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A new study published in the Journal of the National Cancer Institute suggests breast cancer screening isn't as effective in real life as it is in carefully controlled medical studies. The researchers didn't find a statistically significant benefit from mammography when they compared women who died of breast cancer to women who never got breast cancer.

But that doesn't mean women should skip their regular mammograms.

"I want women to continue with current screening as recommended," said lead study author Joann Elmore, MD, MPH, of the University of Washington School of Medicine. "I think they also need to realize that screening is not perfect or foolproof. Women need to pay attention to their bodies and seek medical attention if they notice an abnormality that worries them."

The American Cancer Society recommends yearly mammograms starting at age 40 for women at average risk of breast cancer; women at high risk may want to begin screening even earlier.

ACS recommends regular screening because well-designed studies conducted in many different settings over the past 30 years, using a range of methodologies, have shown that it can reduce breast cancer deaths by finding tumors when they're at an early stage and still highly treatable, said Robert Smith, PhD, director of cancer screening for ACS.

"It's important not to let the occasional study that differs from the long-term trend in the scientific evidence question the value of medical advice that is endorsed by leading, evidence-based guidelines," Smith said.

And though it's important to learn whether screening works as well in the real-world setting as it does in a clinical trial, Smith finds numerous problems with the way this particular study tried to look at that question, including some of the choices made when designing the study.

The format used, known as a case-control study, can skew -- or bias -- the results in a way that makes screening seem less helpful than it actually is, Smith explained. This type of design could also be biased in favor of screening if different choices were made. That makes case-control studies challenging for evaluating screening, Smith said.

"There have been a number of studies with a stronger design that have shown that modern mammography not only achieves the benefits observed in the [earlier] trials, but exceeds those benefits," he said.

That research comes from Sweden, the Netherlands, and other countries that have government-run screening programs that invite all women of the right age to get their regular mammogram. These studies include much larger numbers of women than Elmore's study and were able to collect much more detailed information about who did or did not get screened. They also followed women for a longer period of time, and their design was less likely to bias their results, Smith said.

In those trials, breast cancer deaths dropped by about 40% in the women who got screened. Women who did not get screened saw only a 16% drop in breast cancer deaths.

"It is important to look for guidance from research that derives from better data and that has fewer methodological problems," Smith said.

Elmore and her colleagues did their case-control study by comparing 1,351 women who died of breast cancer ("cases") and 2,501 similar women who never got the disease to begin with ("controls"). The women with cancer had been diagnosed between 1983 and 1993, and died between 1983 and 1998. The researchers looked at the screening history of each woman with cancer in the 3 years before her diagnosis, and at the screening history of her matched control for the same 3 years.

Screening patterns were similar between the two groups, and the researchers concluded that screening had only a modest effect on breast cancer deaths that was not statistically significant.

There could be several reasons for those findings, Elmore said. For one thing, about 1/3 of the women in the study used only a clinical breast exam (a physical exam by a doctor) to screen for breast cancer. But physical exams can't find very small tumors the way a mammogram can, and are not as effective a screening method. Only about 1/4 of the women in the study who had cancer were diagnosed with small tumors (less than 2 cm).

Another possible explanation: Treatments have gotten better over the years, so survival may have improved regardless of women's screening practices. Likewise, women are more aware of breast cancer these days, Elmore said, and they're more likely to consult a doctor if they notice something amiss.

But there's also a chance that screening in the community really doesn't measure up to screening done in a clinical trial.

"Research trials are often done in academic centers [that have a lot of experience], and the people doing them are experts," she explained. "There is a lot of variability in clinical practice."

Smith also points to the low rate of mammography in the study and the high rate of large tumors as possible reasons for the poor findings. But he also has another explanation.

"The study covers a very long period of time," he explained. "This was a period of considerable change during which mammography use increased (after the mid-1980s) and quality steadily improved (in the 1990s). Mortality began to decline in 1989-1990 following a period of rapidly increasing participation in screening."

Smith and Elmore agree that it's important to find ways to improve the quality of mammography and breast cancer screening in the US.

"I hope the results of our study will serve as an impetus to investigators and funding agencies," Elmore said. "We can't become complacent about screening."

She offered some tips for women to improve the quality of screening.

• Get your mammogram in the same clinic each year. That way, films from previous mammograms will be available for comparison.
• Hold as still as possible to get as much compression as possible. This will produce a better image that's more likely to show any abnormality.
• If you're still getting your period, schedule your mammogram mid-cycle when you aren't menstruating. If you go during your period your breasts may be tender and swollen, making compression more uncomfortable and perhaps less effective.

Said Smith, "It's important that women get screened regularly because a mammogram can help find a tumor early, it may spare women from more extensive debilitating treatments, and it could save their life."

Citation: "Efficacy of Breast Cancer Screening in the Community According to Risk Level." Published in the Journal of the National Cancer Institute (Vol. 97, No. 14: 1035-1043). First author: Joann G. Elmore, MD, MPH, University of Washington School of Medicine.