Putting the CAT Scanner Cancer Risk Into Perspective: Risk Compared to What?

After listening to and reading the news media's alarmist interpretations of an Archives of Internal Medicine report on the risk of 'yikes cancer!' from computerized (axial) tomography (CT or CAT) scans, the ever curious Disease Management Care Blog decided to take a look at the basis for all the fuss. Amy Berrington de Gonzales and colleagues used radiation and cancer data to project the future national incidence of additional cancers that can be expected from today's CAT scanners. The bottom line can be found in this table from the paper. Basically, the highest calculated risk of 'yikes cancer!' runs from a high of 7 per thousand CAT scans (or 0.7%) to 1 per ten thousand CAT scans (or 0.01%). There is a companion article that indicates that the dose of radiation may be even higher, suggesting there will be even more cancer down the road.

That all may sound awful, but the DMCB likes to keep the following factors in mind:

1) The lifetime risk of cancer from exposure to the radiation from a CAT scan is highest among children, but that pales compared to the more immediate risks that come from sedating children to keep them from moving. In the real world of clinical practice, this risk threshold is already considered so high that the added 0.7% risk from the radiation is not much of an additional consideration. Informed guardians already have a powerful incentive to keep CAT scan use to an absolute minimum - and they generally do.

2) As for the grown-ups, stumbling over the assessment of the real magnitude of risk may be a function of innumeracy, but the DMCB believes it can be managed by asking 'compared to what?' So while a CAT scan can present a lifetime risk of hundreths to tenths of one percent:

- having high blood pressure gives 40 year old men an additional 4-5% (one in twenty) chance developing heart failure over a lifetime. The DMCB says know your blood pressure, get it treated if it's high and then smile when they take your picture with the CAT scan.

- the risk of a brain hemorrhage from taking daily aspirin is 2 per 10,000 per year which is on the same order of magnitude every year as the lifetime cancer risk from a CAT scan. The DMCB doesn't recall any alarmist media reports on this risk. Hmmm. Once again, take two aspirins and smile for the scanner in the morning.

- the risk of a dying in a car accident during an average life time, according to this report, is 1 in 83, which works out to be more than 1% for every man, woman and child in the U.S. today. The DMCB recommends wearing a seat belt when you drive to your CAT scan appointment.

3) One way of gauging the risk of any health threat that is reported in the media is to examine the number of reports: the greater their number, stridency and alarm, the lower the real risk. The hubub over CAT scans suggests this inverse relationship is holding up.

4) Yet, Radiology Benefit Managers (or 'RBMs') have long regarded the cancer risk associated with CAT scans (example here) as one factor in controlling their utilization. If unecessary CAT scans are 'denied' for coverage by managed care oganizations, is that now no longer evidence of 'evil' managed care putting profits before patients? Were they right all along?

The DMCB has always been less worried about the radiation dose from CAT scanners than their far more dangerous tendency to find abnormalities in the absence of disease. Asymptomatic spots, densities, signals and other lesions of dubious significance have always bedeviled physicians and their patients, leading to ever more sophisticated imaging studies. If patient fear over CAT scan radiation exposure leads to fewer scans with fewer of their inevitable false positives, that may ultimately be a good thing.