Katherine Warrick

Katherine Warrick, 32, photographed at her Montclair, Va., home was 31 when she was diagnosed with breast cancer and has now successfully finished her treatment. (Ryan Rayburn/Photo for the Tribune)

In fact, the discrepancy in survival between younger and older women has become worse over the past 25 years, according to federal data. One possible explanation is that most of therapeutic efforts have targeted middle-aged and older women, and not young women whose cancers may require a different treatment approach.

The good news, said Partridge, is that overall survival is improving among young women.

When to screen

Experts disagree on when screening should begin; some guidelines say at age 40. The U.S. Preventive Services Task Force recommends that women between age 50 and 74 get a mammogram every other year, with the option to start earlier if a woman desires.

Since women younger than 40 aren't routinely screened, their cancer can be missed for months or even years. But, Johnson said, there's also no evidence showing that screening before age 40 would help younger women who are not at high risk and have no symptoms.

Instead, she said, women shouldn't just assume they are “too young” to get cancer. If they notice a lump, pain or other change, they should see a doctor.

Michelle Lamont, a New York City writer and blogger, was diagnosed in December at age 25, four months after another doctor waved off her concern, calling it a cyst.

“I was totally at ease while this cancer was growing inside of me,” she said. “But those four months could have been the difference between life and death.”

Though Johnson's study didn't look at the reasons for the potential increase, one theory to explore looks at whether overeating and lack of exercise are driving up early-life metastatic breast cancer rates, Johnson said. The use of hormonal birth control could play a role, but the risk level goes back to normal about a decade after going off the drugs, according to the National Cancer Institute.

Since the change has been so marked over just a few decades, “We think it's likely related to something external, a modifiable lifestyle-related risk factor or perhaps an environmental toxic exposure, but we don't know what,” Johnson said.

About half of breast cancer cases in all age groups combined are due to known risk factors — genes, reproductive patterns and socioeconomic status — which means environmental factors are likely also related, according to the Centers for Disease Control and Prevention.

Ionizing radiation or X-rays have been clearly linked to breast cancer. Since medical diagnostic X-rays are an increasingly common source of radiation they should only be given when medically necessary, according to an Institute of Medicine committee on breast cancer and the environment.

The widescale use of carcinogenic chemicals tracks tightly with the incidence of breast and other cancers, but so far there's no solid causal evidence in human studies, said sociologist and filmmaker Sabrina McCormick, the author of “No Family History: The Environmental Links to Breast Cancer.”

One growing area of research looks at whether the timing of environmental exposure influences breast cancer risk later in life. Certain time periods are particularly important: in utero, puberty, pregnancy and post menopausal stages of life — when women are particularly vulnerable to environmental causes, according to the National Institute of Environmental Health Sciences.

During early childhood and adolescence, the breast tissue is developing and maturing. Recent studies suggest environmental exposures such as certain chemicals, diet and social factors, during these critical stages of development may affect breast cancer risk later in life, the institute said.

Polycyclic aromatic hydrocarbons, found in air pollution, have been tied to an increase in breast cancer tumors, said McCormick, an associate professor at School of Public Health and Health Services at The George Washington University. Once cancer treatment is over, “it's always good to reduce exposure to carcinogenic chemicals,” she said. “Many accumulate in the body.”

Body fat can also absorb chemicals found in pesticides, food packaging and consumer products that can act as endocrine disrupters. These disrupters concern researchers because they are commonly found in the environment and can mimic or block hormones and disrupt the body's normal functions.

Facing breast cancer

before age 40

Studies have repeatedly shown that younger women have more difficulty adjusting to a diagnosis of breast cancer, in part because the disease and treatment impacts nearly every aspect of their lives, including their evolving identity.

“Cancer stole my independence, my youth, my fun, my fertility, my body, my beauty and my ignorance of death and mortality,” said Lamont. “I used to feel invincible; now I am more aware than ever that my life will end one day.”