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.”

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?’ ”