Answer to Question #4789 Submitted to "Ask the Experts"

How do I know when I have reached my safety limit on radiation exposure for the year as a patient? Who has to decide on my having a fourth test (nuclear radiology test equivalent to six chest x rays) during a period of one year?

My radiation tests for 2005 were:
1. Chest x ray - June (normal)
2. Chest CT scan - September (normal)
3. Mammogram - January (normal)

This is a good question.

First, there are no regulatory limits for radiation doses that are received in medical applications. In each case, the physician considers whether the benefits of obtaining the information (or treating the disease) offset any potential risks of the procedure. This is, of course, true about all medical procedures, including those that do not involve ionizing radiation. When you have several procedures involving radiation over a short time, the physician will generally be aware of this, because he or she will review your medical history. The physician does not take a mathematical approach to this, actually calculating risks and benefits, but makes a general evaluation.

The benefits of medical tests are directly applicable to the subject, and the doses are low, so it is probably rare that a physician will avoid a needed test because of cumulative radiation exposure. Exceptions might occur in high-dose procedures, like interventional fluoroscopy or therapy applications. Also, physicians may opt for diagnostic examinations that do not involve ionizing radiation if they can give the same or an equivalent quality of information.

Most physicians are pleased to speak with patients who are informed and taking an active role in their health care; you can always discuss this upcoming exam with your physician.

Michael G. Stabin, PhD, CHP