The models reproduced the decline in mortality in breast cancer known from SEER data, from 48 per 100,000 women dying of breast cancer each year in 1975 to 27 per 100,000 in 2019 — a decrease of about 44%. The models arrived at a larger estimated reduction in mortality of about 58% because the incidence of breast cancer has risen during the same period and more women would have died had screening and treatments not improved.

The models concluded that about 47% of this reduction in mortality is the result of improved treatments for early-stage breast cancer, and about 25% is attributed to screening mammography. The remainder, or about 29%, is due to improvements in treating metastatic disease.

“Designing the new model, which had to account for individuals with non-metastatic cancer who underwent treatment but later progressed to metastatic cancer, and who may have been treated with multiple drugs over the course of their disease, was extremely complex,” Plevritis said. “It took about four years. But it was really satisfying when we were able to validate the model’s behavior and see that all four models from different institutions, which used the new model inputs in different ways, delivered consistent findings. The models not only make sense, but also produce meaningful insights.”

The impact of treating metastatic disease is exemplified by the increases in median survival time after metastasis: Patients diagnosed in 2000 with metastatic disease lived an average of 1.9 years versus an average of 3.2 years for those diagnosed in 2019. Survival time varies by subgroup status, however. Patients with what are known as estrogen receptor-positive and HER2 positive cancers saw an average increase in survival time of 2.5 years. Those with estrogen receptor-positive and HER2-negative cancers lived an average of 1.6 years longer, but those with cancers that are estrogen receptor-negative and HER2-negative lived about 0.5 years longer in 2019 than in 2000.

“It was meaningful as a breast oncologist to spend time with this history and see real progress over the past decades,” Caswell-Jin said. “There is much more work to be done; metastatic breast cancer isn’t yet curable. But it is rewarding to see that advances have made a difference in these numbers,” she added. “Our scientific and clinical work is helping our patients live longer, and I believe deaths from breast cancer will continue to steadily decline as innovation continues to grow.”

Researchers from MD Anderson Cancer Center, the Dana-Farber Cancer Institute, the University of Wisconsin-Madison School of Medicine and Public Health, the National Institutes of Health, the Albert Einstein College of Medicine, Harvard Medical School, Georgetown University, and the Georgetown-Lombardi Institute for Cancer and Aging contributed to the study.

The study was funded by the National Institutes of Health (grants U01CA253911 and U01CA199218).