Full body scans represent a good example of the conflicting considerations when you need to evaluate a screening test. A full body scan uses a CT (Computerized Tomography) scan to examine the inside of your body. These full body scans are heavily advertised as a way to detect physiologic abnormalities that might provide an early warning of cancer, heart disease, or other illnesses.
Many organizations, including the U.S. Food and Drug Administration strongly discourage the use of full body scans in healthy adults with no obvious symptoms of disease.
Other procedures, such as the use of Prostate Specific Antigen to diagnose prostate cancer, and mammograms to detect breast cancer, are also subject to a lively debate about the risks and benefits of using them for routine screening. The issues here are more complex, and there are both defenders and opponents of the use of these tests for screening in healthy adults.
To evaluate a screening procedure, you need to evaluate the risks of the procedure, and assess the costs of a false positive and the costs of a false negative. You then need to see how frequently the disease occurs in this population, and you might modify this estimate to take into account special features of the types of patients that you yourself see. Finally, is there anything about this particular patient that would cause you to modify the estimates of prevalence further. And do the values of this particular patient warrant a re-evaluation of the costs and benefits of the screening?
A recent publication in Radiology mentions the rather large dose of radiation that a full-body scan produces and estimates an increase in the lifetime risk of risk of cancer mortality of 0.08%. This translates into an NNH of 1,250. For every 1,250 people who receive a single full body scan, you will see one additional death from cancer
This gives a solid reference point for evaluating the possible benefits of whole body scans. If they detect diseases in only one out of every thousand people, then the harms of screening might outweigh the benefits.
The costs of a false positive really represent the costs of additional testing. Beyond just the money, some of the additional tests carry significant risks of side effects. One of the additional tests for prostate cancer for example, a prostate biopsy, carries the risk of bleeding and infection.
The costs of a false negative represents the costs of leaving a disease undiagnosed until it manifests itself in a more overt manner. Sometimes the costs of a false negative is less than you might think. For mammography, as an example, leaving a tumor undetected may not be a bad thing. If the tumor develops slowly, then it will eventually be detected by other means before it has had a chance to spread. If a tumor develops rapidly, then by the time it is detected by mammography, it may be too late. It is, perhaps, only the "Goldilocks" tumors, those that grow not too fast and not too slow, that are worth detecting early.
The prevalence of disease is also very important. In a population of people with no overt signs of disease and no obvious risk factors, whole-body scans may be looking for a needle in a haystack. The few diseases discovered are more than counterbalanced by the costs and risks associated with the screening. In a different population, though, such screening may make more sense. The FDA, for example, mentions on-going research in the use of whole body scans for people at high risk of lung or colon cancer. Regular mammograms are much easier to justify for women who have had a relative who has died from breast cancer.
I discuss some of the controversies associated with the PSA test in my May 31, 2004 weblog entry. When I get the chance, I will discuss some of the controversies associated with mammography.
Estimated Radiation Risks Potentially Associated with Full-Body CT Screening. Brenner DJ, Elliston CD. Radiology 2004: 232(3); 735-8. [Medline] [Abstract] [Full text] [PDF]
Women aged over 40 who are at increased risk of breast cancer should get annual mammograms. Kmietowicz Z. Bmj 2004: 328(7455); 1515. [Medline] [Full text] [PDF]
The Prostate-Specific Antigen (PSA) Test: Questions and Answers. U.S. Department of Health & Human Services, National Cancer Institute. Accessed on 2004-09-06. cis.nci.nih.gov/fact/5_29.htm
Full-Body CT Scans. What You Need to Know. U.S. Food and Drug Administration. Accessed on 2004-09-06. www.fda.gov/cdrh/ct/ctscansbro.html
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You can find an earlier version of this page on my original website.