Breast cancer is the most commonly diagnosed cancer in women and the second-leading cause of death among women with cancer. 1 Whereas outcomes for many breast cancers are favorable, human epidermal growth factor receptor-2 (HER2)-positive breast cancers may have an aggressive clinical course and are associated with higher rates of disease recurrence and mortality. 2, 3 Such tumors are characterized by overexpression of HER2 and/or amplification of the ERBB2 gene. 2, 4 Development of the monoclonal antibody that targets the extracellular domain of HER2, trastuzumab, revolutionized the care of these patients, leading to large improvements in disease-free and overall survival. 5 In addition, development of newer anti-HER2 therapies has led to further improvements in cancer outcomes for this population. 6–9

HER2 targeted therapies, such as trastuzumab, are generally well tolerated. They do not have significant myelosuppressive side effects nor do they cause typical symptoms associated with chemotherapy, such as emesis and alopecia. However, the safety of therapies directed at HER2, in particular trastuzumab, has been questioned by concerns regarding cardiotoxic effects. 10