I TREAT MANY WOMEN WITH BREAST CANCER and was recently struck by a startling new statistic. There is a growing trend of breast cancer in younger females.

In general, breast cancer in young women has a less favorable prognosis compared with older women.

Researchers recently evaluated breast cancer incidence among women ages 20 to 49 in the United States. They examined 20 years of data from the Surveillance, Epidemiology, and End Results program.

We’ll look at the findings in more detail, but here is the bottom line:

There were increases in breast cancer incidence rates among young U.S. women.

Background

I have the privilege of being involved in the medical care of hundreds of patients with breast cancer each year.

In the United States, breast cancer is the leading cause of cancer death among women aged 20 to 49.

That’s according to the American Cancer Society.

Young

Young women tend to develop more breast cancers that are biologically aggressive compared to older women.

What do I mean by “aggressive?” Young folks’ tumors tend to be larger and more advanced in stage.

Moreover, the cancers are less likely to have positive hormone receptor status (estrogen and progesterone receptors) and more likely to have overexpression of HER2, a growth factor receptor.

All of these factors can contribute to a poorer prognosis in young women with breast cancer.

Oh, there is also this: Breast cancer screening programs are not available for women under 40 who are not at very high risk.

Study

While we know that breast cancer has been rising among young women, there is not a lot of data about trend patterns based on cancer stage, race, and cancer biology.

Researchers analyzed data from the U.S. Surveillance, Epidemiology, and End Results registries. SEER is a public database from the National Cancer Institute with coverage of 27% of the U.S. population

They included women ages 20 to 49 who had been diagnosed with invasive breast cancer. The final analytic set included 217,815 women.

The investigators formatted age as five-year age groups (20–24, 25–29, 20–34, 35–39, 40–44, and 45–49 years).

Trends

What are the long-term trends in breast cancer incidence among women aged 20 to 49 years?

The graph comes from this paper, “Breast Cancer Incidence Among U.S. Women Aged 20 to 49 Years by Race, Stage, and Hormone Receptor Status,” appearing in JAMA Network Open.

Look at the recent upward trend line (in blue). Disturbing, to say the least.

Why?

Let’s try to tease out this uptrend in young breast cancer diagnoses.

Period effects

Could it be that breast cancer rates are rising among all age groups, with the younger population riding the incidence wave?

Cohort effects

Or is there something different about the younger population? Could it be environmental exposures or changes in other breast cancer risk factors?

It turns out it is both. The rising incidence of breast cancer among young cancer is due to two effects:

• a general increase in incidence over time
• the special risk of being born in the late 1970s to early 1980s.

Stage

The stage is the extent of breast cancer at presentation. The study offers a fascinating finding:

The rising cancer rates among young women are mostly due to more stage I and IV (metastatic, with spread to distant sites) disease. The increases are not because of stages II or III breast cancer.

The higher cancer rates are due primarily to stage I and stage IV cancers, not stage II and stage III cancers.

The higher rates of stage I are perplexing to me. I don’t think it is because of early detection; women with normal risk levels would not be candidates for screening studies (such as mammograms).

Maybe I am wrong, and women are more aware of being at high risk. More genetic testing might lead them to get early screening.

Estrogen receptor-positive

Much of the reported breast cancer increase among young women is a type known as estrogen receptor-positive (ER +).

The cells of this breast cancer type have receptors that allow them to use the hormone estrogen to grow.

Treatment with “anti-estrogen” (endocrine) therapy can block the growth of such cancer cells.

Why might we see a rise in estrogen receptor-positive breast cancer among young women?

This rise in incidence may reflect women tending to have fewer children and children later in life.

Stage IV (advanced)

The report also showed a rise in young women presenting with later-stage breast cancer.

Why?

This finding does not surprise me.

If breast cancer is rising in all age groups, including among young women, we will see greater numbers of women with advanced breast cancer.

Younger women generally should not have screening mammograms. If a woman presents with a palpable lump in her breast, she is more likely to have an advanced stage.

But why the recent change? The study authors suggest that an increase in overweight and obese women in the U.S. might play a role.

Historical studies associated higher body mass index (BMI) with higher breast cancer stage at presentation.

My take

I don’t know why breast cancer is rising among young women.

But, I am disturbed.

We may need to adjust our screening approaches as the contours of breast cancer epidemiology evolve.

We may need to address weight and other lifestyle factors, environmental toxins, and other potential risk factors.