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Absorbed Dose to Water and Air Kerma Calibrations in Co-60 Therapy Level Gamma Ray Beams


Three dosimetry calibration laboratories accredited by the American Association of Physicists in Medicine (AAPM) routinely perform calibrations of a large number of instruments used in hospitals across the U.S. for medical applications.


Measurements provided by these accredited dosimetry calibration laboratories (ADCLs) are directly traceable to the national standard for absorbed dose to water and air kerma from 60Co therapy level gamma ray beams held at NIST. This is achieved by the use of high quality ionization chambers that are sent by the ADCLs to NIST for calibration. In addition to calibrations, all three ADCLs participate periodically in a proficiency test (blind test) with NIST. The first proficiency test conducted between NIST and the ADCLs after the change in the air kerma standards from 2003 was published in the Medical Physics Journal (Minniti et al, Med. Phys. 33 (4), p. 1074, April 2006). Since then, periodic proficiency tests have been conducted between NIST and the ADCLs to ensure the accuracy of radiation measurements in hospitals nationwide. A total of fifteen proficiency tests involving all three ADCLs (five tests per ADCL) have been conducted in the last 10 years as of 2015 in the NIST therapy level 60Co radiation beam facility. An outstanding agreement of better than 0.3% has been obtained in each and every one of these blind tests which indicates the strength and quality of measurements involved between all these institutions. In addition to measurements invoving the ADCLs, calibrations of ionization chambers performed in the NIST 60Co therapy level reference beams contribute to a number of other ongoing projects at NIST and elsewhere. These include work in support of the development of air kerma and absorbed dose standards in high energy x-ray beams produced by linacs and proton beam accelerators. Passive dosimeters including different types of radiochromic film and thermoluminescent dosimeters are also characterized in these facilities.

Co60 Therapy
Photograph by: Ronaldo Minniti

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Ronaldo Minniti