Part one of a two part series. Part two, next week, will discuss the views taken by the medical community about CT screening. Both are part of an occasional series on new health information.
Have you been invited to have a whole body scan? I have. In fact, I’ve been invited to have a body scan (head, neck, chest, abdomen), a chest scan or a heart scan on the radio, on the internet, by mail and as I walked through the mall.
A whole body scan is a simple CT (computed tomography) scan of the chest, abdomen and pelvis (and sometimes the head and neck). The chest scan is intended to screen primarily for lung cancer. The heart scan is a special scan that looks at coronary artery calcification and is intended to screen for coronary artery disease by deriving a calcium score. This score is said to reflect your overall burden of coronary atherosclerosis (hardening of the arteries).
These invitations are for screening. Screening is the examination or testing of individuals without signs or symptoms of disease for the purpose of early detection of conditions or diseases that have not yet become clinically apparent. Why have a screening CT scan? What do supporters say in favor of it? They say that is a very sensitive tool for uncovering hidden lesions. (Lesions” are simply alterations or abnormalities of tissues or organs.) Experience with these screening scans suggests that they do indeed reveal numerous lesions that individuals didn’t know about before. Some of these lesions represent important disease such as cancer, coronary atherosclerosis, and aortic aneurysms, but most are not clinically important.
Once these images of abnormalities are obtained, follow-up testing is usually required to determine what they mean. The hope, of course, is that early detection of important disease will lead to a better outcome, such as prolongation of life. Furthermore, individuals who have “negative” screening scans, where there are no findings of importance, may feel reassured.
So why not have a screening CT scan, whole body or otherwise? What’s the harm, aside from the cost? Critics point to four main areas of potential harm. One is radiation exposure. CT scans involve x-irradiation. The amount varies with the type of scan, the techniques used, and the equipment. The FDA website estimates that a CT scan of the head is the equivalent to 100 ordinary chest x-rays. For the abdomen, it’s equivalent to about 500. The FDA, the EPA and the American Association of Physicists in Medicine all have expressed concern about the radiation exposure of CT screening and increased risk of future cancers.
A second concern is about the use of contrast to enhance the images. The head and abdominal scans are performed either with or without administration of contrast material. Some radiologists say that an abdominal scan without contrast does not provide enough resolution to be useful and isn’t worth doing. However, there is a risk of adverse, even fatal, reactions to contrast material and radiologists generally consider the risk is justifiable only when there is a specific indication for the procedure. Some have argued that screening is not a specific indication.
Thirdly, CT screening is subject to problems typical of all screening: false positives and overdiagnosis (also called pseudodisease). Both of these lead to follow-up procedures that may be invasive, costly and dangerous. This problem may be especially important in CT screening because it’s so sensitive that it detects many spots or other “abnormalities.”
False positives represent abnormal findings that don’t turn out to be important disease.
In a letter to the medical journal, Radiology, Dr. William Casarella, Professor and Chairman of the Department of Radiology at Emory University School of Medicine, recently described experiencing an excruciatingly painful hospital stay, two weeks at home, continuing pain for another month after that and more than $50,000 in costs, all to follow-up some lesions seen in his lungs, kidney and liver incidental to a negative CT virtual colonography. None of these lesions “lurking” outside of the colon turned out to be medically important — all were false positives — but it took a multiple procedures including liver and lung biopsies to rule out important disease.
How important are false positives? What fraction of people need a follow-up after a whole body scan? Reports vary from 30 to 80 percent. That means anxiety, time, expense and risk for a large fraction of people with the possibility of prolonging life in a very few people. It also means increased expense to all of us paying health insurance premiums and taxes.
Overdiagnosis refers to finding disease that appears to be real, but that would have never affected the individual’s health if left alone: slow-growing or stable prostate, lung, or breast cancers, for example. It may be difficult or impossible to distinguish cancer that is life-threatening from cancer that is not, so commonly cancers uncovered by screening are treated as if they are threatening. Overdiagnosis, then, is more dangerous than a false positive, because it may lead to unnecessary surgery, radiation and chemotherapy.
CT screening will also involve other errors. Radiology journals have numerous articles about the error rates in interpreting radiographic images, including CT scans. Important findings are undoubtedly missed, leading to false reassurance.
Mark H. Zweig, M.D., previously a staff physician at NIH, has more than 35 years of experience in medical research and 25 years in patient care.