Trends in the dose calibrator surveys

Full text: The dose calibrator surveys of 1991, 1994 and 1996 showed that >70 per cent of institutions surveyed were within 5 per cent of the ANSTO standards. Sufficient data have now been collected to assess trends over time, in particular the stability of calibrators and the effects of calibrator replacement. Not all institutions participated in all surveys for all isotopes and approximately one third of the calibrators initially surveyed had been replaced by 1996, reducing the number that could be followed for stability over this period to less than 30. The mean and standard deviation of the differences from the ANSTO standard were determined for each isotope for each year and after grouping by manufacturer. The result of an identifiable individual calibrator at a survey (e.g. 1991) was subtracted from its result at another time (e.g. 1996) to assess long-term stability. Tests for statistical significance were done by paired and non-paired t-tests. Stability over the period 1996-1991 was better than two per cent for 99mTc and 131l (6 GBq and 600 MBq), although the lower result for 131l (6 GBq), -1.6 ± 1.9, was statistically significant (p<0.01). Stability for 67Ga and 201TI over the period 1996-1994 was poorer at -3.4 ± 4.6 (p<0.001) and +1.7 ± 9.5 respectively. At every survey, for all isotopes bar 201TI in 1996, the mean difference between readings obtained at institutions and the standards was negative, especially so for 99mTc in 1994 (-2.5 ± 5.1) and 131l (6 GBq) in 1994 and 1996 (-2.1 ± 2.6 and -2.8 ± 2.4 respectively). Analysis by manufacturer showed that a group of new calibrators in the 1994 and 1996 surveys had readings on average three per cent-five per cent lower than the ANSTO standard for 99mTc, 131l and 201TI. Calibrators from other manufacturers with significant numbers of units in the field on average were within two per cent of most of the ANSTO standards. The ANSTO standards have been compared with those at the BIPM (Paris) and shown to be negligibly different for 131l, about 2 per cent higher for 99mTc and about two cent and three per cent lower for 201TI and 67Ga respectively