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National Air-Kerma Standard for Electronic Brachytherapy


The Dosimetry Group has developed a new facility for the calibration of the Xoft miniature x-ray source, which provides low-energy x-rays (< 50 keV) for electronic brachytherapy applications.


The electronic brachytherapy calibration range incorporates the Lamperti free-air chamber (FAC), a well-ionization chamber, and a high-purity germanium (HPGe) spectrometer to determine the air-kerma of miniature x-ray sources. The 2014 measurement comparison between NIST and a university active in medical physics research resulted in agreement at the 1 % level of the well-ionization chambers. Recent changes to the source measurement geometry resulted in an improved comparison process and a less complicated source alignment procedure. The angular dependency of the source output was incorporated into the value of air kerma for the recent comparison. The new geometry and the ability to directly measure the air attenuation correction was investigated along with the influence of calibration apparatus materials on scattering. Spectra were determined using the HPGe detector with a pinhole collimator. The uncertainty analysis for the recent comparison was updated. Plans to provide traceability to the manufacturer of the sources is underway as well as progress towards the NIST calibration services guide on this project. NIST traceability of clinical source strength measurements will improve both the efficacy and safety of electronic brachytherapy treatments. Establishing the air kerma standard for the current generation of electronic brachytherapy sources will allow baseline dose studies for various medical applications that employ these small x-ray sources. A report describing the air-kerma determination of the electronic brachytherapy sources using the Lamperti chamber is available in the NIST Journal of Research, A description of the service and procedures of the measurement of air-kerma and the calibration of well-ionization chambers using the electronic brachytherapy sources is in draft form. The resulting NIST calibration services guide will support the inclusion of this procedure into the NIST 17025 quality assurance document.

electronic brachytherapy laboratory
Electronic Brachytherapy Laboratory showing Lamperti Free-Air Chamber. Credit: NIST/M. O’Brien

Lead Organizational Unit:


Michelle O'Brien