African American and white women with breast cancer have the same risks of getting cancer in the other breast while on the drug tamoxifen, according to a study in the December 1, 2004, Journal of the National Cancer Institute* by scientists at the National Cancer Institute (NCI), part of the National Institutes of Health. The benefits of tamoxifen, including protection against cancer in the second breast (contralateral breast cancer), have been demonstrated in previous studies whose subjects were predominantly white.

Tamoxifen stops or slows the growth of cancer cells in the breast by interfering with the activity of estrogen, a hormone some breast cancers need to grow. Chemotherapy and tamoxifen are given to women with early stage breast cancer to treat cancer cells throughout the body and to prevent the cancer from coming back. Tamoxifen can also prevent contralateral breast cancers. Large trials have shown that women with early stage breast cancer who are treated with this drug for five years have a 47 percent reduced rate of contralateral breast cancer.

But these studies also have shown a small increase in the number of blood clots in women taking tamoxifen, especially when it is taken during chemotherapy. Blood clots can be deadly, leading to strokes, for example, or blocking the lungs' supply of blood from the heart. Worta McCaskill-Stevens, M.D., the study's lead author and a medical oncologist in NCI's Division of Cancer Prevention, said, "There have been concerns about the association of blood clots with tamoxifen because while few data exist on the effects of tamoxifen on African American women, greater rates of obesity and cardiovascular disease in this population leave them more vulnerable to blood clots than white women."

For their study, McCaskill-Stevens and her colleagues at the National Surgical Adjuvant Breast and Bowel Project (NSABP) analyzed data from 13 clinical trials of tamoxifen performed by the NSABP. On their own, none of these studies included enough African American women for scientists to determine whether tamoxifen affects them differently. Together, these trials include 20,878 women, of whom 1,842 are African American. The NCI scientists did two analyses of the data: one to determine the effect of tamoxifen on contralateral breast cancer in white and African American women, the other to determine the effect of tamoxifen on the rates of blood clot-related events in African American and white patients.

Because of evidence that tamoxifen is primarily effective in treating estrogen receptor-positive breast cancers--cancers whose cells have a surface protein that estrogen can attach to--McCaskill-Stevens' group included only women with this kind of cancer in its analysis of contralateral breast cancer rates. This group included 10,619 women. Race had no effect on the rate of contralateral breast cancers, and the reduced rate of contralateral cancer due to tamoxifen treatment was the same in both races.

For their study of the incidence of blood clot-related events, the researchers analyzed data from all 20,878 women in the trials. Race had an effect on the risk of these events (blood clot-related events were greater in African Americans) but the effect of tamoxifen on the rates was not different according to race.

Body mass index was a risk factor for both contralateral breast cancer and blood clots. Age was associated with an increased risk of blood clots, while the number of lymph nodes infiltrated by cancer cells increased the risk of contralateral cancers. The latter is contrary to previous studies, most of which have shown no such association.

Based on these results, McCaskill-Stevens said, "African American and white women appear to have the same risks of breast cancer and blood clots in response to tamoxifen." She believes their data demonstrate "the need to assess an individual woman's health and personal history to determine the benefits and risks of prescribing tamoxifen to prevent breast cancer."

For more information about cancer, please visit the NCI Web site at http://www.cancer.gov or call NCI's Cancer Information Service at 1-800-4-CANCER (1-800-422-6237).

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