Breast cancer death rates declined almost 40 percent between 1989 and 2015, averting 322,600 deaths, reports the American Cancer Society.

Breast cancer death rates increased by 0.4 percent per year from 1975 to 1989, according to the study. After that, mortality rates decreased rapidly, for a 39 percent drop overall through 2015. The report, the latest to document a long-term reduction in breast-cancer mortality, attributed the declines to both improvements in treatments and to early detection by mammography.

Deanna Attai, a breast cancer surgeon at the University of California, Los Angeles who was not involved in the study, said the advances in treatment included much better chemotherapy regimens — developed in the 1980s and refined ever since — that are administered post-surgery to reduce the risk of recurrence.

Other improvements have included tamoxifen, an anti-estrogen agent that was approved in the late 1970s; Herceptin, a drug used to treat tumors with a higher-than-normal level of a protein called HER2 and drugs called aromatase inhibitors.

More recently, more sophisticated targeted treatments are being used to treat cancer that has spread beyond the breast.

Even with the gains, however, the toll of the disease remains high. Breast cancer is the most common cancer diagnosed in U.S. women and the second-leading cause of death after lung cancer. About 252,000 new cases of breast cancer are expected to be diagnosed in the United States this year, and more than 40,600 women are expected to die of the disease.

A woman in the U.S. has a 12.4 percent, or 1 in 8, lifetime risk of being diagnosed with breast cancer.

Eighty-one percent of breast cancers are diagnosed among women age 50 years and older, the study found, and 89 percent of breast-cancer deaths occur in that age group. The median age at diagnosis for women overall is 62 years.