WASHINGTON (Reuters) - Women who believe mammograms will save their lives may be overestimating the benefits, two experts said in what is sure to be a controversial report published on Wednesday.

They said fewer than five percent of all women whose breast tumors can be detected through screening actually have their lives saved by a mammogram.

The American Cancer Society disputed these figures and said the number was 15 percent.

Experts have long debated whether regular screening tests for breast cancer are worth the trouble, expense and the anguish of waiting for further tests if a woman does have an unusual X-ray.

But most countries have settled on a plan for regular mammograms after age 40 or 50 in the hope of detecting tumors while they are small and easily cured.

Drs. John and James Keen believe women and their doctors do not fully understand why the women are getting mammograms.

"The people who are promoting screening are not explaining it," said John Keen, a radiologist at Cook County Hospital in Chicago and former consumer advocate.

"They are pushing the wrong statistics. I am saying that women need to be told the benefits and the harms and they need to make their own decision."

Keen and his brother James, an epidemiologist at the University of Nebraska, looked at statistics for breast cancer screening, disease rates and deaths in the United States.

Writing in the BioMed Central journal BMC Medical Informatics and Decision Making, they said a woman aged 55 has a six percent risk of developing breast cancer over the next 15 years. "Repeated screening starting at age 50 saves about 1.8 lives over 15 years for every 1,000 women screened," they wrote.

"In other words, 2,970 women must be screened once to save one life."

Robert Smith, director of cancer screening for the Atlanta-based American Cancer Society, said the Keens chose data that would support their ideas.

His team estimated just 465 women would need to get screened for seven years to save one life over 20 years.

And the benefits are real, he said. "Most women will go through their lives getting mammograms every year and not having their lives saved. I say, well, good. They didn't get breast cancer," Smith said in a telephone interview.

John Keen, who conducts mammograms himself, does not dispute the benefit, but he also points out the potential costs. Many women get a "false positive" screening and must wait weeks or months for another mammogram, ultrasound or even a biopsy to learn they do not, in fact, have breast cancer.

Insurance companies and patients alike must pay for these tests, he added. "They are buying something. I just want to tell them what they are buying," Keen said in a telephone interview."

"You can use the word paternalistic and that's what it is. We don't trust women to make their own decisions about whether to screen.

"We just tell them to screen. We just say mammography saves lives."

Breast cancer is the top cancer killer of women globally. It is diagnosed in 1.2 million people every year and kills 500,000.

Mammograms can detect breast tumors when they are too small to be felt and potentially while they are easily removed and before they spread.