by Gina Kolata

The American Cancer Society, which has long been a staunch defender of most cancer screening, is now saying that the benefits of detecting many cancers, especially breast and prostate, have been overstated.

It is quietly working on a message, to put on its Web site early next year, to emphasize that screening for breast and prostate cancer and certain other cancers can come with a real risk of overtreating many small cancers while missing cancers that are deadly.

“We don’t want people to panic,” said Dr. Otis Brawley, chief medical officer of the cancer society. “But I’m admitting that American medicine has overpromised when it comes to screening. The advantages to screening have been exaggerated.”

Prostate cancer screening has long been problematic. The cancer society, which with more than two million volunteers is one of the nation’s largest voluntary health agencies, does not advocate testing for all men. And many researchers point out that the PSA prostate cancer screening test has not been shown to prevent prostate cancer deaths.

There has been much less public debate about mammograms. Studies from the 1960s to the 1980s found that they reduced the death rate from breast cancer by up to 20 percent.

The cancer society’s decision to reconsider its message about the risks as well as potential benefits of screening was spurred in part by an analysis published Wednesday in The Journal of the American Medical Association, Dr. Brawley said.

In it, researchers report a 40 percent increase in breast cancer diagnoses and a near doubling of early stage cancers, but just a 10 percent decline in cancers that have spread beyond the breast to the lymph nodes or elsewhere in the body. With prostate cancer, the situation is similar, the researchers report.

If breast and prostate cancer screening really fulfilled their promise, the researchers note, cancers that once were found late, when they were often incurable, would now be found early, when they could be cured. A large increase in early cancers would be balanced by a commensurate decline in late-stage cancers. That is what happened with screening for colon and cervical cancers. But not with breast and prostate cancer.

Still, the researchers and others say, they do not think all screening will — or should — go away. Instead, they say that when people make a decision about being screened, they should understand what is known about the risks and benefits.

For now, those risks are not emphasized in the cancer society’s mammogram message which states that a mammogram is “one of the best things a woman can do to protect her health.”

Dr. Brawley says mammograms can prevent some cancer deaths. However, he says, “If a woman says, ‘I don’t want it,’ I would not think badly of her but I would like her to get it.”

But some, like Colin Begg, a biostatistician at Memorial Sloan-Kettering Cancer Center in New York, worry that the increased discussion of screening’s risks is going to confuse the public and make people turn away from screening, mammography in particular.

“I am concerned that the complex view of a changing landscape will be distilled by the public into yet another ‘screening does not work’ headline,” Dr. Begg said. “The fact that population screening is no panacea does not mean that it is useless,” he added.

The new analysis — by Dr. Laura Esserman, a professor of surgery and radiology at the University of California, San Francisco, and director of the Carol Frank Buck Breast Care Center there, and Dr. Ian Thompson, professor and chairman of the department of urology at The University of Texas Health Science Center, San Antonio — finds that prostate cancer screening and breast cancer screening are not so different.

Both have a problem that runs counter to everything people have been told about cancer: They are finding cancers that do not need to be found because they would never spread and kill or even be noticed if left alone. That has led to a huge increase in cancer diagnoses because, without screening, those innocuous cancers would go undetected.

At the same time, both screening tests are not making much of a dent in the number of cancers that are deadly. That may be because many lethal breast cancers grow so fast they spring up between mammograms. And the deadly prostate ones have already spread at the time of cancer screening. The dilemma for breast and prostate screening is that it is not usually clear which tumors need aggressive treatment and which can be left alone. And one reason that is not clear, some say, is that studying it has not been much of a priority.

“The issue here is, as we look at cancer medicine over the last 35 or 40 years, we have always worked to treat cancer or to find cancer early,” Dr. Brawley said. “And we never sat back and actually thought, ‘Are we treating the cancers that need to be treated?’ ”

The very idea that some cancers are not dangerous and some might actually go away on their own can be hard to swallow, researchers say.

“It is so counterintuitive that it raises debate every time it comes up and every time it has been observed,” said Dr. Barnett Kramer, associate director for disease prevention at the National Institutes of Health.

It was first raised as a theoretical possibility in the 1970s, Dr. Kramer said. Then it was documented in a rare pediatric cancer, but was dismissed as something peculiar to that cancer. Then it was discovered in common cancers as well, but it is still not always accepted or appreciated, he said.

But finding those insignificant cancers is the reason the breast and prostate cancer rates soared when screening was introduced, Dr. Kramer said. And those cancers, he said, are the reason screening has the problem called overdiagnosis — labeling innocuous tumors cancer and treating them as though they could be lethal when in fact they are not dangerous.

“Overdiagnosis is pure, unadulterated harm,” he said.

Dr. Peter Albertsen, chief and program director of the urology division at the University of Connecticut Health Center, said that had not been an easy message to get across. “Politically, it’s almost unacceptable,” Dr. Albertsen said. “If you question overdiagnosis in breast cancer, you are against women. If you question overdiagnosis in prostate cancer, you are against men.”

Dr. Esserman hopes that as research continues on how to advance beyond screening, distinguishing innocuous tumors from dangerous ones, people will be more realistic about what screening can do.

“Someone may say, ‘I don’t want to be screened’ ” she said. “Another person may say, ‘Of course I want to be screened.’ Just like everything in medicine, there is no free lunch. For every intervention, there are complications and problems.”