Routine quality control of computer designed and fabricated intensity modulation filters

For anatomy compensating filters the authors' approach is to confirm the shape in the two dimensions perpendicular to the beam axis by visual inspection and confirm the third dimension parallel to the beam) at one point, corresponding to the Helax reference point, by direct relative dose measurement. This approach has proved satisfactory in the over 100 pairs of filters designed by Helax and fabricated by the Compuformer. The authors' routine quality control results together with their initial commissioning measurements indicate a high degree of reliability in the fabrication of these filters. However, for modulating filters to be used in IMRT, the authors' present quality control protocol will have to be extended. Visual inspection of the milled but unfilled mould is likely to be of little value as it will not be possible to relate the modulation of the filter and hence the photon fluence to surface anatomy. Profiler measurements have been shown here to be of adequate accuracy to confirm the filter profile in one dimension. Added confidence in the accuracy of fabrication of an IMRT filter could be gained by measuring several linear profiles although,it has to be remembered that such measurements and the associated Helax calculations do absorb resources. Ideally, however, verification of modulating filters for IMRT will employ an area detector such as an electronic portal imager which, with known dose response characteristics, could yield the two dimensional profile of the filter.