Mammography and decision making

A little history for the non-US readers: the U.S. Preventive Services Task Force (USPSTF) is an independent panel that reviews evidence and issues recommendations for preventive health care services. They are sponsored by the U.S. Agency for Healthcare Research and Quality (AHRQ) but the panelists are physicians, nurses, and public health researchers employed by universities and state health departments.

From 1989-2002, annual screening mammography was recommended for women at low risk of breast cancer starting at age 50 in the US. In 2002, USPSTF changed that recommendation to recommend mammography every 1-2 years for women starting at age 40. This change was scientifically contentious – there were questions about the data – but endorsed by cancer organizations. This month, USPSTF changed the recommendation back to starting at age 50. You can read the published recommendation here. This change is less scientifically contentious, but has been even more upsetting to cancer organizations and, in many cases, to women.

I’m going to tackle just a few of the key decision making questions here to try to clarify what’s going on. For example, I’m going to set aside questions of cost and of insurance coverage, which are significant issues, to focus only on the health questions, and only on women at low risk for breast cancer.

Is there a threshold for screening?

Assumption: A false positive mammogram is harmful.

Why? A positive mammogram creates a decision: ignore the finding or confirm the finding? Ignoring the finding may mean ongoing anxiety, which can range from mild concern to debilitating worry. Confirming the finding requires a breast biopsy, which is uncomfortable, and, depending on the technique required, can involve small risks of breast damage, infection, anesthesia, etc. A positive biospy (which may also be false positive in rare cases) generally leads to treatment for breast cancer (lumpectomy, mastectomy, lymph node biopsy, radiation, chemotherapy, and drug therapies). Notably, some breast cancers can be very slow-growing, so that treating them at all is unnecessary (you are more likely to die with these cancers than of them), but, again, ignoring them may mean ongoing anxiety.

If you read that description and you don’t think that the possibilities of anxiety, unnecessary biopsy, and potentially unnecessary treatment of indolent cancer are harmful, then guidelines for starting ages for mammography are unnecessary — mammography should begin after puberty. After all, any chance of detecting cancer, however small, is worthwhile if there’s no harm in looking.

Most people, however, and USPSTF in particular, don’t discount those harms. Therefore, the question is: When do the benefits of screening mammography exceed the harms? As women get older, they are more likely to have breast cancer, so positive mammographies are less likely to be false positives. On the other hand, cancers found in younger women may be more beneficial to treat. (And as women get much older, they’re more likely to die of other causes, so it’s sometimes more harmful to find and treat breast cancer than to stop looking, which is why all these guidelines refer to women 75 and younger).

Put like this, the question isn’t “Is mammography a good thing?” Overall, it undoubtedly is. The question is “Knowing that there’s *some age* under which mammography is not worthwhile for the average woman, what age is that?” And, “How sure are we?”

It’s entirely possible, for example, that that age could be 45. But by convention, patients are usually studied in 10-year age ranges, so if the true “best age” for the average patient is 45, some studies will look like the right answer is 40 and some will look like the right answer is 50.

How sure are we?

So, how sure are we? In 2002, the USPSTF gave their recommendation (40-75) a certainty grade of “B”, which means (from their web site): “The USPSTF recommends the service. There is high certainty that the net benefit is moderate or there is moderate certainty that the net benefit is moderate to substantial.”

What is high certainty to USPSTF? “The available evidence usually includes consistent results from well-designed, well-conducted studies in representative primary care populations. These studies assess the effects of the preventive service on health outcomes. This conclusion is therefore unlikely to be strongly affected by the results of future studies.”

What’s moderate certainty? “The available evidence is sufficient to determine the effects of the preventive service on health outcomes, but confidence in the estimate is constrained [by concerns about the quality of the evidence]…As more information becomes available, the magnitude or direction of the observed effect could change, and this change may be large enough to alter the conclusion.”

Today, in 2009, USPSTF is effectively saying that they were overconfident in 2002. Or, if you like, they were moderately certain in 2002, and more information has become available that was large enough to alter the conclusion. The new 2009 recommendation (50-75) has a certainty grade of “C”:

“The USPSTF recommends against routinely providing the service. There may be considerations that support providing the service in an individual patient. There is at least moderate certainty that the net benefit is small.”

The new scientific evidence since 2002 hasn’t made us hugely more certain. But it has tipped USPSTF’s beliefs about the net benefit of screening mammography for 40-49 year old women far enough to cross back over their threshold for recommendations.

Why are people so upset about it?

New evidence changes recommendations all the time, and not just in health care. In my childhood, dinosaurs were depicted as brown and scaly. Evidence now suggests many were feathered, probably colorful, and the recent discovery of fossilized melanin-producing cells makes it likely that we will know what some of those colors were. This bothers no one. What’s different about mammography?

First, of course, health evidence affects the quality and length of our lives in a way that dinosaur colors don’t.

Second, because mammograms have been urged on women for years, they are considered a valuable health good. Extending a good (as in the 2002 recommendations) seems like a gain. Removing a good (as in the 2009 recommendations) seems like a loss, and losses are psychologically more painful than equivalent gains. Of course, the debate itself is over whether mammography really is a good in younger women, not about whether we should provide goods or not.

Third, there is evidence that people may actually think well of false positives. In a 2002 study of prostate cancer screening (which may have even less benefit than mammography for women 40-49), Cantor and colleagues found that several patients were willing to endure the anxiety of a hypothetical false positive PSA test and the pain of a biospy to be reassured that they did not have prostate cancer. On the other hand, Katz and colleagues surveyed patients who had actually had a (false) positive PSA test and negative biopsy, and found increased worry and decreased sexual function among these patients as compared to those with a negative PSA test. It may be that we underestimate the downside of a false positive. (Hat tip to Rob Hamm and Scott Cantor for the references.)

Fourth, the recommendations have changed and changed back within a short enough period that the USPSTF appears indecisive and inconsistent, and this leads to distrust of their recommendations. It is easier to see the recommendations change than the evidence behind them, and it’s certainly a lot harder for USPSTF to communicate the changing evidence, but we need to develop better strategies for doing so.

Finally, USPSTF recommendations are for the population — what I’ve called the average woman. There are women, however, who had true positive mammographies in their 40’s, and on the basis of their experience are vocal supporters of earlier screening. On the other hand, the many women who had (retrospectively) unnecessary mammographies from 40-49 and didn’t experience a false positive are not a loud voice in the discussion. This returns us to the threshold question — if a single 25-year old woman turns up to say that she had a mammogram that led to discovery and treatment of a dangerous cancer that saved her life, is that sufficient reason to begin screening mammography for women in their 20’s?

With respect to individual women in their 40’s, Diana Petitti, the Vice Chair of USPSTF, says “You should talk to your doctor and make an informed decision about whether a mammography is right for you based on your family history, general health, and personal values.” That, at least, is a recommendation that has always been true.

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