Thyroid Intercomparison Program

The National Calibration Reference Centre for Bioassay and In Vivo Monitoring (NCRC) has specially designed neck phantoms that allow a facility to establish that the quality of their thyroid monitoring program, where necessary, meets the requirements of Regulatory Standard S-106. The phantoms, provided to participants in the NCRC's intercomparison program, can also be used to establish calibration factors.

The benefit of participation in the thyroid intercomparison program is two-fold. First, facilities can compare their results to other Canadian facilities and judge their performance based on the results. Second, and more important, the participation in the NCRC's intercomparison program allows the facility to show that their in-house calibrations are accurate and that their quality assurance program is performing as expected. The use of an outside independent standard gives any quality assurance program more credibility than it would otherwise have if all results were based on in-house data.

Each year, participants in the thyroid intercomparison program are mailed a kit that contains the following: a neck phantom modeled from a human neck made of material to closely approximate human tissue, inserts designed to mimic a real thyroid gland containing simulated ¹³¹I, and/or ¹²⁵I, an overlay plate to allow for different thickness of tissue over the thyroid gland, instructions for the proper use of the phantom (including a training video), a report form, and a telephone "hot-line" number for assistance. The NCRC provides advice and assistance to participating facilities to solve problems identified through their participation in the thyroid intercomparison program and maintains a historical, confidential data base for efficient information retrieval and trend analysis.

The tests that can be performed with the neck-thyroid phantoms are:

• Accuracy of counting
• Determination of the Minimum Detectable Activity (MDA)
• Effect of overlaying tissue
• Precision of counting

Accuracy of Counting

The accuracy of counting B is obtained by evaluating the bias of the facility for any given phantom. The bias is given by the following expression:

Where:

The acceptable limit for bias, B(%), is that it should be greater than or equal to -25% and less than or equal to +50%. The activity in the test phantom must be greater than five times the MDA specified in Table 2 of Regulatory Standard S-106 for that nuclide.

The actual activities of the inserts used and the bias results are sent to the participating facility in the form of a short report. The facility's contact person is telephoned, when necessary, to discuss remedial action that should be taken to improve the performance of their monitoring system.

Determination of the Minimum Detectable Activity

The Minimum Detectable Activity should be determined using an uncontaminated subject, and applying the following expression to the results.

Where:

BCKND:   is the total number of counts in the region of interest for a given radionuclide.

E:   is the calibration factor used to convert the count rate to activity and includes geometry factors.

T   is the count period, usually in seconds (assumed to be the same for sample and background).

Effect of Overlying Tissue

The facility can use the overlay plate to investigate the change in efficiency for their counting system. The change in counting efficiency will depend on detector type, detector size, counting geometry, and collimation.

Precision of Counting

Precision of counting is performed by counting the neck-thyroid phantom repeatedly. Between each count the phantom is removed and replaced in the counting position. The phantom is counted five times. Precision S is then estimated by the following expression:

Where:

The acceptable limit for precision, (S%), is that it should be less than or equal to 40%.

Historical Data

The Thyroid Intercomparison Program (TIP) has been in existence since 1989. The number of participating facilities in 1989 was 17. Participation has steadily grown so that currently there were 300 participants in 2001. As Figure 1 shows, the number of intercomparison tests has remained approximately constant since 1993.

Figure 1

Confidentiality of Results

The results contained in this report will not be released to any other facility. Any reference to the results contained in this report will maintain the anonymity of your facility. The only exception is the results that are reported to the Canadian Nuclear Safety Commission (CNSC) without anonymity.

Previous Results

Figure 2 shows how facilities have improved in their measurement capability since 1989.

Figure 2

Summary Data for 1989 - 2004

Year	Pass (%)	Fail (%)	Calibration (%)
1989	28.1%	71.9%	0.0%
1990	33.9%	66.1%	0.0%
1991	31.5%	68.5%	0.0%
1992	51.3%	48.7%	0.0%
1993	80.1%	19.9%	0.0%
1994	80.9%	19.1%	0.0%
1995	73.3%	15.5%	11.3%
1996	83.6%	13.0%	3.4%
1997	81.7%	12.8%	5.5%
1998	88.0%	6.5%	5.6%
1999	84.6%	9.0%	6.3%
2000	79.8%	2.8%	17.4%
2001	85.0%	4.0%	11.0%
2002	85.0%	4.00%	11.00%
2003	91.1%	2.69%	6.18%
2004	89.6%	0.87%	9.57%

In 2004, 89.6% of the measurements satisfied the CNSC's bias criteria.