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The discovery had unusual origins in a Houston book group about seven years ago. Three of the women in the group were diagnosed with breast cancer. Alison Henning, a geologist and mother of two young boys, was one of them.
"The fact that I know two other people in my circle of friends who've been diagnosed with breast cancer under 40 is amazing," Henning tells Shots. "I mean, it's ridiculous in an otherwise very healthy population."
One of the women was Dr. Rebecca Johnson, who was diagnosed at age 27. She's now a pediatric cancer specialist at Seattle Children's Hospital.
Johnson kept in touch with Henning after she moved to Seattle, and she wondered about the bigger picture.
"The going wisdom is that breast cancer is uncommon in young women compared to older women," Johnson says. "But I wondered how common it actually was."
She's not the only one.
"There was an impression among doctors who treat women with breast cancer that they were seeing more young women who had advanced disease," Dr. Len Lichtenfeld of the American Cancer Society tells Shots.
“ It suggests to us that the trend is real. And it certainly suggests that the acceleration is happening at an exponential rate. It tells us nothing about why the increase is occurring, of course.
- Dr. Rebecca Johnson, Seattle Children's Hospital
But apparently, no one ever investigated.
Johnson decided to do a national study. It's published this week in the Journal of the American Medical Association.
It found that metastatic breast cancer — disease that spread to the bones or other organs — tripled in incidence among women younger than 40 between 1976 and 2009. These are women whose cancer had already spread by the time it was diagnosed.
But the actual numbers are small. About 800 women younger than 40 are being diagnosed with advanced cancer nowadays, compared with 250 a year in the mid-1970s.
The research has uncovered other troubling things. Incidence has gone up fastest in younger women — ages 25 to 34. The trend affects women of all ethnic backgrounds, in rural areas as well as cities, and it has been accelerating in recent years.
What does Johnson think this all means? "Well, it suggests to us that the trend is real. And it certainly suggests that the acceleration is happening at an exponential rate," she says. "It tells us nothing about why the increase is occurring, of course."
Lichtenfeld, who is the cancer society's deputy chief medical officer, says one thing that famously distinguishes women of this generation is that they've been delaying childbirth. And most of the cancer increase involves tumors that are sensitive to the hormone estrogen, levels of which soar during pregnancy.
"There is some thinking on our part that this is related to perhaps delay in childbirth or to the actual effects of pregnancy itself in this age group," he says. "That may have something to do with the hormonal relationship."
Lichtenfeld says another possible cause is toxic chemicals in the environment. Or possibly increasing obesity — though obesity in adolescents and young women may actually protect against breast cancer.
Lichtenfeld says women shouldn't overreact to these findings.
"When people hear about research like this, they tend to become far more concerned than the numbers reflect," he says. "These are very small numbers. Yes, this is a very serious problem for women impacted by this disease and their families."
But he says scientists should and will investigate what's going on.
"When we see trends that continue to increase over time, we have to be concerned," Lichtenfeld says.
And Henning, the Houston woman who helped inspire the study, says young women should pay attention.
"If you think that something's wrong or feels funny, follow through yourself," she says. "Don't allow your doctors to dismiss it just based on your age. You have to be your own advocate."