VALIDATION OF RECONSTRUCTED BREMSSTRAHLUNG SPECTRA BETWEEN 6 MV AND 25 MV FROM MEASURED TRANSMISSION DATA

We have previously investigated a method of high-energy x-ray spectral reconstruction from transmission data by direct resolution of a matri x system. This technique uses the spectral vectorial algebra formalism . The resolution has previously been tested on a 12 MV photon beam. To extend and to test the validity of the results to the entire radiothe rapy energy range, we have performed the method on photon beams with n ominal energies of 6, 12, 15, and 25 MV. The influence on the 6 MV spe ctrum of a 60 degrees built-in wedge has also been investigated to tes t the sensitivity of the method and the results are reported. To valid ate our reconstructed spectra, dosimetric quantities such as tissue ph antom ratios (TPR), water-to-air stopping power ratios (S/rho)(air)(wa ter), and quality indexes TPR1020 have been calculated. The results sh ow good agreement between the measured and calculata data. Mean mass e nergy absorption coefficient ratios for different materials have also been computed and compared to data published recently and the results are very close (within +/-0.5%). Primary depth dose functions in water have also been computed to deduce primary dose attenuation coefficien ts. (C) 1997 American Association of Physicists in Medicine.