224
18 Radiological Assessment and Public Health Response
In vitro bioassays begin with collecting a sample, typically of excreta. For most
radionuclides, especially those that are in a chemical form that is not insoluble, this
will be a urine sample, and an aliquot will be extracted from this sample, or the
sample will be evaporated to dryness or incinerated to concentrate the radionuclide
in order to enhance detection capability. The aliquot will then be counted using an
appropriate detector depending on the type of radiation emitted by the nuclide in
question and the final activity concentration in the excreta will be used to determine
the amount of uptake.
For example, if studies have shown that, say, 1% of ingested radioactivity is
excreted in the urine in the first 24 h following an inhalation of a particular nuclide
and then we measure 100 Bq of radioactivity in the urine then we can conclude
that the person had an intake of 10 kBq of radioactivity, and we can calculate their
radiation exposure accordingly (Fig. 18.2).
Sensitivity and accuracy: In vivo bioassays are obtained by counting samples with
radiation detection equipment for a given period of time. The sensitivity and accuracy
of the analysis depends on a number of factors, including the accuracy of the radiation
detector, the amount of time for which the sample is counted, the counting efficiency
of the radiation detector for the nuclide(s) in question, and various inaccuracies
involved in collecting and measuring the sample. The sensitivity of in vivo bioassay
varies from about 1 to 100 Bq and measurement accuracy varies between about
±10–30%, depending on the factors noted above and the nuclide(s) being counted
[1].
In vitro bioassays can be more sensitive than in vivo bioassays because there is
no tissue providing shielding between the sample and the detector; in vitro bioassays
can quantify samples with much less than 1 Bq per sample.
Limitations: This technique requires properly calibrated radiation detectors and
counting equipment and the calculated results can vary considerably depending on
the person(s) being counted for in vivo bioassay. In addition, every person’s body
processes radionuclides differently and biokinetic models developed for a reference
person likely differ from those exhibited by any individual.
18.2 Lymphocyte Depletion
Principle: Lymphocytes are blood cells that are sensitive to radiation; when exposed
to high levels of radiation, lymphocytes begin to accumulate DNA damage and to
die. At the same time, the blood-forming organs are also sensitive to radiation so
as the lymphocytes die they are not replaced. Since the rate and degree to which
the lymphocyte count drops is related to the radiation dose received, measuring the
change in the patient’s lymphocyte count can be used to estimate the radiation dose
to which the patient was exposed (Fig. 18.3).
18 Radiological Assessment and Public Health Response
In vitro bioassays begin with collecting a sample, typically of excreta. For most
radionuclides, especially those that are in a chemical form that is not insoluble, this
will be a urine sample, and an aliquot will be extracted from this sample, or the
sample will be evaporated to dryness or incinerated to concentrate the radionuclide
in order to enhance detection capability. The aliquot will then be counted using an
appropriate detector depending on the type of radiation emitted by the nuclide in
question and the final activity concentration in the excreta will be used to determine
the amount of uptake.
For example, if studies have shown that, say, 1% of ingested radioactivity is
excreted in the urine in the first 24 h following an inhalation of a particular nuclide
and then we measure 100 Bq of radioactivity in the urine then we can conclude
that the person had an intake of 10 kBq of radioactivity, and we can calculate their
radiation exposure accordingly (Fig. 18.2).
Sensitivity and accuracy: In vivo bioassays are obtained by counting samples with
radiation detection equipment for a given period of time. The sensitivity and accuracy
of the analysis depends on a number of factors, including the accuracy of the radiation
detector, the amount of time for which the sample is counted, the counting efficiency
of the radiation detector for the nuclide(s) in question, and various inaccuracies
involved in collecting and measuring the sample. The sensitivity of in vivo bioassay
varies from about 1 to 100 Bq and measurement accuracy varies between about
±10–30%, depending on the factors noted above and the nuclide(s) being counted
[1].
In vitro bioassays can be more sensitive than in vivo bioassays because there is
no tissue providing shielding between the sample and the detector; in vitro bioassays
can quantify samples with much less than 1 Bq per sample.
Limitations: This technique requires properly calibrated radiation detectors and
counting equipment and the calculated results can vary considerably depending on
the person(s) being counted for in vivo bioassay. In addition, every person’s body
processes radionuclides differently and biokinetic models developed for a reference
person likely differ from those exhibited by any individual.
18.2 Lymphocyte Depletion
Principle: Lymphocytes are blood cells that are sensitive to radiation; when exposed
to high levels of radiation, lymphocytes begin to accumulate DNA damage and to
die. At the same time, the blood-forming organs are also sensitive to radiation so
as the lymphocytes die they are not replaced. Since the rate and degree to which
the lymphocyte count drops is related to the radiation dose received, measuring the
change in the patient’s lymphocyte count can be used to estimate the radiation dose
to which the patient was exposed (Fig. 18.3).