The US radiation dosimetry standards for {sup 60}Co therapy level beams, and the transfer to the AAPM accredited dosimetry calibration laboratories

• Published in: Medical physics (Lancaster) 2006-04, Vol.33 (4)
• Main Authors: Minniti, R., Chen-Mayer, H., Seltzer, S.M., Huq, M. Saiful, Bryson, L., Slowey, T., Micka, J.A., DeWerd, L.A., Wells, N., Hanson, W.F., Ibbott, G.S., University of Pittsburgh Cancer Institute, Department of Radiation Oncology, Pittsburgh, Pennsylvania 15232, K and S Associates, Nashville, Tennessee 37210, The University of Wisconsin, Radiation Calibration Laboratory, Madison, Wisconsin 53706, University of Texas MD Anderson Cancer Center, Department of Radiation Physics, Houston, Texas 77030-4095
• Format: Article
• Language: English
• Subjects: CALIBRATION STANDARDS; COBALT 60; DOSIMETRY; GAMMA RADIATION; IONIZATION CHAMBERS; KERMA; RADIATION PROTECTION AND DOSIMETRY; RADIATION SOURCES; RADIOTHERAPY; SAFETY STANDARDS
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.2184442

This work reports the transfer of the primary standard for air kerma from the National Institute of Standards and Technology (NIST) to the secondary laboratories accredited by the American Association of Physics in Medicine (AAPM). This transfer, performed in August of 2003, was motivated by the recent revision of the NIST air-kerma standards for {sup 60}Co gamma-ray beams implemented on July 1, 2003. The revision involved a complete recharacterization of the two NIST therapy-level {sup 60}Co gamma-ray beam facilities, resulting in new values for the air-kerma rates disseminated by the NIST. Some of the experimental aspects of the determination of the new air-kerma rates are briefly summarized here; the theoretical aspects have been described in detail by Seltzer and Bergstrom [''Changes in the U.S. primary standards for the air-kerma from gamma-ray beams,'' J. Res. Natl. Inst. Stand. Technol. 108, 359-381 (2003)]. The standard was transferred to reference-class chambers submitted by each of the AAPM Accredited Dosimetry Calibration Laboratories (ADCLs). These secondary-standard instruments were then used to characterize the {sup 60}Co gamma-ray beams at the ADCLs. The values of the response (calibration coefficient) of the ADCL secondary-standard ionization chambers are reported and compared to values obtained prior to the change in the NIST air-kerma standards announced on July 1, 2003. The relative change is about 1.1% for all of these chambers, and this value agrees well with the expected change in chambers calibrated at the NIST or at any secondary-standard laboratory traceable to the new NIST standard.