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)