The purpose of this study is to find the safest and most effective dose for delivering proton beam to the space containing CSF, brain, and spinal cord, in treating leptomeningeal metastases. The researchers think that using proton beam radiation to treat the space containing CSF, brain, and spinal cord, instead of treating only the areas where the metastasized tumor cells are causing symptoms, would improve the treatment of this disease.

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.