Utilizing a Real-Time Permanent-Magnet Energy Spectrometer for Spectra-Based Therapeutic Electron Beam Tuning
Abstract
Purpose
Therapeutic electron beams are normally characterized by a percent dose versus depth (PDD) curve. Beam tuning based on PDD features can produce large variations in the resulting energy spectra, potentially leading to premature maintenance requirements. A study was conducted utilizing a real-time electron energy spectrometer to investigate the effects of common tuning parameters on spectral shape. Subsequently, each of five beams of different nominal energy were tuned to prioritize a narrow full-width half-maximum (FWHM) while PDDs remained within clinical matching criteria.
Methods
Using an Elekta Agility linac, a real-time spectrometer measured the energy spectrum of the 11 MeV beam while an accelerator engineer adjusted the high and low-power phase shifters, the magnetron current, and the currents of the bending magnets. Subsequently, these parameters were adjusted to tune beams of 7, 9, 11, 13, and 16 MeV. “Synthetic” PDDs were generated in real-time from precomputed Monte Carlo simulations using the energy spectra. Tuning prioritized a narrow FWHM while maintaining the synthetic PDDs R50 to within 0.5 mm of its original value.
Results
Single parameter adjustments produced large differences in the most probable energy (Ep,0), FWHM, and general characteristics of the spectra. Changes in the magnetron and bending magnet currents produced monotonic, non-linear differences, while the low and high-power phase shifters produced non-monotonic changes due to phase-dependent resonance within the accelerating structure. Beam tuning prioritizing minimal FWHM for the five different energy beams resulted in decreases ranging from 0.16 to 0.75 MeV.
Conclusion
Energy spectra are sensitive to parameters of internal linac components. Features of an energy spectrum can be tuned by adjusting these parameters, while maintaining synthetic PDDs within beam matching criteria. A real-time spectrometer is a useful tool for rapidly tuning electron beams. Development of a spectrum-specific tuning scheme would be of further benefit.