In Vivo Count Results - doe-std-1128-98

DOE-STD-1128-98

5.7.1 In Vivo Count Results

In vivo plutonium or americium measurements are generally relatively insensitive

with regard to levels of occupational exposure concern. This applies particularly to

routine chest or lung counting, skeleton counting, and liver counting. For that reason,

any detection of plutonium or americium should be investigated. The investigation

should address the validity of the measurement by reviewing the spectrum and its

associated background subtraction. These reviews are particularly important if the

result is near the Lc. Follow-up to a positive result should include a confirming

measurement. Ideally, this should be an immediate (same day) recount of equal or

higher sensitivity. The farther removed in time a verification measurement is from

the original measurement, the more important it becomes to factor in potential lung

clearance in comparing the two measurements. A follow-up measurement taken 30

days after an initial high-routine may not be capable of providing verification if the

material of concern exhibits class W behavior.

Chest-wall thickness has a significant impact on chest counting. Corrections are

commonly made using a height-to-weight ratio or ultrasonic methods (Kruchten and

Anderson, 1990).

Corrections may be required to address apparent detection in one tissue resulting

from photon crossfire from another tissue. For example, chest counting is performed

primarily to estimate activity in the lung. Yet, there is substantial bone over the lungs

(rib cage, sternum) and behind the lungs (vertebrae). Plutonium and americium are

both bone-seeking radionuclides which will deposit on those bone surfaces and can

interfere with chest counting. It is possible for a person having a systemic burden of

plutonium from a wound in the finger to manifest a positive chest count from

material translocated to the skeleton, axillary lymph nodes, or liver (Carbaugh et al.,

1989; Graham and Kirkham, 1983; Jeffries and Gunston, 1986). Interpreting such a

chest count as a lung burden can render dose estimates somewhat inaccurate.

When comparing in vivo measurements made over many years, it is important to

make sure that the measurements are, in fact, comparable. One consideration is to

make sure that corrections have been consistently applied to all similar

measurements. It is not unusual for measurement systems to be replaced or to change

the algorithms used for calculating results over time. Step changes in data can occur

and should be addressed by monitoring long-term detectable trends (Carbaugh et al.,

1988).

5-26