Purpose: The HyperScint scintillator dosimetry system from MedScint is a small-field dosimeter with reported energy independence down to 100-keV. This work investigates the energy dependence of the scintillator between a monoenergetic photon source and polyenergetic orthovoltage beam.
Methods: The Monte Carlo particle simulator code, Geant4, was employed via its wrapper, TOPAS, to calculate energy correction factors between a 180-kVp field (60-keV effective energy) and several monoenergetic beams (20 to 100-keV). All beams were modeled as square, uniform fields. Two simulations were run for each beam; the first measured dose to a water volume in air, while the second measured energy deposited in a scintillator of equal size. Energy correction was calculated as the ratio of energy deposited in the scintillator per dose to water for the orthovoltage beam to that of the monoenergetic beam. All simulations were performed for 1e9 histories using a particle cut 50x smaller than the detector dimension. Quenching was modeled in the scintillator using Birks' Law with a Birks' factor of 0.197-mm/MeV. All outputs include history-by-history variance calculations for uncertainty analysis. Results were compared against an analytical model derived using exponential attenuation and Birks' Law.
Results: Correction factors were collected for 20 to 100-keV with a mean uncertainty of 0.4% (95% confidence interval). Factors ranged from almost 2 at the lowest energy down to 0.8 at the highest energy. Simulation showed good agreement with the analytical model above 30-keV.
Conclusion: Large energy correction factors were observed to good precision in the HyperScint system between monoenergetic beams and a polyenergetic orthovoltage beam. The monoenergetic simulation with the same effective energy as the orthovoltage beam had the smallest correction. Future work will include adding insensitive components to the simulation (such as the stem) that may scatter dose into the active volume in an energy-dependent manner.