Breast cancer death rates declined almost 40 percent between 1989 and 2015, averting 322,600 deaths, the American Cancer Society reported Tuesday. From a report: 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 at 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.