Answer to Question #4779 Submitted to "Ask the Experts"
Category: Nuclear Medicine Patient Issues — Diagnostic Nuclear Medicine
The following question was answered by an expert in the appropriate field:
I had a thallium stress test. Two weeks later, my body is still
giving off radiation according to the personal radiation
detectors in my office. Isn't this abnormal? A cardiologist that I met
informally said that there are two different types of thallium, and one
is stronger than the other.
It is entirely normal to have residual radioactive material in your
body at 13 days after a thallium stress test. There are several reasons
The first reason is based on how thallium clears from the body.
Thallium stress tests utilize thallium-201, with a physical half-life
of about three days. This means that the activity or emissions from a
vial of thallium-201 will decrease by half every three days. Thus,
after three days, the activity will be one-half of the initial
activity. After six days it will be about one-fourth of the initial
activity, and so forth. It could take up to about 30 days for the
activity in this vial to decrease to levels that are no longer
measurable above natural background radiation levels.
Second, when thallium-201 is injected into the blood during a
stress test, the thallium rapidly leaves the blood by uptake in the
cells of all organs and tissues in the body. Over time, the thallium
eventually is excreted by way of the gastrointestinal tract (~80%) and
through the urine (~20%). The International Commission on Radiological
Protection notes that 63% of injected thallium chemical will leave the
body with a biological-half life, which is how long it takes for half
of the amount of a chemical to clear from the body, of seven days. The
remaining 37% will leave the body with a much longer biological
half-life of 28 days. Therefore, thallium chemical will clear from the
body with about a 10-day overall biological half-life.
The effective half-life based on both the radiological decay and
biological clearance of thallium-201 in the body is on the order of
about 2.3 days. Assuming that only thallium-201 was present in the
injection, radiation dose measurements would continue to be cut in half
about every 2.3 days and could potentially be measurable for a few
weeks after the stress test.
However, an additional reason that activity is measurable for an
extended period of time is based on very small amounts of radiochemical
impurities in the thallium preparation. Many radioactive substances for
medical use contain small amounts of radionuclides other than those
intended. This is especially true when the other radionuclides have the
same element as the intended radionuclide, making complete separation
of the impurities especially difficult. In the case of
thallium-201, typical radionuclide contaminants include thallium-200 or
thallium-202. Thallium-200 has a physical half-life of 26 hours
and thallium-202 has a much longer physical half-life, just over 12
days. Even at very small levels of radiochemical impurity, the
thallium-202 will become the dominant contributor to the measured
activity in the body several weeks after a thallium-201 stress test as
it takes longer to decay. It is quite normal for thallium-202 to be
detected with sensitive detecting equipment for up to five or more
weeks following a thallium stress test.
The following list of literature resources provides more detailed information on the subject:
- Stabin M, Schlafke-Stelson A. A list of nuclear medicine
radionuclides and potential contaminants for operators of in-vivo
counters. Health Phys 61(3):427-430; 1991.
- Kol R, et al. The interference of medical radionuclides with
occupational in-vivo gamma spectrometry. Health Phys 84(6):756-763;
- International Commission on Radiological Protection. Radiation dose
to patients from radiopharmaceuticals. Oxford: Pergamon Press; ICRP
Publication 53: 371-373; 1988.
Lawrence T. Dauer, CHP
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