The number of women living with metastatic breast cancer (MBC) is increasing. But that’s not necessarily a bad thing, say researchers, noting that today’s better treatments allow patients to live longer than ever with the disease

The National Cancer Institute (NCI) study, “Estimation of the number of women living with metastatic breast cancer in the United States,” appeared in the journal Cancer Epidemiology, Biomarkers & Prevention.

“Even though this group of patients with MBC is increasing in size, our findings are favorable,” Angela Mariotto, data analytics chief at NCI’s Division of Cancer Control and Population Sciences, said in a press release. “This is because, over time, these women are living longer with MBC. Longer survival with MBC means increased needs for services and research. Our study helps to document this need.”

To estimate the number of women living with MBC, researchers need to include those who have metastatic disease at diagnosis, as well as those in whom the cancer spreads later on in the disease. U.S. data on patients who develop MBC after the initial diagnosis is lacking. In this group of women, cancer spread is detected as the disease progresses or recurs — but most cancer registries don’t routinely report recurrence.

To overcome this, NCI researchers, along with colleagues at the New York-based Metastatic Breast Cancer Alliance and Seattle’s Fred Hutchinson Cancer Research Center turned to the Surveillance, Epidemiology and End Results (SEER) Program to collect U.S. cancer statistics. Their estimates show that on Jan. 1, 2017, more than 150,000 women were living with MBC — about 75 percent of whom had developed the disease after diagnosis.

Available data suggest this is more than in previous years, and that the number is still rising. From 1990 to 2000, the number of women living with metastatic disease edged up by 4 percent, but rose by 17 percent from 200o to 2010. Researchers project that by 2020, 31 percent more women will be living with MBC than in 2010.

These numbers closely mirror survival statistics. From 1992-94 to 2005-12, the five-year survival among women aged 15-49 with a MBC diagnosis doubled from 18 to 36 percent.  with metastatic disease at diagnosis doubled to 36 percent.

This group also saw an increase from 22.3 months to 38.7 months in median relative survival during the same period. Increased relative survival could also be seen in older women aged 50-64 — from 19.1 months to 29.7 months. In addition, more than 11 percent of women under the age of 64 and diagnosed between 2000 and 2004 survived 10 years or more.

“These findings make clear that the majority of MBC patients, those who are diagnosed with non-metastatic cancer but progress to distant disease, have never been properly documented,” said Mariotto. “This study emphasizes the importance of collecting data on recurrence at the individual level in order to foster more research into the prevention of recurrence and the specific needs of this growing population.”