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 (K-40,Y-88,Cs-137). 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 Y-88 and Cs-137 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.