Dosimetry intercomparison using a 35-keV x-ray synchrotron beam

Abstract

Significance: Photon activated Auger electron therapy utilizes a keV-ranged, monoenergetic x-ray beam, and radiobiological and animal experiments studying this therapy require accurate dosimetry techniques. However, there exist few dosimetry protocols for low-energy x-ray beams. This research intended to use the CAMD synchrotron as a source of monochromatic, 35-keV x-rays and test dosimetry techniques of film and ion chamber. The hypothesis of my research was that depth-dose measured in a PMMA phantom using an air-equivalent ionization chamber and radiochromic film dosimeters in a PMMA phantom irradiated by a 35-keV, monochromatic x-ray beam will agree to within 5% of each other and to within 5% of dose determined from fluence-scaled Monte Carlo dose simulations. Methods: The narrow beam produced on the CAMD tomography beamline (0.1×2.8 cm2) was effectively broadened (2.5×2.8 cm2) by vertically oscillating experimental apparatus. Beam energy selected by the monochromator was verified using a Compton scatterer and powder diffraction methods. Depth-dose in PMMA was measured by an air-equivalent ion chamber using a modified AAPM’s TG-61 (100-300 kV) formalism and by Gafchromic EBT film using 125-kVp calibration curves. Depth-dose was also determined from scaling MCNP5 Monte Carlo output by fluence measured using a NaI detector. Results: The powder diffraction energy measurement agreed closest to the monochromator’s setting (mean = -0.1±0.3 keV). Depth-doses performed on 5 separate experimental dates showed that beam fluence did not accurately scale to synchrotron ring current between dates. Depth-dose measurements from ion chamber and film at 2 cm resulted in film-measured dose underestimating ion-chamber measured dose by an average of 5.0±2.1%. Fractional Monte Carlo depth-dose simulations agreed well with ion chamber and film measurements, with maximum disagreements of 3.9% at 9.0-cm depth and 0.9% at 8.25 cm, respectively. Fluence-scaled, Monte Carlo dose determination overestimated ion chamber-measured depth-dose by 6.4±0.8% and overestimated film-measured depth-dose by 9.1±0.7%. Conclusions: Results of this research were unable to prove or disprove the hypothesis regarding 5% agreement of ion chamber and film dose measurements. Results also proved the hypothesis false for achieving 5% agreement between either ion chamber-measured dose or film-measured dose and dose determined from fluence-scaled Monte Carlo simulations.