Comparison of therapy benefit from standard anti-HER2 directed approaches in metastatic breast cancer (MBC) between initially HER2-positive patients and patients initially HER2-negative with switch to HER2-positive.

1040Background: Metaanalyses have demonstrated that 5% of initially HER2 negative breast cancer patients switch to HER2 positive during the course of the disease. Whether there is a difference in b...