18 Radiological Assessment and Public Health Response
223
here) and in a special issue of Health Physics devoted to the subject, including a
summary by Swartz and several colleagues [14].
18.1 Bioassay
Principle: Bioassay is the process of measuring the amount of radioactivity that has
entered the body. There are two major forms of bioassay: in vivo (“in the body”)
bioassay involves directly counting the amount of radioactivity in the body, typically
using scintillation detectors to measure gamma radiation emitted by radionuclides
within the body. A chest count, a thyroid count, or whole-body count—holding a radiation detector over the chest or sitting in a whole-body counter—and measuring the
count rate emanating from gamma-emitters within the body—are examples of in vivo
bioassay. The other, in vitro (“in the glass”) bioassay, involves taking samples—
generally urine, feces, or blood—and extrapolating the amount of intake from the
concentration of radioactivity in the sample.
Procedure: An in vivo bioassay, as noted above, is performed by placing a radiation
detector on or near the body near where the radioactivity is expected to be. A person
who has inhaled radioactivity that is in an insoluble form, for example, should have the
majority of the radioactivity remaining in the lungs; if the person inhaled radioactive
iodine then the iodine will enter the blood rapidly and will then collect in the thyroid.
Thus, holding the radiation detector close to the body over the lungs or the thyroid
(respectively) will produce a count rate; if that count rate is significantly higher than
the normal background count rate then we can say that there is radioactivity in the
lungs or thyroid. Further, if the counting efficiency of the radiation detector for I131 is known and the radiation detector has been characterized with respect to the
attenuation of I-131 gamma photons as they pass through the tissue overlying the
thyroid then we can calculate the amount of radioactivity that has been absorbed by
the thyroid as shown in this text box.
Interpreting thyroid bioassay results
Let’s assume that a radiation detector has a 10% counting efficiency for I-131
gamma ray photons and 90% of the I-131 gammas emitted from the thyroid are
absorbed by the tissues of the neck (for an overall counting efficiency of 1%)
then 1000 cps above background would represent 100,000 dps of iodine; since
1 Bq = 1 dps, the thyroid in question would contain 100 kBq of I-131. From
Federal Guidance Report Number 11 [16] we can find that 2 MBq (2000 kBq)
of I-131 will produce a radiation exposure of 0.5 Sv to the thyroid so we can
easily calculate that 100 kBq (0.1 MBq) will produce a thyroid dose of 25 mSv
to the thyroid.
223
here) and in a special issue of Health Physics devoted to the subject, including a
summary by Swartz and several colleagues [14].
18.1 Bioassay
Principle: Bioassay is the process of measuring the amount of radioactivity that has
entered the body. There are two major forms of bioassay: in vivo (“in the body”)
bioassay involves directly counting the amount of radioactivity in the body, typically
using scintillation detectors to measure gamma radiation emitted by radionuclides
within the body. A chest count, a thyroid count, or whole-body count—holding a radiation detector over the chest or sitting in a whole-body counter—and measuring the
count rate emanating from gamma-emitters within the body—are examples of in vivo
bioassay. The other, in vitro (“in the glass”) bioassay, involves taking samples—
generally urine, feces, or blood—and extrapolating the amount of intake from the
concentration of radioactivity in the sample.
Procedure: An in vivo bioassay, as noted above, is performed by placing a radiation
detector on or near the body near where the radioactivity is expected to be. A person
who has inhaled radioactivity that is in an insoluble form, for example, should have the
majority of the radioactivity remaining in the lungs; if the person inhaled radioactive
iodine then the iodine will enter the blood rapidly and will then collect in the thyroid.
Thus, holding the radiation detector close to the body over the lungs or the thyroid
(respectively) will produce a count rate; if that count rate is significantly higher than
the normal background count rate then we can say that there is radioactivity in the
lungs or thyroid. Further, if the counting efficiency of the radiation detector for I131 is known and the radiation detector has been characterized with respect to the
attenuation of I-131 gamma photons as they pass through the tissue overlying the
thyroid then we can calculate the amount of radioactivity that has been absorbed by
the thyroid as shown in this text box.
Interpreting thyroid bioassay results
Let’s assume that a radiation detector has a 10% counting efficiency for I-131
gamma ray photons and 90% of the I-131 gammas emitted from the thyroid are
absorbed by the tissues of the neck (for an overall counting efficiency of 1%)
then 1000 cps above background would represent 100,000 dps of iodine; since
1 Bq = 1 dps, the thyroid in question would contain 100 kBq of I-131. From
Federal Guidance Report Number 11 [16] we can find that 2 MBq (2000 kBq)
of I-131 will produce a radiation exposure of 0.5 Sv to the thyroid so we can
easily calculate that 100 kBq (0.1 MBq) will produce a thyroid dose of 25 mSv
to the thyroid.