SU-GG-T-428: Photon Beam Spectrum Characterization Using Scatter Radiation Analysis

Purpose: A method is described to characterize a linac bremsstrahlung spectrum. The method employs a combination of monoenergetic scatter simulations, polyenergetic scatter measurements, and matrix solutions. Method and Materials:Monte Carlo simulations were used to predict the ratio of primary to scatteredphotons for narrow mono‐energetic photon beams at 9 different locations, with 10 degree increments and 15 cm from a scattering material. Measurements were performed in the same geometry using the photon beams produced by linear accelerators. A linear matrix system, A×F=T, was developed to describe the scattering interactions and their relationship to the primary spectrum. Matrix “A” is the monoenergetic scatter kernel determined from the Monte Carlo simulations, “F” is the incident photon spectrum, and “T” represents the scatter distribution characterized by empirical measurement. Direct matrix inversion methods produce results that are not physically consistent due to errors inherent in the system. Tikhonov regularization methods were applied to address the effects of these errors and solve the system for physically consistent bremsstrahlung spectra. The technique was applied to two photon energy spectra ‐a helical tomotherapy accelerator operating at therapeutic energy and at imaging energy. Results: The resulting spectra were qualitatively and quantitatively similar to other published data and consistent with expected behavior such as maximum photon energy. Conclusion: The results of this research provide a method to empirically characterize the primary radiation energy spectrum produced by a linear accelerator.Key words: photon energy, spectrum, scatteranalysis, regularization, Tikhonov