Researchers at the National Cancer Institute and Fred Hutchinson Cancer Research Center have used a “mathematical recipe” to solve a longstanding puzzle: How many women in the U.S. are currently living with metastatic breast cancer (MBC).

Published in the journal Cancer, Epidemiology, Biomarkers & Prevention, the study estimates that as of January 2017 there are nearly 155,000 women living with the disease in the U.S., a quarter of whom were diagnosed from the very start with advanced cancer – known as de novo – and three quarters who progressed to MBC from early stage disease.

This is the first time the number of women living with MBC has been estimated, and it is part of an ongoing effort by the NCI to provide data on what it calls an “understudied population.”

All told, 3.5 million U.S. women have a history of breast cancer — from those who were just diagnosed, to those who’ve gone through treatment for early stage disease, to those who were initially or eventually diagnosed with MBC.

The study found women are living longer with the disease and younger women diagnosed with MBC de novo are surviving twice as long as in years past. This group’s five–year survival rate doubled, from 18 percent from 1992–1994 to 36 percent from 2005–2012, most likely due to improvements in treatment.

This translates into “an increase of approximately one third in the number of women living with MBC,” the authors wrote. Part of that increase is related to the aging of the U.S. population.

Fred Hutch biostatistician Dr. Ruth Etzioni, co–author of the study, said the new research reveals the “magnitude of the MBC problem,” which has been difficult to measure due to gaps in cancer databases, namely the Surveillance, Epidemiology, and End Results Program, known as SEER. The registries, created and maintained by the NCI, are a “national treasure,” according to Etzioni. An ever–unfolding flower of data, SEER provides us with critical cancer statistics: who gets it, where they live, what kind of cancer they get, the stage and morphology of their tumor, the first course of treatment and, finally, whether the cancer kills them or not.

But like most of us, the set of registries has issues.

The biggest one, according to many patient advocates, is that it does not collect recurrence rates; if a person’s cancer comes back, the SEER registries don’t fold that information into the mix. Doing that would be “very resource intensive,” Etzioni said, because you’d have to be “continually checking on people to see if they’ve had a recurrence” which she said, is “not feasible at this point on a national scale.”

This lack of data makes it difficult to accurately assess how many people are actually living with terminal cancer. But there are creative workarounds, which is how Etzioni and lead author Dr. Angela Mariotto of the NCI attained their MBC estimates.

“It’s important to have more accurate estimates,” said Mariotto, who for the last 17 years has squeezed the SEER registries for every last ounce of useful data.

Mariotto said the MBC study is all part of a larger effort to beef up SEER, led by Dr. Lynne Penberthy, associate director for the Surveillance Research Program, within the NCI’s Division of Cancer Control and Population Sciences. Penberthy is also one of the authors of the new study, along with Dr. Marc Hurlbert of the Breast Cancer Research Foundation and Musa Mayer of the Metastatic Breast Cancer Alliance.

"We're trying to capture more data items and give a more complete picture of cancer treatment and cancer care," Mariotto said. "For example, we capture information on treatment, but we don’t have information on the agents for chemotherapy.”