Women with breast cancer often develop metastases in the brain. Currently, treatment of these metastases is difficult and relies on radiotherapy or surgery which often fail. Therefore, development of new methods of treatment for breast cancer with brain metastasis is very important. T-DM1 is a drug that is already in everyday use for a specific type of breast cancer called HER2-positive breast cancer. The objective of this study is to investigate whether T-DM1 is also effective in brain metastasis and can help patients to live longer and better