Comparison of dosimetry calibration factors at the NRCC and the NIST

In early 1998, three transfer ionization chambers were used to compare the air-karma and absorbed-dose-to-water calibration factors measured by the Na tional Research Council of Canada (NRCC) and the National Institute of Stan dards and Technology (NIST). The ratios between the NRCC and NIST calibrati on factors are 0.9950 and 1.0061 in the case of the absorbed-dose-to-water and air-kerma standards, respectively. In the case of the standard of absor bed dose to water, the combined uncertainty of the ratio between the standa rds of the two laboratories is about 0.6% and consequently, the observed di fference of 0.5% is not significant at the one sigma level. In the case of the standard of air karma, the combined uncertainty of the ratio between th e standards of the two laboratories is about 0.4%, and so the observed diff erence of 0.61% is significant at the one sigma level. However, this discre pancy is due to the known differences in the methods of assessing the wall correction factor at the two laboratories. Taking into account changes impl emented in the standards that form the basis of the calibrations, the prese nt results are consistent with those of the previous comparison done in 199 0/91. As a direct result of these differences in the calibration factors, c hanging from an air-kerma based protocol following TG-21 to an absorbed-dos e-to-water based protocol following TG-51, would alter the relationship bet ween clinical dosimetry in Canada and the United States by about 1%. For cl inical reference dosimetry, the change from TG-21 to TG-51 could result in an increase of up to 2% depending upon the ion chamber used, the details of the protocol followed and the source of traceability, either NRCC or NIST. (C) 2000 American Association of Physicists in Medicine. [S0094-2405(00)01 007-5].