Answer to Question #12521 Submitted to "Ask the Experts"

I recently had nuclear stress tests consisting of a single photon emission computerized tomography (SPECT) stress test using a radionuclide dose of ^99mTc sestamibi (resting) of 248 MBq and ^99mTc sestamibi (under stress) of 966 MBq. Now my doctor recommends a hepatobiliary (HIDA) scan.

Your question reflects the concern we all have when looking at how much risk medical imaging procedures present to us as patients. In this particular case, the risks are small and your physician has decided that the medical benefits of obtaining the study information greatly outweigh them.

Nuclear medicine studies involve introducing—by injecting, inhaling, or swallowing—a radioactive drug into the body that can be seen using a particular type of medical scanner (the gamma camera) and which will highlight how the body is functioning. This differs from computed tomography (CT) scans and other types of x rays, which can show the shape of our internal organs, but not necessarily whether they are working properly. The HIDA scan your doctor is proposing, more properly called a cholescintigraphy scan, can determine whether your gall bladder and bile ducts are working normally. The nuclear stress test you already had is used to find out whether your heart muscle is receiving the blood flow it needs.

The risk we worry about in connection with radiation exposure is an increased chance of developing cancer in the future. Of course, we are all susceptible to cancer, even in the absence of additional medical radiation exposures. Overall, we have a lifetime baseline risk (with no additional medical radiation involved) of developing cancer of some sort of about 40% and a risk of dying from cancer of about 20%. 

We use a measure called the effective dose to estimate the radiation risk of a procedure. For the HIDA scan, the value of the effective dose is approximately 2.5 milliievert (mSv) and we would estimate that the risk of developing a fatal cancer as a consequence of this radiation dose is about 0.012%, or one chance in 8,000. This means that your chances of developing a fatal cancer would shift from about 20% (the baseline value, no medical radiation added) to, say, 20.012% as a result of having the scan. This is a very small change in the odds.

Another way of understanding the amount of risk associated with the scan is to look at how much radiation we are all exposed to during the course of everyday living. On average, all of us are exposed to about 3 mSv (more than the amount associated with the HIDA scan) every year as a consequence of natural radiation sources that surround us. The natural background arises from cosmic rays (radiation coming through the atmosphere from outer space), external terrestrial sources (rocks and dirt have radioactive materials in them), internal sources (our food has natural radioactive materials that have been taken up from the soil), and radioactive gases (radon) which we breathe and which have come from natural sources.  

All of us are exposed to these sources all of the time, regardless of whether we work or stay home. Thus, the radiation risk of the HIDA scan is less than this natural "radiation risk" that we take on every year while we live, work, and play.

You will have noticed I have talked about the HIDA scan, but not much about the cardiac scan. The model we use to estimate cancer risks treats different radiation exposures as independent. This is like flipping coins: If I flip the coin and get "heads," I will still have a 50% chance of getting heads when I flip the coin again—the first coin toss does not influence the second. Each procedure carries its own risk, but it does not influence the risk of a following procedure. In this case, the cardiac scan carries its own risk of about four times that associated with the HIDA scan; this is still a small risk compared to our overall chances of contracting cancer.

Finally, the risks associated with these and other diagnostic procedures are very small—so small that we cannot even reliably measure them. We are estimating these risks based on data in which the radiation exposures were much larger. The Health Physics Society position statement Radiation Risk in Perspective states, "Substantial and convincing scientific data show evidence of health effects following high-dose exposures (many multiples of natural background). However, below levels of about 100 mSv above background from all sources combined, the observed radiation effects in people are not statistically different from zero."

Similarly, the American Association of Physicists in Medicine has issued a position statement on radiation risk which states: "At the present time, epidemiological evidence supporting increased cancer incidence or mortality from radiation doses below 100 mSv is inconclusive. As diagnostic imaging doses are typically much lower than 100 mSv, when such exposures are medically appropriate, the anticipated benefits to the patient are highly likely to outweigh any small potential risks."

To summarize, the radiation risk involved in your HIDA scan is very small, and the scan can potentially provide your physician with information they need to treat your current medical conditions.

Jon A. Anderson, PhD, FAAPM
Professor and Chief of Diagnostic Medical Physics