Update: see also this story.
The U.S. Preventive Services Task force (whatever that is), in the Annals of Internal Medicine, yesterday advised women not to begin mammogram screenings for breast cancer until 50, and then conducting them only every other year and not annually, as had been recommended previously.
It’s about time. But why the change?
Docs have finally woke up to the effects of false positives. Mammograms are far from perfect test instruments, and a false positive is when the mammogram says you have cancer when you truly do not. These happen all the time with mammograms, especially in younger women who are not at special risk (no family history of cancer, no smoking, etc.). That is, mammograms are far too often saying that women have cancer when they truly do not. And when that happens, that costs something.
The worst false positive cost for many women—though it might not seem like it until it happens to them—is the stress and anxiety that follows a positive mammogram. “Oh, no! I have cancer!” is what most women think after they are told that “The mammogram found something.” Days, even weeks can go by until further tests can be taken—biopsies, CT scans, X-rays, and so forth which carry their own risk of infection and radiation exposure. Meanwhile, many women are consumed with fear that this is it!
But it is almost always not it. A standard calculation, done in every introductory statistics class, shows that the actual probability that you have cancer given the mammogram “found something” is only about 9%—only 9%!. That’s how crappy mammograms really are (for healthy, not-at-risk females).
What other costs are there? (from the paper)
False-positive results are common with mammography and can cause anxiety and lead to additional imaging studies and invasive procedures (such as biopsy or fine-needle aspiration)…Biopsies may occur as a consequence of false-positive mammography results…Anxiety, distress, and other psychosocial effects can exist with abnormal mammography results…Other potential harms, such as pain caused by the procedure, exist…
Overdiagnosis can occur when screening detects early-stage invasive breast cancer or DCIS [ductal carcinoma in situ] in a woman, typically older, who is likely to die from another cause before the breast cancer would be clinically detected. Overdiagnosis can also occur in younger women if a detected DCIS or other early-stage lesion never progresses to invasive cancer…Similarly, unnecessary earlier treatment can occur at any age when screening detects a slower-growing cancer that would have eventually become clinically apparent but would never have caused death.
Radiation exposure may increase the risk for breast cancer, but usually only at much higher doses than those used in mammography, although regular mammography could contribute to cumulative radiation doses from additional imaging for other reasons.
How to make a decision whether to have a mammogram
Regular readers will know that I have written about this subject often. For those who are mathematically inclined, I have a formal paper on this that appeared in Biometrics a couple of years back. A PDF is available here.
I also have a Decision Calculator that you can play with. This allows you to input your own false positive and false negative costs and discover whether mammograms are worth it. The example data built in is for healthy (American) women, aged 40 to 60-ish, undergoing their first mammogram screening. Women who have additional risk factors (like family history of cancer, who smoke, etc.) should consult their doctors.
I point out that the Naive-o-gram—a device which proclaims every women does not have cancer—beats the success rate of accurate diagnoses of professional mammograms (for the healthy population of which we speak). And therefore, the Naive-o-gram is a viable alternative to a mammogram. In other words, don’t bother with a mammogram unless your costs (of false positives and negatives) are very different than most women’s.
What distresses me about the operatives of the right (and the left) is their penchant for grasping onto every bit of news that bobs up and then finding a way to interpret it negatively. I am already hearing reports that these new mammogram guidelines are the result of Obama’s new health care system. “Rationing!” goes the cry. “They are preparing us for limited availability of care now!” John Gambling on WOR radio (710 AM, NYC) is saying these things as I write.
Nonsense, of course. But we can suppose it’s only a matter of time before conspiracies of the left are touted. Drug companies—no, insurance companies—are trying to find a way to reduce costs and they don’t want to pay for all those mammograms for women!
Wait and see if I’m right.
Update: see also this story.