Calculations for beam-flattening filters for high-energy x-ray machines

A flattening filter is an important component in a medical accelerator to modify the photonbeam 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.