Doses from Medical Radiation Sources
Michael G. Stabin, PhD, CHP
Radiation exposures from diagnostic medical examinations are generally low and are almost always justified by the benefits of accurate diagnosis of possible disease conditions. There is no direct evidence of radiation ever causing any harm at the exposure levels encountered with diagnostic radiological examinations. Therapeutic uses of radiation naturally involve higher exposures and physicians will consider the risks of the treatment against the potential benefits. In diagnostic uses, we have only theoretical models that suggest the possibility of cancer risks, but these models all extrapolate results from higher exposures to these low levels, and no one is certain about whether or not any real risks are involved. The exposures are usually comparable to those that we receive routinely from natural radiation all around us, and the benefits of the medical exams are of course very significant.
Standardized radiation dose estimates can be given for a number of typical diagnostic medical procedures. It is impossible to give accurate radiation dosimetry for procedures involving radiation therapy; these need to be handled very carefully on a case-by-case basis. Doses are thus suggested below for some typical diagnostic radiology and nuclear medicine studies. It is important to note that these are only typical values. Doses will change, depending on a number of variables, including the specific machine and manufacturer (in the case of radiology), study techniques (in the case of radiology on the settings of the machine used to produce the radiation, in nuclear medicine on the amount of activity administered and the patient's metabolism), and other issues.
Typical Doses from Diagnostic Radiology Exams
As noted above, the tables below give dose estimates for typical diagnostic radiology exams. For comparison, we all receive about 3 mSv of exposure annually to natural background radiation.
The following table shows the dose an individual might receive if undergoing an entire procedure, e.g., a lumbar spine series typically consists of five films.
The following table shows typical doses from nuclear medicine exams.
The Pregnant or Breast-Feeding Patient
Special attention is paid to the pregnant, potentially pregnant, or breast-feeding patient. The developing embryo or fetus is particularly sensitive to radiation. If an examination that involves radiation exposure can be postponed or replaced with another study, this is always desirable. If the study is needed, precautions to eliminate or reduce the dose to the embryo/fetus should be followed. Most radiation therapy studies are contraindicated in pregnancy, but at times may be necessary. Any women of childbearing years should be given a pregnancy test before any therapeutic study involving radiation is undertaken. Breast-feeding patients are of concern in nuclear medicine studies (diagnostic or therapeutic), because the compounds given to the mother may be taken up and excreted into the breast milk, and thus possibly ingested by the nursing infant.
Doses from some important nuclear medicine exams for exposures to the fetus occurring in early pregnancy are shown in the following table. An article by J.R. Russell and colleagues7 gives doses at other stages of pregnancy and for other radiopharmaceuticals.
The following table shows estimated fetal radiation dose for 0, 3, 6, and 9 months gestational age based upon "typical" exposure values.
In the case of the breast-feeding mother who receives a radiopharmaceutical, an article in 2000 in the Journal of Nuclear Medicine8 discussed the issues associated with possible exposures of the nursing infant. The authors noted that individual levels of activity in breast milk vary widely and that the best approach is to make individual measurements of breast milk activity and individual-specific projections of dose and suggested interruption schedules. Nonetheless, they also offered a general set of recommendations for possible interruption of breast-feeding for a number of compounds, for which some experience with breast milk excretion had been reported in the literature. The following table summarizes these recommendations.
Doses in Radiation Therapy
In radiation therapy, much higher doses are given, with the intent of destroying cancer tissues. The trick is to give enough dose to kill the unhealthy tissues while not giving too much to normal, healthy tissues. This is accomplished in a number of ways. With external radiation, many techniques are used to focus the radiation dose in the area of interest and give lower doses to normal tissues. Small sources may be placed very near to or in direct contact with cancer tissues (brachytherapy) and only left in place for defined periods of time. Therapy is also performed with internal radioactive sources, like are used in diagnostic nuclear medicine. These sources are labeled to special molecules that are designed to be taken up preferentially by cancer cells, and less by other organs and tissues, and thus lead to a positive outcome without compromising the health of the patient. It is not possible to give specific dose calculations for these cases. Each situation is studied carefully by a radiation physicist before the therapy is given, and a specific dose plan is devised. Doses are typically on the order of hundreds or thousands times higher than in the diagnostic studies described above.
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