The accusation comes about six months after the breast cancer charity came under fire for cutting Planned Parenthood funding, which it claimed was not a politically motivated decision. According to the foundation's most recent Form 990, it netted about $114 million in 2010.
The ad in question ran last October in several major magazines, and said, "The five-year survival rate for breast cancer when caught early is 98 percent. When it's not? 23 percent."
"It sounds like you'd have to be crazy not to get screened. It sounds like a huge benefit," said Dr. Steven Woloshin, co-author of the article in BMJ, the British medical journal. "The statistic is totally distorted."
The problem is that a five-year survival rate is easy to manipulate, he said. The ad compares five-year survival rates for early-stage cancers and late-stage cancers, which Woloshin said is not a meaningful way to measure the benefits of screening. Here's why:
Suppose three women are diagnosed with breast cancer at age 67 when a doctor finds a small lump, and they die of the disease three years later, when they're 70. That five-year survival rate is a miserable zero percent because no one lived five years past their diagnosis dates.
Now, suppose the same women were diagnosed when they were 64 because the cancer showed up on mammograms, but they still die of breast cancer at age 70, he said. The new five-year survival rate is a triumphant 100 percent, even though the women actually survived cancer the same length of time. They just didn't know how long they had it.
"If there were an Oscar for misleading statistics, using survival statistics to judge the benefit of screening would win a lifetime achievement award hands down," Woloshin and his co-author Lisa Schwartz wrote.
More useful numbers, he said, are derived from randomized trials. They show that 0.53 percent of women in unscreened trial groups died over 10 years -- compared with 0.46 percent of the women who were screened. That's not much of a difference, the authors said.
This problem is further exacerbated by overdiagnosis, which happens when mammograms detect cancers that never grow or cause symptoms, he said.
For every woman saved by an early screening, two to 10 are falsely diagnosed, the authors wrote. This means patients endure biopsies, chemotherapy and radiation even though they don't have a threatening cancer.
"There's no way for an individual to know they've been overdiagnosed," Wolostin said, explaining that the only way to tell whether this has happened to a specific person is if that person is diagnosed, does not seek treatment and eventually dies of something else.
Doctors have seen evidence of overdiagnoses in long-term follow-ups to randomized trials and analyses of population data, he said.
In response to the article, Komen's Vice President of Research, Chandini Portteus, stood by the foundation's stance on mammograms, calling the screenings "the best widely available detection tool that we have today."
She said Komen has contributed funds toward even earlier detection.
"We think it's simply irresponsible to effectively discourage women from taking steps to know what's going on with their health," Portteus said, adding that the foundation is also funding research to determine which tumors will spread.
Woloshin said his article is not saying mammograms are bad.
"Some people benefit while other people are harmed," he said. "If you don't know, you can't make an informed decision."