Study of Proton Radiation to the Brain and Spinal Cord for Patients With Leptomeningeal Metastases

Inclusion Criteria: * Female subjects must either be of non-reproductive potential (i.e. post-menopausal by history: \>/= 60 years old and no menses for 1\> year without an alternative medical cause; OR history of hysterectomy, OR history of bilateral tubal ligation, OR history of bilateral oophorectomy) or must have a negative serum pregnancy test within 2 weeks prior to starting treatment. * Patient with solid tumor malignancy with leptomeningeal metastases established radiographically and/or through CSF cytology. * KPS ≥ 60. * Age ≥ 10 years. * For adult patient, the patient is able to provide informed consent. For pediatric patient, a parent is able to provide informed consent. * Patient at reproductive potential must agree to practice an effective contraceptive method. * Adequate bone marrow function: * Hemoglobin ≥ 8g/dL * Absolute neutrophil count ≥500/mm\^3 * Platelet count ≥ 100,000/mm\^3 Exclusion Criteria: * Patient with multiple, serious major neurologic deficits including encephalopathy. * Patient with extensive systemic disease and without reasonable systemic treatment options. * Patient who is unable to undergo MRI brain and spine with gadolinium contrast. * Previous radiotherapy to the intended treatment site that precludes developing a treatment plan that respects normal tissue tolerances. * Pregnant or lactating women.