Despite increased demand for contralateral prophylactic mastectomy (CPM), the survival benefit of this procedure remains uncertain.

We used the Surveillance, Epidemiology, and End Results database to identify 107 106 women with breast cancer who had undergone mastectomy for treatment between 1998 and 2003 and a subset of 8902 women who also underwent CPM during the same period. Associations between predictor variables and the likelihood of undergoing CPM were evaluated by use of ² analyses. Risk-stratified (estrogen receptor [ER] status, stage, and age) adjusted survival analyses were performed by using Cox regression. Statistical tests were two-sided.

In a univariate analysis, CPM was associated with improved disease-specific survival (hazard ratio [HR] of death = 0.63, 95% confidence interval [CI] = 0.57 to 0.69; P < .001). Risk-stratified analysis showed that this association was because of a reduction in breast cancer–specific mortality in women aged 18–49 years with stages I–II ER-negative cancer (HR of death = 0.68, 95% CI = 0.53 to 0.88; P = .004). Five year–adjusted breast cancer survival for this group was improved with CPM vs without (88.5% vs 83.7%, difference = 4.8%). Although rates of contralateral breast cancer among young women with stages I–II disease undergoing CPM were independent of ER status, women with ER-positive tumors in the absence of prophylactic mastectomy also had a lower overall risk for contralateral breast cancer than women with ER-negative tumors (0.46% vs 0.90%, difference = 0.44%; P < .001).

CPM is associated with a small improvement in 5-year breast cancer–specific survival mainly in young women with early-stage ER-negative breast cancer. This effect is related to a higher baseline risk of contralateral breast cancer.