Answer to Question #4779 Submitted to "Ask the Experts"

It is entirely normal to have residual radioactive material in your body at 13 days after a thallium stress test. There are several reasons for this.

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; 2003.
  
   
• International Commission on Radiological Protection. Radiation dose to patients from radiopharmaceuticals. Oxford: Pergamon Press; ICRP Publication 53: 371-373; 1988.

Lawrence T. Dauer, CHP