Among women who survive breast cancer, the most frequent new cancer occurs in the opposite, or contralateral, breast. Since 1985, a new study reports, the incidence rate of contralateral breast cancer has declined steadily—at a pace of more than 3 percent per year. Although the causes of this trend are not known, the decline occurred as drugs such as tamoxifen were coming into widespread use to help prevent a recurrence of the disease, researchers reported online in the Journal of Clinical Oncology on March 14.

To assess the incidence rate of contralateral breast cancer over time, Dr. Amy Berrington de González of NCI’s Division of Cancer Epidemiology and Genetics (DCEG) and her colleagues analyzed statistics from the SEER database for the years 1975 to 2006.

The downward trend they observed was driven by declining rates of contralateral breast cancer among women whose first breast cancer tested positive for the estrogen receptor (ER). No clear decrease was seen among women with ER-negative tumors. Although the details of hormone treatments were not available in SEER, the annual declines of more than 3 percent after ER-positive first cancers “suggest an important role for the widespread usage of adjuvant therapies, especially hormone treatments,” the study authors noted.

“We know from randomized trials that drugs like tamoxifen significantly reduce the risk of a contralateral breast cancer, by about 40 percent,” said Dr. Berrington de González. The combination of the trial results, the timing of the decline, and the restriction to cancers following ER-positive first breast cancers all point to adjuvant hormonal therapy as a key factor in the decline, she added.

Tamoxifen was widely adopted in the United States after the results of the Nolvadex Adjuvant Trial were published in 1983. In addition to adjuvant hormone therapies, other factors such as the increased use of chemotherapy may also have contributed to the decline, the study noted. “Newer hormonal therapies like aromatase inhibitors may be even more effective than tamoxifen at reducing contralateral breast cancer rates, and so we would hope to see further reductions in the future,” said Dr. Berrington de González.

Although the decline represents “a notable success,” the study authors cautioned that overall rates of contralateral breast cancer remain high, especially among women whose first breast cancer is ER-negative. “We still need new strategies for reducing risks after an ER-negative cancer, because rates can be as high as 1 percent per year for these women,” said Dr. Berrington de González.

source: NCI bulletin