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.
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