The second international in-vivo monitoring intercomparison program for whole body counting facilities by Canadian and United States agencies.

The Canadian National Calibration Reference Centre for In-Vivo Monitoring and the United States Department of Energy collaborated to offer a second international in vivo intercomparison program to whole body counting facilities in 1996. This program used a Reference Female phantom shell filled with radioactive tissue-substitute polyurethane to simulate a uniform fission-product distribution in soft tissues. The nuclides used were 137Cs and 60Co. The phantom also contained 40K homogeneously distributed in an amount similar to a Reference Female to produce a representative Compton background in the resulting spectra. Participants were asked to identify the nuclides and report activities for all except 40K. They were also asked to measure the precision of counting and supply the MDA for 137Cs and 60Co. The bias results were in the range of -30% to +80% with most facilities falling inside the range of -25% to +50% (Canadian and U.S. acceptable performance criteria). Results indicated that there was no measurable size dependency for this phantom. All reported precisions were less than 5% but NaI detector based systems seemed to have a systematic uncertainty in addition to Poisson variability. Contrarily, this was not found for Ge detector based systems. MDA data was scattered (14-3,500 Bq for 137Cs and 9-460 Bq for 60Co) and only suggested that lengthening the counting time improves MDA.

An internal dosimetry intercomparison study.

Pacific Northwest Laboratory performed a study to evaluate the consistency of internal dosimetry assessments. A total of eleven laboratories, including DOE sites and NRC licensees, participated in this intercomparison study. Participants were asked to respond to five actual exposure scenarios, previously used in a similar European study. The participating dosimetrists assessed the data of the test scenarios and calculated results in terms of estimated radionuclide intake and the resulting internal doses. To maintain confidentiality, results are given without identifying any site. Except for one scenario, the results showed that the standard deviation of the final results on committed effective dose equivalent for each exposure scenario was about 30-50% of the mean value, giving a consistency slightly greater variant than that of the European study. The discrepancies can be attributed to variations in 1) the interpretation and statistical treatment of the bioassay data; 2) the biokinetic models applied; and 3) the computational tools used. This represents a preliminary study; further intercomparison testing is needed to fully evaluate the problem of dose-assessment inconsistency.