The Doctor Weighs In

by Pat Salber

CT scans are a miracle of modern medicine. We can see things inside the human body that simply were not possible to image prior to the advent of this technology. Because of diagnostic power of this technology as well as the ease of obtaining this diagnostic test, the number of CTs used in the US has increased rapidly since its introduction in the 1970s.

When I was a medical student in the early 70s, obtaining a CT scan on a patient was a really big deal – it was reserved for the most difficult diagnostic cases. By 1980, approximately 3 million CTs were being performed each year in the US. Today, according to an article in the New England Journal of Medicine (NEJM, November 29, 2007), more than 62 million CT scans are obtained in the US each year. Somewhere between 6% and 11% of these scans are obtained on children.  According to the NEJM article, the unnecessary use of this imaging modality has been estimated to be about 30% of the scans.

Many clinicians believe CT scans pose no more health risk than a simple x-ray. A recent survey of radiologists and emergency physicians found that 55% of radiologists and 91% of emergency physicians believed that CT scans did not increase life time risk of cancer. However, According to the NEJM article, this is wishful thinking on the part of these clinicians.

Studies from survivors of atomic bombs have helped us to understand the long term risks of low dose radiation exposure. About 25,000 survivors of A-bomb attacks received radiation doses similar to the organ doses (OD)  from CT scanning that we see in some patients. For example, a typical CT study that involves two or three scans in an adult can result in a mean radiation exposure of about 40 mSV (mili- Sieverts). This is about the same radiation dosage that was found to be related to a significant increase in overall risk of cancer in the A-bomb survivors.

There has not been and hopefully will not be, a prospective comparison group study that definitively proves that exposure to radiation causes cancer. If we wait until enough data is available from retrospective studies, then many people may have developed radiation-related cancers that could have been avoided.

The conclusion section of the excellent review of CT and associated radiation risks in the NEJM suggests a much more reasonable approach:

1. Radiologists should reduce the CT-related dose in individual patients to a level that still allows the production of usable images. "The automatic exposure control option on the latest generation of scanners is helping to address this issue."
2. CT use should be replaced, whenever practical, with other imaging options, such as ultrasonography and magnetic resonsance imaging (MRI).
3. According to the article, “the third and most effective way to reduce the population dose from CT is to simply to decrease the number of CT studies that are prescribed."