CT Scan “Risks”

Every 2-3 years a “well-meaning” newspaper reporter will write an article about the potential risks of CT scans.  The final decision whether or not to get the CT scan resides with the patient.  The potential benefits of CT scan, especially in an adult usually far outweigh the risks.

  • Compared to MRI, CT is quicker (important for claustrophobics), and the quality of the images less affected by patient movement.
  • CT can still be performed if you have an implanted medical device of any kind (e.g. pacemaker), unlike MRI.
  • A diagnosis determined by CT scanning may eliminate the need for exploratory surgery and surgical biopsy. The CT scan images the bone that surrounds the eye much better than MRI.
  • No radiation remains in a patient’s body after a CT examination.

Radiation dosages are measured in millisievert (mSv). Each year, people who live in the Toronto / New England area are exposed to about 3-5 mSv of radiation.

A CT scan of the head/orbit will expose you to about 2 mSv, a scan of the chest is about 8 mSv, and a study of the abdomen and pelvis is about 10 mSv.  At these dosages the risk to you as a patient is absolutely minimal.

The problem arises if you have many studies ordered and you get into dosages of greater than 50 mSv and are less than 40 years old. The 50-mSv number has been derived from a number of historical sources, such as an analysis of data from the Atomic Bomb explosions, which may have a limited applicability to CT scans given the circumstances under which some of the individuals living in and around Hiroshima and Nagasaki were exposed to radiation.  We use the 40-year-old cut-off point because patients who are this age or younger have a long life ahead of them and may be more sensitive to radiation than older patients. This is particularly true of children.

What is the exact radiation risk?  No one can be certain but in adults, the estimated risk is 1 to 2 per 2,000 patients who are exposed to 10 mSv of radiation developing a fatal cancer. (Because there is less radiation with a HEAD CT, the risk of cancer is probably in the order of 1 to in 8,100 for a head scan.)

Much of the above information is dervied from an article by Dr. Richard Semelka and Dr. Steven Birnbaum.

This seems horrible but realize that if you take 2000 individuals in the general population, 400 of them will die of a fatal cancer NOT induced by radiation.  For this reason, the risk for 10 mSv of exposure would then increase to 401- 402 for every 2000 patients.  This difference would be difficult to detect in the population, and the power of CT scanning would in the interim save lives from other diseases. Most of the time, this risk is certainly worth taking. We all take risks every day of our lives from driving our cars, walking down the street, flying in airplanes, etc.

According to the National Safety Council the lifetime probability of dying:

in a car accident is 1 in 83,walking across the street 1 in 625, drowning in 1 in 1100, in plane crash is 1 in 5000.

Patients should take responsibility for their healthcare and be aware of relevant risks so that appropriate decisions can be made in a calm and reasonable fashion.