Lyman said the benefits of mammography have been clear, particularly since insurers and Medicare began reimbursing for them, noting that mortality rates have fallen by 25 percent since then.
"I can't say all that's due to mammography, but it's pretty clear that part of that and maybe most of that is due to early detection with routine mammographic screening," he said.
Doctors' offices and hospitals have been fielding calls from women wanting to know what these new guidelines will mean for them.
One of these calls came not from a woman concerned about getting breast cancer but from one who has already had it. Beth Thompson, 44, a mother of four who lives in the suburbs of Baltimore, was first diagnosed with breast cancer following a mammogram at age 40.
"I had no risk factors and no family history," Thompson told ABCNews.com. "Under the new guidelines I wouldn't be screened. That's why I'm so upset about this. I firmly believe I would not be here today if I had not had a screening mammogram at 40."
Thompson explained that in removing the tumor detected by the mammogram, doctors found a faster-growing tumor underneath, one that would not have been found until it became a palpable lump.
"It really makes me shudder to think of what a different situation I would have been in if that were the case," she said, explaining that she needed four surgeries, four rounds of chemotherapy and took Herceptin, an adjuvant for an aggressive form of breast cancer, for a year.
"That's the treatment that I needed even for an early-stage cancer," Thompson said. "There's just no reason that I would have been screened and no way that it would have been found at the time, except for mammography."
While false positives may create problems for some patients, many seem to believe that those are outweighed by the deaths that can result if the screening is not done, something that would present a challenge to any desire to change screening guidelines.
"At the Methodist Breast Center, we diagnose and/or treat about 500 patients with breast cancer every year," said Dr. Luz Venta, medical director of the Methodist Breast Center in Houston and fellow at the Society of Breast Imaging, in a statement. "And about 21 percent of these are women under age 50. Should these women be sent away and told the cost of screening for breast cancer is not justified in the number of lives that can be saved?"
That sentiment was echoed by many, some of them survivors of breast cancer, who flooded the message boards of breast cancer groups like breastcancer.org and the Susan Love Foundation to protest the new guidelines.
Thompson, whose sentiments run along those same lines, said she worries about getting her own daughters, the oldest of whom is in her teens, proper screening when they reach the right age, and is concerned that future recommendations might change how soon they get screened.
"I will move heaven and earth to have them screened at 30," said Thompson, affirming the recommendation that women with a first-degree relative with breast cancer get screened 10 years before cancer first appeared in that relative.
But Thompson, reflecting the sentiments of other survivors of breast cancer, also said she worries that any recommendation for screening that would have excluded her also devalues the lives of other potential breast cancer survivors.