How long should we maintain anti-HER2 therapy for metastatic breast cancer patients with complete remission?

Metastatic HER2-positive breast cancer is typically estrogen and progesterone receptor negative, moderate-to-high tumor grade, DNA aneuploid with high S phase fraction, and featuring ductal rather than lobular histology. Trastuzumab, the monoclonal antibody to the extracellular domain of tyrosine kinase HER2 (HER2/neu, erbB-2), has been routinely administered to patients with HER2-amplified metastatic breast cancer since its US FDA approval in October 1998. [...]before the initiation of HER2-targeted therapy, HER2-positive breast cancers were more likely to spread early to major visceral sites including the axillary lymph nodes, bone marrow, lungs, liver, adrenal glands and ovaries.