Reconstruction of 4-MV bremsstrahlung spectra from measured transmission data

Abstract

Transmission data for 4-MV bremsstrahlung beams were measured with a combination of lead and aluminum attenuators. From these data, the original energy spectra were reconstructed using an iterative least-squares technique, previously evaluated by simulation studies. The spectra on the central axis for 3 similar 4-MV linear accelerators indicated no significant differences. When studying the spectra at 5.degree. and 9.degree. off the central axis, that at 9.degree. showed the expected increase of low-energy photons. All these spectra showed a maximum photon energy of 4.5 .+-. 0.2 MeV. When the magnetron power was reduced, the spectrum on the central axis shifted to lower energies and the maximum photon energy decreased to 3.5 .+-. 0.2 MeV. The result of this experimental study confirms the conclusions from the previous simulation, that the numerical technique for analysis of transmission data can accurately represent 4-MV bremsstrahlung spectra and detect differences in energy distribution with changes in machine tuning and position in the radiation field for a 4-MV bremsstrahlung beam. [The use of this method in radiation therapy is discussed.].