After lung cancer, metastatic breast cancer is the second most common cancer associated with brain metastases in the United States. As patients with advanced breast cancer live longer, the incidence of brain metastases appears to be increasing, with one meta-analysis suggesting that approximately one-third of patients with human epidermal growth factor receptor 2 (HER2)-positive, one third of those with triple-negative, and 15 percent of those with hormone receptor-positive, HER2-negative metastatic breast cancer will develop brain metastases. In a subset of women, progression in the CNS has become the major life-limiting problem.