RATIONALE: Osmotic blood-brain barrier disruption uses certain drugs, such as mannitol, to open the blood vessels around the brain and allow tumor-killing substances to be carried directly to the brain. Drugs used in chemotherapy, such as methotrexate and carboplatin, work in different ways to stop the growth of tumor cells, either by killing the cells or by stopping them from dividing. Monoclonal antibodies, such as trastuzumab, can block tumor growth in different ways. Some block the ability of tumor cells to grow and spread. Others find tumor cells and help kill them or carry tumor-killing substances to them. Trastuzumab may also help methotrexate and carboplatin work better by making tumor cells more sensitive to the drugs. Giving osmotic blood-brain barrier disruption together with methotrexate, carboplatin, and trastuzumab may kill more tumor cells.