Proton Cranio-spinal Irradiation for Leptomeningeal Metastasis (CSI ProLong)

Inclusion Criteria: Metastasis to the leptomeningeal space (LM) surrounding the brain and/ or spinal cord Metastasis verified by MRI or CSF cytology Karnofsky >=60 (requiring some help, can take care of most personal requirements) Adequate bone marrow function Haemoglobin > 5 mmol/l Absolute neutrophil count >1 10^9/l Platelet count > 100 10^9/l Patient consent Female subjects must either be of non-reproductive potential ( ≥ 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 pregnancy test within 2 weeks prior to starting treatment. Patient at reproductive potential must agree to practice an effective contraceptive method. Exclusion Criteria: • Previous radiotherapy to the intended treatment site that precludes developing a treatment plan that respects normal tissue tolerances (Yang) 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 Pregnant or lactating women.