Hippocampal Avoidance in Craniospinal Irradiation for the Treatment of Leptomeningeal Metastases From Breast Cancer or Non-small Cell Lung Cancer

Inclusion Criteria: * Patients with breast cancer or NSCLC malignancies with leptomeningeal metastases established radiographically and/or through CSF cytology * Patients who are candidates for radiation therapy for the treatment of leptomeningeal metastases * Patients ≥ 18 years old * Karnofsky performance status (KPS) ≥ 60 or Eastern Cooperative Oncology Group (ECOG) ≥ 2 * The patient is able to provide informed consent * Hemoglobin \> 8 g/dL * Absolute neutrophil count \> 1,000/mm * Platelet count \> 100,000/mm * Participants born female at birth must either be of non-reproductive potential (i.e. post-menopausal 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 /urine pregnancy test within 3 weeks prior to starting radiation therapy (RT) * Patients with reproductive potential must agree to practice two highly effective contraceptive methods Exclusion Criteria: * Patients with multiple, serious major neurologic deficits per physician/investigator assessment including encephalopathy * Patients with extensive systemic disease and without reasonable systemic treatment options * Patients who are 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 * Gross ventricular disease * Brain metastases within 5 mm of the hippocampal contours not previously treated * Pregnant or lactating women