Methods
The electronic medical record data at the time of trastuzumab administration were retrospectively reviewed. The following exclusion criteria were applied to 229 breast cancer patients who received trastuzumab at Osaka Medical and Pharmaceutical University Hospital during the 4-year study period: missing information on human epidermal growth factor receptor type 2 (HER2) status (n =1); missing information on eosinophils (n =11); or use of treatments other than trastuzumab (n =41).