The estrogen receptor (ER) status of a primary breast cancer is associated with the receptor status of a breast cancer in the opposite breast for women who do not take tamoxifen, a new study has found.

In addition, patients with an ER-positive primary cancer who receive tamoxifen have fewer ER-positive contralateral breast cancers, possibly as a result of tamoxifen treatment.

The findings were published in the Journal of the National Cancer Institute.

About 6% to 9% of women with breast cancer will develop breast cancer in the opposite breast (called contralateral breast cancer). Tamoxifen reduces the risk of contralateral breast cancer and primarily benefits women with ER-positive disease.

Sandra Swain, M.D., of the National Cancer Institute, and colleagues examined data from earlier breast cancer clinical trials to determine the relationship between the ER status of primary and contralateral breast cancers and whether tamoxifen treatment affects this relationship.

Among patients who did not receive tamoxifen, 89% with an ER-positive primary cancer had an ER-positive contralateral breast cancer and 70% with an ER-negative primary cancer had an ER-negative contralateral breast cancer.

Among patients who received tamoxifen, 56% with an ER-positive primary cancer had an ER-positive contralateral breast cancer and 78% with an ER-negative primary cancer had an ER-negative contralateral breast cancer.

SOURCE:
Journal of the National Cancer Institute, April 7, 2004