Among breast cancer patients, both chemotherapy and tamoxifen independently reduced the risk of developing a second cancer in the other breast, according to a study published online December 25 in the Journal of the National Cancer Institute. The risk reduction persisted for at least 10 and 5 years, respectively.

For breast cancer patients the risk of developing cancer in the other breast is two to six times greater than the breast cancer risk of the general public. Studies have shown that taking tamoxifen for five years reduces the risk of cancer in the opposite breast among women who have estrogen receptor-positive breast cancer, but the studies did not clarify how long the protective effect lasts.

Lisbeth Bertelsen, M.D., of the Danish Cancer Society in Copenhagen and colleagues investigated the relationship between tamoxifen and chemotherapy - either alone or in combination - and the risk of cancer in the opposite breast among American and Danish women who were first diagnosed with breast cancer before age 55. The study included 1,158 women who developed cancer in one breast and an additional 634 who initially had cancer in one breast then developed a second cancer in the other breast.

The chemotherapy treatment was associated with a 43 percent reduced risk for developing cancer in the opposite breast, compared with no chemotherapy. This risk reduction lasted up to 10 years after the initial cancer diagnosis and was stronger among women who entered menopause within a year of their diagnosis. Tamoxifen use was associated with a 34 percent reduced risk of a second breast cancer, compared with no tamoxifen use, and this reduction continued for five years after diagnosis.

"Ovarian suppression caused by chemotherapy may have a role in the association, possibly in combination with a cytotoxic effect on [breast tumor cells]," the authors write.