"FASTEN YOUR seat belts, it's going to be a bumpy night."

Bette Davis famously delivers that line in the film "All about Eve" while she surveys partygoers congregating in her home, aware of an explosive conflict that is about to detonate in their midst. That line always helps me to prepare for the turbulence that predictably arises whenever medical practice guidelines change, whenever diagnostic or treatment standards flip-flop.

It's been a bumpy week indeed since the U.S. Preventive Services Task Force (USPSTF) issued revised guidelines for breast cancer screening. They now counsel women without known risks for cancer to begin routine screening at age 50 — a decade later than was recommended in 2002. They advise women age 50 to 74 to undergo mammography less frequently — every two years instead of annually. For women over 74 years of age, they remain indecisive about the value of screening.

But perhaps the greatest turbulence ensued from the task force's advisement that doctors cease instructing female patients to self-examine their breasts. Whoa! Whiplash! Wasn't it just a minute ago that American medicine was "empowering" women to catch breast cancer early by regular self-examination? Weren't many of us health care providers loudly drumming that message in the clinic?

The updated guidelines from the USPSTF — an independent panel of experts appointed by the federal Department of

Health and Human Services — rocked people in dissimilar ways. Some reacted with suspicion, alleging that the task force was simply trying to save costs for the Obama administration. A few Republicans diagnosed the situation as a malignant Democratic plot to enforce rationing. To others, less screening for breast cancer just seemed counterintuitive and dangerous — why would anyone opt for less intensive surveillance against such a potentially serious threat?

Some, but not all, professional cancer societies rebutted or opposed the task force revisions. For example, the American Cancer Society, the American Medical Association, and the American College of Radiology announced their continued endorsement of prior guidelines calling for annual mammography screening to commence at age 40.

On the other hand, many women welcomed the USPSTF guideline update. They appreciated that experts were continuously monitoring new research to assess the sustaining validity of prior recommendations — even if that meant change. After all, the shadow cast in this country by prior medical promotion of hormone replacement therapy remains uncomfortably warm.

Advocacy groups like the National Breast Cancer Coalition and the National Women's Health Network welcomed the new guidelines that embraced less frequent mammograms. As succinctly stated on the Breast Cancer Action Web site: "This is good news."

As the USPSTF also pointed out, mammography screening is not a completely benign procedure. Potential patient harm may accrue when false alarms appear on mammograms, triggering anxiety and further unnecessary testing or breast biopsies.

The USPSTF weighed the frequency and intensity of those harms against these benefits of screening: one breast cancer death prevented for every 1,900 women age 40 to 49 who were screened over 10 years, comparing to one of every 1,300 women age 50 to 59, and one of every 377 women age 60 to 69 under the same surveillance regimen. The task force drew its guidelines where the scales tipped.

Still, what's a woman supposed to do when experts weighing the same facts read the scale so differently?

My advice? First, fasten your seat belt and stay calm. This will help you ride out the bumpy uncertainty that pervades most medical practice, beyond any singular issue like breast cancer screening.

When it comes to cancer, we all want a clear guideline drawn in the sand — one that can unambiguously separate us into two neat categories: those of us with, and those of us without cancer.

But we have to realize that cancer is much more complex than that. It often straddles that line, unbeknown to us and under the radar of mammograms. Mammograms sometime spot transgressors that intend no harm, and in the process of responding defensively to them we suffer the backfire of over-diagnosis and over-treatment.

Secondly, be aware that once someone yells "cancer" in a public theater, it is hard for everyone to stay seated and look dispassionately at the big screen. The "C" word is embedded with fear, and it is highly politicized. It has the power to shake the most substantial statistic, blurring lucid views of the data. Experts may — and do — make different interpretations and recommendations based upon what they see.

Which brings us to this final advisement posted on the Susan G. Komen Web site: "That fact that the USPSTF and the American Cancer Society now have different screening recommendations for women in their 40s simply reinforces the importance of educating yourself about the potential risks and benefits of screening, talking with your physician, and making the decision that's right for you."

Kate Scannell is a Bay Area physician and syndicated columnist.