Measurement and reconstruction of high-energy photon beam spectra using Compton spectrometry

A fast and efficient method for measuring and reconstructing the megavoltage photon beam spectra of clinical machines used for treating cancer patients is developed using Compton scattering. The construction and characterization of a novel prototype of a Compton spectrometer are performed to measure the pulse height spectra (PHS) of radiotherapy machines in routine clinical practice. In this work, we measured the PHS for a Co-60 radiotherapy machine using our Compton spectrometer. Based on the measured PHS and by employing the response matrix that had been previously constructed for the spectrometry system with a deconvolution method, we recovered the clinical machine photon fluence spectra. First, the characterization of one NaI(Tl) detector was performed to obtain the response functions for the Compton spectrometer with the purpose of constructing the spectrometry system response matrix. Mechanical and radiation tests were performed to ensure and verify the functionality and operation of the spectrometry measurement system. Next, the PHS for an X-ray unit with energies of 70-100 kV was measured at a Compton angle of 90 degrees to obtain the response of the spectrometer at these energies. Finally, the PHS for a Theratron-780 Co-60 clinical machine at different Compton angles was measured, and the corresponding photon fluence spectra were recovered. The results for the fluence spectra of the Theratron-780 show two well-defined peaks of 1173 and 1332 keV corresponding to the Co-60 source. The recovered spectra show good correlation with the spectra obtained by Monte Carlo simulation and with the spectra recovered for a standard Co-60 calibration source used as benchmarks in the corresponding megavoltage range. Our designed and built Compton spectrometer can be used in the clinical routine to measure and reconstruct megavoltage photon beam spectra for clinical machines.