Looking at the United States and Britain, the researchers credit the drug tamoxifen.
Better treatment over the last decade has dropped breast cancer death rates in the United States and Britain and will save the lives of 14,000 women this year in the two countries, researchers said.
Most of the credit goes to the drug tamoxifen, taken by about 1-million women worldwide, said Sir Richard Peto, a professor of epidemiology at Oxford University who headed the study, published this week in the Lancet medical journal.
"This is the first time that improvements in the treatment of any type of cancer have ever produced such a rapid fall in national death rates," Peto said. "They really are remarkable trends."
While "a lot of things contribute, I think the key one is tamoxifen," he said, adding it is much too sudden a drop to be due to changes in the causes of the disease.
Britain and the United States were studied mostly because they had the most current, detailed statistical information and because they were among the first to use tamoxifen, Peto said.
An analysis of other Western countries would show they already are starting to see a similar trend, said Dr. Kent Osborne, director of the breast center at Baylor University in Waco, Texas, who was not connected with the study.
Osborne agreed with Peto that breast cancer screening, which only started to become widespread in the 1980s, has not been around long enough to cause the death rates to drop.
Peto said death rates will continue to decline once the benefits of screening emerge and other countries begin to more widely use tamoxifen.
His research showed that breast cancer deaths will be down 30 percent in Britain this year, compared with the late 1980s, before chemotherapy and tamoxifen became widely used. It indicated 13,000 women would die of breast cancer this year, compared with 17,000 in the late 1980s.
The United States will experience an estimated 20 percent drop in death rates, with about 40,000 women dying of cancer this year, Peto said. If treatment methods were at their pre-1989 stage, 50,000 would die this year, he said.
Britain is slightly ahead, probably because tamoxifen was invented there and used there first, Peto said.
Breast cancer is particularly problematic because remnants may remain after a tumor has been cut out and can cause a recurrence of cancer years later.
There are three main treatments after surgery. Radiotherapy targets the remnants of cancer lurking in or near the breast. Chemotherapy is a cocktail of cell-killing drugs, and tamoxifen and other hormonal drugs block hormones from nurturing the cancer.
Tamoxifen alone has been found to head off one in six recurrences and cut the risk of relapse by more than one-third. It also halves the risk of a new cancer occurring in the other breast, according to Peto's analysis, which examined the largest-ever collection of existing evidence on cancer therapy.
While tamoxifen fights cancer, it also can cause womb cancer and blood clots in the lungs. But using the drug for five years does 30 times more good than harm, the study found.
For every 1,000 women who take it, tamoxifen leads to 80 fewer deaths from breast cancer, two more deaths from womb cancer and one more death from blood clots in the lungs, it found.
Chemotherapy seems to work as well as tamoxifen in young women, but only about a third as well as tamoxifen in middle age.
When the two are used together, the chances of survival are even better.
Radiotherapy, the oldest treatment, prevents about two-thirds of recurrences, but has yielded only a 1 percent decrease in the overall death rates. While it works well in clearing up rogue bits of cancer in the breast, it also increases the risk of heart attacks and strokes.
Overall, radiotherapy has reduced the death rate from breast cancer by 5 percent since the late 1980s, but has increased the rate of death from other causes by 4 percent.
"This radiotherapy effect is certainly not going to explain the trend," said Jack Cuzick, a professor of mathematics, statistics and epidemiology at the Imperial Cancer Research Fund in London, which funded the analysis.
Death rates from lung cancer have plummeted, but that has mainly been due to people quitting smoking. Cervical cancer rates have also fallen, but mostly because of early detection.
While improved treatment has led to better survival in rare cancers such as Hodgkin's disease and childhood leukemia, none of the other major cancers have benefited as much as breast cancer, Peto said.