Calibration Service: Absorbed Dose to Water and Air Kerma Calibration in ⁶⁰Co Therapy Level Gamma Ray Beams

Radiation dose measurements provided by 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 using 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 journal Medical Physics (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. In the more than 20 + years since the 2003 test, regular proficiency tests have been conducted between NIST and all three ADCLs in the NIST therapy level ⁶⁰Co radiation vertical beam facility. An outstanding agreement of better than 0.3 % has been maintained through the years with these blind tests which indicates the quality of measurements involved between all these institutions. In addition to measurements involving the ADCLs, calibrations of ionization chambers performed in this NIST ⁶⁰Co therapy level measurement facility contribute to other metrological applications at NIST and elsewhere in support of radiation protection, security measurements and other research projects to support Megavoltage Photon beam metrology.