Matching the 6-MV photon beam characteristics of two dissimilar linear accelerators.

A new prototype 6-MV flattening filter was designed by the manufacturer for use in a popular dual energy linear accelerator. To satisfy the contract demands, this filter was designed to produce a beam whose characteristics matched precisely with those of the 6-MV beam produced from a single photon peak energy unit from the same manufacturer and already in operation in the department. A single set of 6-MV dosimetric files for both units can now be maintained. The new filter has forced percent depth values over a wide clinical range of field sizes and depths to agree within 1.3%. Beam profiles now agree to within 1% over the useful area. For wedges with similar wedge angles, transmission factors now agree to within 1%. Standard acceptance testing performance specifications provided by the manufacturer were not adequate for clinical beam matching. The purchase contract for these units included our own specifications, which were more rigid and pertinent to our goal. Details of the effort are discussed.