Whole Body Counting Intercomparison

The National Calibration Reference Centre for Bioassay and In Vivo Monitoring (NCRC) has a series of BOttle Manikin ABsorption (BOMAB) phantoms to allow a facility to establish that the quality of their whole body counting program meets the requirements of Regulatory Standard S-106.

Each year the staff of the NCRC will visit the user's site to perform the intercomparison. There are many tests that 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
• Effect of Overlaying Tissue

Identification of Unknown Internal Contaminants

Each year the NCRC creates one or two new unknown phantoms that will be used in the intercomparison. The radionuclide is chosen to be fairly representative of a possible contaminant that might be found in the workplace; however, sometimes an unusual radionuclide or mixture of radionuclides are chosen to test the facilities resources. Under this test the facility must correctly identify the radionuclide(s).

Identification of the Distribution of the Contaminants

The unknown radionuclides may be distributed homogeneously throughout the phantom or it maybe localized in some sections. The localization usually follows the International Commission on Radiological Protection (ICRP) metabolic recommendations (e.g. localization 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. Under this test the facility must identify the location(s) of the radionuclide.

Size Dependency

The NCRC has created the BRMD (Formerly the Bureau of Radiation and Medical Devices, now know as the Radiation Protection Bureau) BOMAB phantom family which currently consists of the following phantoms:

• PM series: Reference male. Designed using ICRP 23 data.
• PF series: Reference female. Designed using ICRP 23 data.
• P10 series: Reference ten (10) year old. Designed using ICRP 23 data.
• P4 series: Reference four (4) year old. 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.
• PMacc addition: A chest piece that better simulates lung and/or liver depositions in males.
• PFacc addition: A chest piece that better simulates lung and/or liver depositions in females.
• Overlay plates to simulate extra adipose tissue. Activity is not deposited in the plates. They are available for the PM and P4 series.

Location Dependency

This test is performed using two phantoms: one phantom is blank or contains only 40K and the other phantom, known as the active phantom, will contain 40K 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

Determination of the Minimum Detectable Activity

The Minimum Detectable Activity (MDA) should be determined using a phantom containing 40K, or preferably an uncontaminated subject, The following expression is used to determine the MDA:

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).

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. An example of positioning for a scanning bed shadow shield whole body counter can be viewed.

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%.

Accuracy of Counting

The accuracy of counting is obtained by evaluating the bias of the facility for any given phantom series. The bias, B, 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.

Effect of Overlying Tissue

A series of overlay plates, constructed from high density polyethylene, has been manufactured for the PM series. There is a plate for each of: chest section, gut section, each thigh section. Each plate is approximately 1.25 cm thick. The curvature of each plate is such that the plates can be stacked upon each other to give a total overlay thickness of five cm. The overlay plates are designed to be used to simulate individuals that have a thicker adipose tissue layer than Reference Man. The use of the overlay plate series makes the assumption that any internally deposited activity does not reside in this tissue layer.