Photon Craniospinal Irradiation for the Treatment of Leptomeningeal Disease Secondary to Breast Cancer or Non-small Cell Lung Cancer

Inclusion Criteria: * Documented informed consent of the participant and/or legally authorized representative * Assent, when appropriate, will be obtained per institutional guidelines * ≥ 18 years * Karnofsky performance status (KPS) ≥ 60 * Ability to read and understand English or Spanish for questionnaires, or ability to complete questionnaires using certified interpreter * Histologically confirmed breast cancer or non-small cell lung cancer * Leptomeningeal disease established either radiographically and/or CSF cytology * Absolute neutrophil count (ANC) ≥ 1,000/mm\^3 * Hemoglobin ≥ 8 g/dL * Platelet ≥ 100,000/mm\^3 * Creatinine clearance of ≥ 50 mL/min per 24 hour urine test or the Cockcroft-Gault formula * Women of childbearing potential (WOCBP): negative urine or serum pregnancy test * If the urine test is positive or cannot be confirmed as negative, a serum pregnancy test will be required * Agreement by females and males of childbearing potential to use an effective method of birth control or abstain from heterosexual activity for the course of the study through at least 1 month after the last dose of protocol therapy * Childbearing potential defined as not being surgically sterilized (men and women) or have not been free from menses for \> 1 year (women only) Exclusion Criteria: * Chemotherapy, biological therapy, immunotherapy within 7 days prior to day 1 of protocol therapy * Any prior radiation that in the opinion of the investigator unable to respect normal tissue tolerances and preclude craniospinal irradiation * Patients with multiple or serious major neurologic symptoms (including encephalopathy) per physician / investigator assessment * Patients with extensive, uncontrolled extracranial systemic disease * Patients without reasonable systemic treatment options per physician / investigator * Other clinically significant uncontrolled illness per opinion of physician / investigator * Patients with a history or evidence of HIV infection (unless on effective anti-retroviral therapy with undetectable viral load based on prior tests within 6 months are eligible for this trial) * Patients with history or evidence of chronic hepatitis B virus (HBV) infection (unless HBV viral load is undetectable based on prior tests and on suppressive therapy) * Patients with a history or evidence of hepatitis C virus (HCV) infection unless treated and cured. (Patients with HCV infection who are currently on treatment are eligible if they have an undetectable HCV viral load based on prior tests) * Other active malignancy. Patients with a prior or concurrent malignancy whose natural history or treatment does not have the potential to interfere with the safety or efficacy assessment of the study are eligible for this trial * Females only: Pregnant or breastfeeding * Any other condition that would, in the investigator's judgment, contraindicate the patient's participation in the clinical study due to safety concerns with clinical study procedures * Unable to undergo MRI brain and spine with gadolinium contrast * Prospective participants who, in the opinion of the investigator, may not be able to comply with all study procedures (including compliance issues related to feasibility/logistics)