The effect of flattening filter design on quality variations within an 8-MV primary x-ray beam

Single material flattening filters supplied by manufacturers of medical linear accelerators are designed to produce the desired primary dose profile while maintaining output at a maximum level. This design criterion tends to produce substantial quality variations within the primary beam. Quality variations, as expressed by half-value layer in brass and polystyrene, were measured for an 8-MV primary beam both unfiltered and with the flattening filter supplied by the manufacturer. Most of the quality variation was introduced by the filter. Two approaches were then used to reduce this quality variation, each at a cost of a 25% reduction in output. First, a hardening filter was added to the manufacturer's flattening filter. The second approach was to design a new composite flattening filter made from brass and lead. For both approaches, the increase in quality variation over the intrinsic (no filter) variation was reduced by one-half.