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Interpreparation of Bioassay Results
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DOE Standard Guide of Good Practices for Occupational Radiological Protection In Plutonium Facilities
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Urine Sample Results - doe-std-1128-98_ch10137


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).
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