It’s one of the most often-cited – and misunderstood – numbers in the daunting world of cancer risk.

One in eight.

One out of eight U.S. women gets breast cancer, goes the shorthand version.

What does that number mean to you?

Let’s start with what it doesn’t mean: It doesn’t mean that one in eight women has breast cancer. It doesn’t mean that if you’re a woman reading this, you have a one-in-eight chance of developing breast cancer this year, or during the next 10 years, or the rest of your life. It doesn’t mean that one-eighth of your women friends or classmates or co-workers will get breast cancer, either.

Verbal shorthand

It means that a girl born today has a cumulative one-in-eight risk of developing breast cancer during the course of her life – assuming she lives to 85. But that qualification sometimes gets lost in the verbal shorthand.

Breast Cancer Action, a national support group, lists as one of its Top Ten Breast Cancer Myths the notion that “one in eight women have breast cancer.” Advocacy groups say the one-in-eight figure is cited not to scare people but to remind them of the ubiquitous risk of breast cancer and keep the issue in public view.

But the National Cancer Institute avoids using the one-in-eight number on its Web site and in statistical handouts because of possible confusion, says Mike Miller, a senior science writer for the institute.

Reducing a complex disease such as breast cancer to a simple number has other problems, Miller says. Research these days is moving away from the idea of cancer as a disease of a specific organ and focusing more on cancer at the molecular level.

“While this number gives an overall guesstimate, a particular woman’s risk is going to depend on a lot of genetic and molecular aspects that are just beginning to be teased out,” Miller says.

Breast cancer doesn’t need statistical hype to get attention; the reality is plenty frightening. Nationwide, an estimated 182,000 women will find out this year they have invasive breast cancer – meaning it has spread into breast tissue or outside the breast. More than 40,000 women will die.

That doesn’t count the nearly 68,000 new cases of breast cancer in the earliest stage, or about 2,000 cases diagnosed in men. More than 2 million U.S. women are living with breast cancer.

Age and risk linked

Breast cancer risk rises sharply with age. That’s one key to putting the cumulative one-in-eight statistic into perspective.

At the age of 20, a woman’s statistical risk of getting breast cancer during the next decade is one in 2,000, NCI figures show. At age 30, her chance of getting breast cancer in the next decade rises to one in 233. At 40, it rises to one in 69, and by 70, it rises to one in 27.

The link between age and risk results in a paradox.

A young woman has a lower chance of getting breast cancer in the next decade than an older woman. But as she ages, her lifetime risk of breast cancer also drops below one in eight because she no longer has a whole life span of 85 years longer to live.

A 60-year-old woman, for example, has a slightly lower lifetime risk of breast cancer – about one in 11 – if she has lived 60 years without it.