SU-D-BRCD-03: Spectroscopic Characterization of a 6 MV Linear Accelerator Field Using Compton Spectrometry Measurements and Monte Carlo Techniques.

PURPOSE Effective treatment planning for radiotherapy is dependent on accurate spectra determination; however, direct measurements of spectra arecomplicated by fluence rates that exceed detection system limitations. This work demonstrates the potential to use a Compton scattering technique to measure the spectrum of a 6MV linac. These spectra are further characterized using Monte Carlo (MC) simulations. METHODS A high-purity germanium detector was used to measure the photon spectrum scattered at35 degrees from the central axis of 3cmx3cm and lOcmxlOcm 6MV fields from a Varian linac. Photons were scattered using aluminum rods positioned at isocenter, and were admitted to the detector through a 30cm-long collimating aperture. The measured Compton-scattered spectra were corrected for background. An MC model of the linac was developed in MCNP5 to calculate central- and off-axis spectra. The model geometry was verified by comparisons with percentage depth-dose and profilemeasurements. The spectroscopic effect of the mean energy, radius, and divergence of the electron beam incident on the target was tested for twofield sizes. RESULTS The count rate of the scattered beam increased with field size and scattering rod diameter. Preliminary measurements indicate that the spectrum was shifted to lower energies using this technique; however, the signal-to-noise ratio was poor due to leakage and room scatter. MC simulations demonstrate that the central- and off-axis spectra were sensitive to changes in mean electron energy; however, changes in beam diameter and angular divergence did not substantially affect either the central- or off-axis spectra. CONCLUSIONS This work demonstrates that the spectrum from a 6MV linac can be measured using Compton spectrometry. Further work is required to increase the signal-to-noise ratio and correct fordetector response. MC simulations indicate that the spectra were sensitive to variations in the parameters used to define the primary electron beam incident on the target.