The 1993 Intercomparison of the Measurement of In Vivo Radioactivity

The Canadian National Calibration Reference Centre for In-Vivo Monitoring and the United States Department of Energy collaborated to offer an international intercomparison programme to whole-body counting facilities in 1993. The Human Monitoring Laboratory fabricated a phantom shell corresponding to a reference four-year-old child and Battelle Pacific Northwest National Laboratory filled the shell with radioactive tissue substitute polyurethane to simulate a uniform natural radioactivity and fission product distribution in soft tissues ( 40 K, 88 Y, 137 Cs). Participants were not informed of the radioactive content and were asked to determine the identity and amount of the radionuclides in the energy range 200-2000 keV. Although all nuclides were identified correctly, most participants reported activities for 88 Y and 137 Cs only. Each facility was asked to make an estimate of the 'worst case' precision and estimate their minimum detectable activity. The programme had 35 participants from 18 countries for a total of 43 counting systems. The programme began in April 1993 and ended on 1 Aug. 1995. The intercomparison has shown that all whole-body counters are phantom-size dependent and that size-dependent calibration factors should be used. The scanning bed geometry is less size-dependent than the other geometries when only the standard (man-sized) calibration factors are used; however, when applied, size correction factors remove this geometrical advantage and all configurations perform equally well.