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Calculations for beam‐flattening filters for high‐energy x‐ray machines
Author(s) -
Larsen Ronald D.,
Brown Lisa Hartnagel,
Bjärngard Bengt E.
Publication year - 1978
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 - 10.1118/1.594430
Subject(s) - flattening , optics , physics , beam (structure) , energy (signal processing) , x ray , materials science , medical physics , quantum mechanics , astronomy
A flattening filter is an important component in a medical accelerator to modify the photon‐beam properties. To simplify the calculations of the flattening‐filter profile, we have developed a computer program which sums primary and scatter and then makes iterations in the primary component to produce a desired total‐dose profile. The program can account, to first order, for radial spectral changes by using an effective primary attenuation coefficient which varies with radius. Calculations made to model the Clinac‐4 dose profiles using the measured variation of half‐value layer with radius show good agreement with the measured data. It is shown that the variation of quality within the beam impairs the flatness that can be achieved over a range of depths. Since perfect flatness cannot be achieved for small and large fields with one flattening filter, one may choose a primary profile which is a compromise over a range of field sizes and depths. A compromise profile for a 4‐MV beam is discussed.