Proton Craniospinal Radiation Therapy vs. Partial Photon Radiation Therapy for Leptomeningeal Metastasis From Solid Tumors

Inclusion Criteria: * Patient with solid tumor malignancy with leptomeningeal metastases established radiographically and/or through CSF cytology * Patients who are candidates for radiation therapy for the treatment of leptomeningeal metastases * Patients of all ages * KPS ≥ 60 for participants ≥ 16 years old, and Lansky ≥ 60 for participants \< 16 years old * For adult patient, the patient/ legally authorized representative is able to provide informed consent. For pediatric patient, a parent is able to provide informed consent. * Adequate bone marrow function: * Hemoglobin \> 8g/dL * Absolute neutrophil count \>1,000/mm * Platelet count \> 100,000/mm * Female subjects must either be of non-reproductive potential (i.e. pediatric patients, OR postmenopausal by history \[≥ 60 years old, or with no menses for \>1 year without an alternative medical cause\], 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 at reproductive potential must agree to practice an effective contraceptive method. Exclusion Criteria: * Patient with multiple, serious major neurologic deficits per physician/investigator assessment 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.