Wholebody Counting Intercomparison - 2003

Protocol of Testing

Each year the staff of the HML visits the user's site to perform the intercomparison. Many tests can be performed with the BOMAB phantoms and the protocol of testing of whole body counting facilities varies from year to year. The variability of testing is due to the inability of the NCRC to transport all of the necessary phantoms to the user's site. The practical maximum number of phantoms that can be used in a given test is six.

The tests that can be performed with these phantoms are:

• Identification of unknown internal contaminants
• Identification of the distribution of the contaminants
• Size dependency
• Location dependency
• Determination of the Minimum Detectable Activity
• Precision of counting
• Accuracy of counting

Identification of unknown internal contaminants.

What is the radionuclide(s)?

Each year the NCRC creates one or two new unknown phantoms that will be used in the intercomparison. The radionuclide is chosen to represent a possible contaminant that might be found in the workplace; however, sometimes an unusual radionuclide is chosen to test the facility's resources.

Identification of the distribution of the contaminants.

Where is the radionuclide(s)?

The unknown radionuclides may be distributed homogeneously throughout the phantom or it maybe localised in some sections. The localisation usually follows the ICRP metabolic recommendations (e.g., localisation in the thyroid for radioiodine) or may be inspired by a hypothetical accident scenario. In the latter case the facility is informed of the hypothetical accident.

Size dependency.

The NCRC has created the BRMD BOMAB phantom family (Kramer at al. 1991) that currently consists of the following phantoms:

P4 and P10 are NOT used for the Intercomparisons.

• P4 series: Reference four year old. Designed using ICRP 23 data.
• P10 series: Reference 10 year old. Designed using ICRP 23 data.
• PF series: Reference female. Designed using ICRP 23 data.
• PM series: Reference male. Designed using ICRP 23 data.
• PM5 series: Five percentile male. This phantom was designed using Canadian anthropomorphic data and represents the 5th percentile weight in the 5th percentile height category.
• PM95 series: Ninety-five percentile male. This phantom was designed using Canadian anthropomorphic data and represents the 95th percentile weight in the 95th percentile height category. This phantom also has two chest sections (due to the weight) that simulate a lung and lower chest.
• PM_acc addition: A chest piece that better simulates lung and/or liver depositions. It is used with the reference male series (PM).
• PF_acc addition: A chest piece that better simulates lung and/or liver depositions. It is used with the reference female series (PF).

The BOMAB phantom family also has a series of overlay plates for the PM and P4 series. The plates compensate for individuals that have more adipose tissue on their chest, gut, and thighs.

Determination of the Minimum Detectable Activity.

The Minimum Detectable Activity (MDA) should be determined using a phantom containing ⁴⁰ K, or better still an uncontaminated person. The MDA will be function of the region of interest defined in the spectrum and several regions should be selected at high and low energies. Providing that the counting time of the subject and the background is equal a simplified equation may be used to calculate the MDA.

The MDA (HPS 1996) is given by:

Where: BCKGND = the background counts in the region of interest (ROI), E = counting efficiency for that ROI, and T = counting time.

Location dependency.

This test can be performed using two phantoms: one phantom is blank or contains only ⁴⁰ K and the other phantom, known as the active phantom, will contain ⁴⁰ K and other homogeneously distributed radionuclides. During the test the active phantom is first counted and then selected sections of the phantom are replaced with sections from the blank phantom.

Typical tests that can be performed with the phantoms include:

• Activity in the extremities only.
• Activity in the chest only.
• Activity in the gut only.

Precision of counting.

Precision of counting is performed by counting a selected phantom repeatedly. Between each count the phantom is removed and reassembled in a slightly different position. The phantom is counted five times. Precision, S (%), is then estimated by the following expression:

Where: Ai is the observed value, M is the mean of the data set, and N is the number of measurements (usually 5).

The relative precision of the measurement process can also be expressed in terms of the bias by calculating the relative dispersion of the bias values, B, from their mean, B_r, and is defined to be:

The absolute values of the precision of the bias, S_B, should be less than or equal to 40% (HPS 1996) and S_B is included in the results analysis.

Accuracy of counting.

The accuracy of counting is obtained by evaluating the bias of the facility for any given phantom series. The bias for each phantom and radionuclide, B_i, is given by the following expression:

Where A_i is the observed value, and A is the true value.

The acceptable limit for bias, B(%), is given by (CNSC 1998) as greater than or equal to -25% and less than or equal to 50% for activities greater than five times or more the facility's MDA.

The Overall Relative Bias, Br, can be calculated from the individual bias values, B_i, by the following expression:

This quantity should also lie within the range -25% to 50% (HPS 1996) and Br is included in the results analysis.

This Year's Tests

The test performed this year were:

• Size dependency (PFB, PFC,PF-_Acc).
• Identification of unknown internal contaminants (PFB, PF-_Acc, PFC, PME, PM-_Acc1, PMF).
• Accuracy of counting (PFB, PF-_Acc, PFC, PME, PM-_Acc1, PMF).
• Location dependence (PM-_Acc1, PF-Acc).
• Effect of an interferent ( ²²Na in PFC and ¹³⁴Cs in PMF).