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If the brouhaha following a government advisory panel's recent change in breast-cancer-screening recommendations has proved anything, it is that even modern medicine does not rely on statistics, scientific facts and clinical outcomes alone.

That's the hard lesson that the U.S. Preventive Services Task Force (USPSTF) learned when it changed course on its recommendations for mammography screening and advised women to delay having the screen until they are 50, rather than beginning evaluations at 40, as they have recommended previously. Over the past two decades, annual mammograms for women over 40 had become a standard of preventive care in the U.S. — right up there with daily exercise, quitting smoking and getting a flu shot.

But after taking a more in-depth look at the numbers, the task force decided that the risks of mammography for women in their 40s do not outweigh the small benefit that the screens provide. On top of that, the panel recommended that doctors no longer urge women to perform monthly breast self-exams at home, citing a lack of scientific evidence to support that they save lives.

Immediately, almost every major cancer organization and physicians' group — including the American Cancer Society, the Susan G. Komen Breast Cancer Foundation and the American College of Ostetricians and Gynecologists — questioned the new recommendations. So did women. "I'm just shocked, absolutely shocked," says Deana Rich, a clinical-research associate in Seattle. The 47-year-old has no family history of breast cancer but has been dutifully getting an annual mammogram for the past seven years in order to reduce her risk of dying from the disease. One of her friends recently received a breast-cancer diagnosis, and several other friends are breast-cancer survivors; all of them learned of their disease thanks to a routine mammogram they got during their 40s. "I can't imagine what would have happened if they didn't have that. The cancer would have just had more time to grow," says Rich.

That is the biggest worry boiling up among doctors and women across the country — that a procedure that undeniably reduces the risk of breast cancer is no longer being recommended for millions of women. Another worry: will insurance begin denying coverage of breast-cancer screens in women under 50 who want them? The Obama Administration quickly disputed that notion, as well as the suggestion that the panel's advisory was a government strategy to cut costs by rationing health care. "The U.S. Preventive Task Force is an outside, independent panel of doctors and scientists who make recommendations," said Secretary of Health and Human Services Kathleen Sebelius in a statement. "They do not set federal policy, and they don't determine what services are covered by the Federal Government."

Sebelius added that private insurance companies were unlikely to change their policies and that mammograms are a valuable lifesaving tool. She advised women to "keep doing what you have been doing for years. Talk to your doctor about your individual history, ask questions and make the decision that is right for you."

Indeed, the mammogram is one of doctors' most powerful tools against breast cancer. There is a robust body of clinical-trial evidence showing that routine screening reduces breast-cancer deaths; the task force attests to that as well. But while everybody, to varying extent, agrees that mammograms are beneficial, what's less clear is the age at which routine mammography screening should begin. That depends in part on breast cancer risk, which increases with age — for every 100,000 women, the risk of developing breast cancer is 1 in 69 in women in their 40s, 1 in 38 in women in their 50s, and 1 in 27 among women in their 60s.

Consensus on this question would be helpful because professional cancer organizations, cancer hospitals and doctors base their screening guidelines on the advice of nationally recognized groups — like the American Cancer Society and the National Comprehensive Cancer Network (or NCCN, a coalition of National Cancer Institute–designated hospitals), and the USPSTF. Neither the ACS nor the NCCN intends to modify its guidelines for yearly breast-cancer screening in all healthy women over 40.

So how exhaustive was the task force's deliberation? How definitive are its guidelines? And which set of recommendations should women follow?

The USPSTF, a volunteer group of 16 health professionals, is often considered to issue the most conservative recommendations compared with other national groups. In 2002, for instance, it called for breast-cancer screening every one or two years for women ages 40 to 49, while other guidelines advocated yearly tests. For its updated 2009 recommendations, the USPSTF analyzed clinical trials on the benefits of mammography — much of that same research was also evaluated for the task force's 2002 decision — while folding in new data on the risks and harms of screening. Those risks include false positive results, over-diagnosis, patient anxiety and unnecessary biopsies, tests and doctor's visits.

The panel also commissioned computer-modeling studies that weighed the benefits of routine screening (reduction in death rate) against its risks, depending on the ages of the women being screened and how often they were tested — every year or every other year.

Overall, based on a review of mammography trials, the panel found that having a yearly mammogram screening cuts the risk of breast-cancer death 15% in women ages 40 to 49. That reduction, it should be noted, is relative, not absolute. The absolute risk of breast-cancer death after age 40 is 3% without annual screening, according to the computer models. That means that with routine screening, which leads to a 15% lower risk of death from breast cancer, a woman's absolute risk drops to 2.6%. Small numbers in either case. Put another way, the panel concluded, the benefit of routine mammograms for women in their 40s is one fewer death for every 1,904 women screened annually for up to a decade.

That benefit increases, however, with the age of the women being screened, as the risk of breast cancer rises: among women 50 to 59, one death is averted for every 1,339 women routinely screened; among women 60 to 69, 377 mammograms would be needed to prevent one death. The task force's computer models further showed that shifting women's screening schedule from yearly to once every two years retains 81% of the benefit of screening while reducing the harms like false positives by half.

Combined, the findings led the panel to reverse their 2002 recommendations on mammography, which extended the advice, originally targeting women over 50, to also include women in their 40s. The new recommendations, published in the Nov. 17 issue of the Annals of Internal Medicine, once again leave out the younger women and suggest that those over 50 get screened biennially. But the recommendations do not instruct women under 50 never to get screened, says Dr. Diana Petitti, vice chair of the task force. The new guidelines were meant to trigger and inform discussion between women in their 40s and their doctors about routine screening. "We thought we were saying that the evidence shows that there is this amount of benefit and this amount of potential harm for women in their 40s," she says. "Which suggests that routine screening is not appropriate. But the word routine clearly got lost."

Such details were bound to get lost in a heated — and highly politicized — discussion of a topic that is for most women more emotional than medical. Add to that an immediate offensive blitz by some cancer doctors who were concerned that the new guidelines would essentially limit their patients' options for preventing breast-cancer death. "I am appalled and horrified," says Dr. David Dershaw, director of breast imaging at Memorial Sloan-Kettering Cancer Center. "We have something that saves lives, and to say we are not going to do it anymore is unconscionable."

The panel stands by its new recommendations, relying on the data, which simply do not support the benefit of routine screening when balanced with risks, among younger women. The new recommendations are also backed by some prominent physicians, including the cancer surgeon Dr. Susan Love, who agree there's insufficient data to show that screening under 50 works. The debate, says Dr. Len Lichtenfeld of ACS, is not likely to end soon. "This is the beginning of a discussion that will likely continue vigorously over the next several months, if not years," he predicts.

But the more immediate issue for many cancer doctors is not that mammograms may work better in some age groups than in others. What worries experts is that the new guidelines could result in fewer women getting screened overall. Already one-third of American women who should be getting annual mammograms do not get screened. Since 1990, the death rate from breast cancer among women under 50 has been declining, 3% each year, in large part because of the expanded screening guidelines. "[The new recommendations] may erode some of the advances we had made in reducing breast-cancer mortality," says Dr. Therese Bevers, a professor of clinical cancer prevention at M.D. Anderson Cancer Center in Houston.

For Deana Rich's part, she plans to continue with her annual screenings, even if at some point she ends up paying for them herself. "It's just too scary not to get mammograms," she says. "I know it's not the be all and end all, but it is one screening tool that we do have."

• Alice Park