It's a medical adage that seems simple and sensible: Early detection of cancer saves lives.

So when an influential federal panel of independent experts last week advised most women in their 40s to forgo regular mammograms — and pronounced self exams largely useless as well — it not only defied medical convention but also seemed to buck common sense.

Heightening the confusion, many cancer experts and other specialty doctors immediately protested the recommendations, saying that cutting back on mammograms would lead to more deaths from breast cancer.

Despite the intensity of the controversy, it's actually not a new one. Researchers for more than a decade have questioned the value of routine breast-cancer screening for women under 50, who stand to reap less benefit and more harm from mammograms than older women.

As public reaction roiled on last week, federal health authorities on Wednesday attempted to assuage the widespread anxiety by stressing that the guidelines were merely advisory and that screening decisions are best left up to women and their doctors.

To that end, here are some of the key questions many have been asking:

Q: How can less screening possibly be a good thing?

A: Women face risks in looking for cancer, as well as in not finding it.

Out of roughly 1,000 women screened, four will be found to have breast cancer and one will have it but the mammogram — or the radiologist reading the mammogram — won't detect it, said William Barlow, biostatistics research professor at the University of Washington School of Public Health.

But about 90 other women will suffer false alarms that often lead to additional mammography, ultrasound or even biopsies to rule out breast cancer, Barlow said.

And other researchers have estimated that as many as five out of 1,000 women will undergo cancer treatments — enduring radiation, chemotherapy and even mastectomies — for cancers that would not have killed them. That's because doctors cannot distinguish which small cancers may grow to become lethal, so they treat them all as potential killers.

Q: Why is the advice to skip routine mammograms directed only at women in their 40s?

A: Both cancer and breast tissue appear white on mammograms. Because younger women have denser breast tissue, it can be more difficult for radiologists to read the images accurately.

That's one reason women in their 40s have the highest false-positive rates, even though their overall risk of breast cancer is lower than for older women.

While an American woman in her 40s has just a 1-in-69 chance of being diagnosed with breast cancer, the odds more than double for a woman in her 60s. At the same time, the group that issued the new mammography guidelines concluded in a paper published Monday in the Annals of Internal Medicine that 9.8 percent of mammography for women 40 to 49 will detect potential cancers where there are none. For women 60 to 69, the false-positive rate is 20 percent lower, at 7.9 percent.

In its guidelines, the group — the United States Preventive Services Task Force — also called for biennial, not annual, mammograms for women 50 to 74.

Dr. Alfred Berg, a professor in the Department of Family Medicine at UW, chaired the task force in 2002, when it urged mammograms every one to two years for all women starting at age 40.

With the updated information, "a woman in her 40s needs to decide what is important to her," Berg said.

"If a 1-in-1,800 chance of direct benefit 1/8estimated screenings required to prevent one breast-cancer death3/8 looks like a good deal compared to the much higher chance of false positive … she should go for it."

Q: What's the harm in checking your breasts for lumps?

A: The task force concluded that self exams haven't been shown to make a difference in the rate of breast-cancer deaths, yet they lead to higher numbers of biopsies that turn out to be benign. It recommended that doctors no longer teach examination to patients.

One shortcoming of self exams is that they typically miss early breast cancers and noninvasive cancers, those that haven't moved out of the ducts — the types that are most treatable and curable, said Dr. Steven Scallon, medical director of Overlake Hospital Medical Center in Bellevue.

That said, it's hard to discount the benefits of self checks when they clearly have been lifesavers for some women, said Dr. James Park, a family physician with Valley Medical Center's primary-care clinic in Newcastle.

"One of the more difficult parts of my job is explaining how doing tests can actually be more harmful than not doing tests," Park said. Yet a 38-year-old patient of Park's recently found a lump that turned out to be invasive cancer.

So despite the lack of evidence that self exams save lives, Park said, "Here's where I sometimes use common sense and say, 'Well, maybe you don't have to do a monthly self check in the mirror, but if you notice any golf balls be sure to let me know.' "

Q: To which women do the new guidelines not apply?

A: The task force's recommendations are meant only for women without particular risks for breast cancer.

The risk of breast cancer increases as a woman ages. Breast cancer also clusters in families — women whose mothers, sisters or daughters have had breast cancer may be several times more likely to get it than women without a first-degree family history.

Delaying childbirth, use of hormone-replacement therapy and carrying genes known to be susceptible to breast cancer are also known to elevate a woman's risk.

But even for those women, Berg asserts, a mammogram "is never 'necessary' unless the woman looks at the benefits and harms and decides that she wants it."

Q: How big a problem is overdiagnosis of breast cancer?

A: Perhaps the most compelling argument against overuse of mammograms is that they result in women undergoing treatments for breast cancers that would not have killed them.

This is the notion behind "watchful waiting" for prostate cancer, some types of which are so slow-growing that men will die of something else first.

In breast cancer, overdiagnosis — detection of cancers that would not have caused symptoms or death — has been estimated to run as high as 30 percent, said Diana Buist of Group Health Research Institute. She's an investigator in the Breast Cancer Surveillance Consortium, a national network of researchers backed by the National Cancer Institute.

Truth is, Buist said, researchers still don't have a good idea what proportion of detected breast cancers would have turned out to be nonlethal, or how many survivors would have been alive even without treatment.

"Women need to make an informed decision based on their personal risk and their willingness to have a cancer detected and treated" that in fact would not have harmed them, Buist said.

Q: So why are so many medical professionals opposed to the new guidelines?

A: For many whose job is to battle cancer, early detection remains a singular goal.

Dr. Constance Lehman, medical director of radiology at Seattle Cancer Care Alliance, said breast cancers come in many types, with unpredictable trajectories.

Some are so aggressive they crop up between two annual mammograms. Others grow so slowly that some patients complain about having had a lump in their breast for months, even years.

All breast cancer originates in the breast ducts. About 20 percent are noninvasive, meaning they are still contained in the ducts and are almost always beatable, Lehman said. But odds of survival get dimmer the farther the cancer is from its origin when it's detected.

Women with an invasive cancer that has reached, but is confined to, the lymph nodes, for instance, have about a 75 percent survival rate, Lehman said. But once it has migrated to the organs and bones, odds plummet to 25 percent and the goal becomes less a cure than palliative care.

That's why Lehman is emphatically insisting on annual mammograms for all women starting at age 40.

To her, when it comes to breast cancer, "Don't ask, don't tell doesn't make any sense."