics) , optics , physics , range (aeronautics) , computational physics , proton , air gap (plumbing) , modulation (music) , beam (structure) , materials science , nuclear physics , mathematics , nuclear medicine , acoustics , medicine , pure mathematics , composite material
At the Francis H. Burr Proton Therapy Center field specific output factors (i.e., dose per monitor unit) for patient treatments were modeled for all beamlines (two gantries, fixed stereotactic, and fixed eye beamline). The authors evaluated the accuracy of dose calculation and output model for small fields. Measurements in a water phantom were performed in three of our beamlines quantifying the dependency of the output factor on the field size for a variety of proton ranges. The influence of snout size, air gap, modulation, and second scatterer was investigated. The impact of field size on output depends strongly on the depth of interest. The air gap has a notable influence on small field outputs. A field size specific correction factor to the output is necessary if the latter was modeled or measured without the custom hardware in place. The output was shown to be field size dependent even for large fields, indicating an effect beyond charged particle disequilibrium caused by lateral scatter.