<qa>  What is the side effect of methotrexate, mannitol,This phase III trial studies the side effects of methotrexate, mannitol, rituximab, and carboplatin and to see how well they work in treating patients with primary central nervous system lymphoma. Drugs used in chemotherapy, such as methotrexate and carboplatin, work in different ways to stop the growth of cancer cells, either by killing the cells, by stopping them from dividing, or by stopping them from spreading. Osmotic blood-brain barrier disruption uses mannitol to open the blood vessels around the brain and allow cancer-killing substances to be carried directly to the brain. Monoclonal antibodies, such as rituximab, may block cancer growth in different ways by targeting certain cells. Giving methotrexate, mannitol, rituximab, and carboplatin together may be an effective treatment for primary central nervous system lymphoma. 