The Effect of Large-Field Wedge Filters on Stopping Power Ratios

Over the past few decades, linear accelerators have been used for the treatment of cancer. These accelerators produce a spectrum of x-ray energies, with the maximum energy determined by the accelerating potential in the accelerator waveguide. Traditionally, the beams produced by these accelerators have been modified for certain treatment schemes to improve the overall dose distribution in the tumor volume. One of the beam modifiers has been the use of wedge filters. Although it has been accepted for some years that the introduction of a wedge filter hardens the x-ray beam from a linear accelerator, little or no correction for this effect has been routinely performed in the typical clinic. The results of this research will demonstrate that (1) a detectable change in the x-ray fluence energy distribution results with the introduction of a large field wedge, and (2) the change in the photon fluence results in a change in the average stopping power ratio for water to air used in the conversion of ionization chamber reading to absorbed dose. These effects are demonstrated for a variety of configurations including central axis and off axis points, with and without the wedge. To demonstrate the change in the x-ray fluence energy distribution, a reconstruction of bremsstrahlung spectra from measured transmission data technique was used, utilizing a Laplace Transform Pair Model. Following determination of Phi(E) for various beam configurations, with and without a wedge filter, average stopping power ratios of water to air were determined for each spectra. The results presented indicate that although a significant change in the photon fluence energy distribution results with the introduction of the wedge filter into the beam, the change in stopping power is <=q 0.5 %. This small change, however, is on the order of the chamber perturbation factors normally incorporated when using national or international dosimetry protocols. Thus this small change should be considered in future developments of ionization to dose formulations.