Room: AAPM ePoster Library
Purpose: To assess the accuracy of ion chamber air kerma measurements in synchrotron-produced monochromatic X-ray beams and determine the output calibration of the beams for therapeutic applications.
Methods: A C-552 walled, Exradin A1SL ion chamber was calibrated at the NRCC in a series of narrow X-ray beam qualities, using a primary standard free air chamber. The effective photon energy was chosen to correspond to the synchrotron beam energies being investigated. The ion chamber was then irradiated at a synchrotron facility using monochromatic beams of 65, 83, 100 and 115 keV energy from the BMIT-ID beam line at the Canadian Light Source. Standard methods were used to evaluate ion recombination, polarity and beam stabilization effects and EBT3 Gafchromic film was used to measure the uniformity of the field over the sensitive volume of the chamber. The results were compared to the nominal kerma rates provided by the synchrotron facility which were based on a manufacturer-calibrated PTW PinPoint 31014 chamber.
Results: The results indicate that the ion recombination and polarity correction factors are an order of magnitude less than the non-uniformity correction of the field over the chamber volume. The differences between the nominal kerma rates provided by the synchrotron facility and these measurements ranged between 4 % and 35 % once both detectors were corrected for the non-uniformity of the field. The estimated standard uncertainty on the measurements is 2.3 % at the k=1 level.
Conclusion: This investigation has indicated that the dominant correction factor for ion chamber measurements in the monochromatic synchrotron beams investigated is field non-uniformity. Although the current nominal beam calibration is acceptable for imaging applications, these results suggest that a different detector type and geometry is required for the accurate determination of air kerma, and ultimately, absorbed dose for therapeutic investigations.