Lung Counting

The National Calibration Reference Centre for Bioassay and In Vivo Monitoring (NCRC) has two torso phantoms and various lung phantoms that allow a facility to establish that the quality of their lung 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 lung counting intercomparison. There are several tests that can be performed using a realistic torso phantom containing a lung phantom. These tests enable the NCRC to evaluate the performance of a lung counting facility.

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 tests the facility on their ability to identify an unknown radionuclide(s) in the lung. The lung phantoms used are either manufactured by the NCRC or approved contractors to the NCRC.

Size Dependency

The NCRC has two realistic torso phantoms of different sizes that can be used to test the facility's size dependency. The phantoms are the Lawrence Livermore National Laboratory (LLNL) Torso phantom and the Japan Atomic Energy Research Institute (JAERI) Torso Phantom.

LLNL Torso Phantom

This phantom is manufactured by Radiology Support Devices Long Beach, California, USA (RSD). The phantom extends from the base of the neck to just below the lower margin of the liver. It has no head, neck, arms or lower torso. It is molded about a plastic skeleton, chosen because variations in cadaveric human skeletons would distort the very low energy outputs of the transuranic nuclides for which it is designed. The interior of the thorax is hollow (except for the skeleton), and it must be filled with organs or spacer blocks to eliminate air spaces with the thorax. Access to the interior of the thorax is gained through a cover plate which contains the sternum and the balance of the ribs. This is molded as a separate unit to make a proper fit with the thorax and to avoid loss of material which might occur if the cover plate were cut from the thorax. The phantom is designed for couch counting only.
The phantom available from RSD is a third generation phantom. The NCRC has a second generation phantom (only 16 exist worldwide). The International Atomic Energy Agency (IAEA) and the United States Department of Energy (USDOE) Phantom Library have a first generation phantom (only three exist worldwide).

Lung Sets Available

Filled by a manufacturer	Filled by the NCRC
Americium 241	Carbon 14
Plutonium 238	Low Enriched Uranium (23%)
Natural Uranium	Americium 241
Natural Thorium	Cobalt 57
Enriched Uranium (97%)	Natural Uranium
Americium 241/Europium 152	Europium 152
Sectioned Americium 241/Europium 152	Other nuclide by request
Blank Lung Sets (2)
Sectioned Blank Lung Set
Sliced Blank Lung Set

JAERI Torso Phantom

This phantom is manufactured by Kyoto Kagaku Co. Ltd., Japan. It was developed to simulate Japanese Reference Man. The NCRC uses this phantom to better represent female workers. The phantom contains removable model organs (lungs, liver, kidneys and heart), model trachea and artificial rib cage, and also includes chest plates that can be placed over the chest to simulate a wide range of adipose/muscle ratios. The lungs and liver are sliced so planar sources can be inserted in the organs to simulate a wide variety of distributions.

Lung Sets Available

Filled by a manufacturer	Filled by the NCRC
Americium 241/Europium 152	Americium 241
Plutonium 239	Natural Uranium
Natural Uranium	Europium 152
Sectioned Americium 241/Europium 152	Other nuclide by request
Sectioned Blank Lung Set
Americium 241
Enriched Uranium (99.8%)

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 geome try 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 torso phantom repeatedly. Between each count the phantom is removed and replaced in the counting position. The phantom is counted five (5) 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%.

Accuracy of Counting

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

Where:

The acceptable limit for bias, B(%), is that it should be <= -25% and <= +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 Overlaying Tissue

Thickness

Lung counting efficiency varies greatly with chest wall thickness. A facility must have chest wall thickness correction factors available to make an accurate estimate of activity in a subject's lung.
The LLNL phantom can be used with an overlay plate that will increase the chest wall thickness. The range of thicknesses that can be simulated varies from about 1.6 cm (no overlay) to 6.5 cm (thickest overlay plate added).
The JAERI phantom is more limited in its range of chest wall thickness. The range of thicknesses that can be simulated varies from about 2.0 cm (no overlay) to 3.5 cm (thickest overlay plate added).

Composition

Low energy photons (< 60 keV) are attenuated differently by adipose and muscle tissue. Both phantoms have overlay plates of differing adipose-muscle ratios available to quantify this effect.