Novel, preclinical radiotherapy modalities are being developed at synchrotrons around the world, most notably stereotactic synchrotron radiation therapy and microbeam radiotherapy at the European Synchrotron Radiation Facility in Grenoble, France. The imaging and medical beamline (IMBL) at the Australian Synchrotron has recently become available for preclinical radiotherapy and imaging research with clinical trials, a distinct possibility in the coming years. The aim of this present study was to accurately characterize the synchrotron-generated x-ray beam for the purposes of air kerma-based absolute dosimetry.
The authors used a theoretical model of the energy spectrum from the wiggler source and validated this model by comparing the transmission through copper absorbers (0.1–3.0 mm) against real measurements conducted at the beamline. The authors used a low energy free air ionization chamber (LEFAC) from the Australian Radiation Protection and Nuclear Safety Agency and a commercially available free air chamber (ADC-105) for the measurements. The dimensions of these two chambers are different from one another requiring careful consideration of correction factors.
Measured and calculated half value layer (HVL) and air kerma rates differed by less than 3% for the LEFAC when the ion chamber readings were corrected for electron energy loss and ion recombination. The agreement between measured and predicted air kerma rates was less satisfactory for the ADC-105 chamber, however. The LEFAC and ADC measurements produced a first half value layer of 0.405 ± 0.015 and 0.412 ± 0.016 mm Cu, respectively, compared to the theoretical prediction of 0.427 ± 0.012 mm Cu. The theoretical model based upon a spectrum calculator derived a mean beam energy of 61.4 keV with a first half value layer of approximately 30 mm in water.
The authors showed in this study their ability to verify the predicted air kerma rate and x-ray attenuation curve on the IMBL using a simple experimental method, namely, HVL measurements. The HVL measurements strongly supports the x-ray beam spectrum, which in turn has a profound effect on x-ray dosimetry.
P.A.W.R. and R.A.L. were in receipt of a project grant (Grant No. APP606614) from the National Health & Medical Research Council (NH&MRC) of Australia. J.C.C. is in receipt of an Early Career Researcher Fellowship from the NH&MRC of Australia. The authors acknowledge technical assistance and advice from the following individuals: K. Wootton, P. Bennetto (University of Melbourne), D. Hausermann, A. Maksimenko (The Australian Synchrotron), T. Ackerly, F. Gagliardi, N. Brouwer (Alfred Hospital), and D. Butler (ARPANSA). The authors thank the reviewers and Associate Editor of the paper for providing insightful and constructive comments.
I. INTRODUCTION II. MATERIALS AND METHODS II.A. X-rayspectrum calculation II.B. Experimental measurements II.B.1. Ionization chambers and air kerma rate II.B.2. Assessing the cross-sectional area of the beam II.B.3. Electron loss and ion recombination correction factors II.B.4. Experimental uncertainty and error bars II.C. Comparison with theory III. RESULTS IV. DISCUSSION AND CONCLUSIONS