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

The purpose of this study was to compare dose measurements using ion chamber and radiochromic film dosimetry for a 35-keV synchrotron beam useful for Auger electron therapy. A 1.3-GeV electron beam, transported through a 3-pole superconducting wiggler magnet, produced a polychromatic photon beam from which a 35-keV beam (3.3 mm Al HVL) was selected using a monochromator. A 2.8 cm × 2.5 cm field was produced by vertically oscillating a polymethylmethacrylate phantom in which dose to water was measured as a function of depth. Charge, measured using a 0.23-cm3 cylindrical, air-equivalent ionization chamber, was converted to dose using American Association of Physicists in Medicine TG-61 protocol for 40-300 kV X-ray beam dosimetry with minor assumptions. Optical density of radiochromic film (Gafchromic® EBT) was converted to dose using a 125 kVp X-ray beam (2.9 mm Al HVL) calibration curve. Fractional depth-dose curves measured using ion chamber and film agreed well with each other, the maximum difference being 4.5% at 8.85 cm. Both agreed well with that predicted by MCNP5 Monte Carlo calculations. At 2.0-cm depth, film doses from five independent measurements predicted 0.952 ± 0.022 of dose measured using the ion chamber. Dose measurements using two independent methods, ionization chamber and radiochromic film dosimetry, showed good agreement and should be suitable for future dosimetry necessary for cell and small animal irradiations. Improving agreement will require additional investigations of methods for converting ionization and film optical density to dose.