This is a randomized study to determine if not treating planning target volume (PTV) margins during radiation therapy worsens progression free survival rates in patients with brain metastases.

Inclusion Criteria:

• Adult patients (≥ 18 years old) with an ECOG Performance Status 0-2 and a life expectancy of 3 months or more.
• Histologically confirmed systemic malignancy with gadolinium contrast-enhanced MRI scan demonstrating 1-5 intraparenchymal brain metastases.
• Well-circumscribed, measureable intraparenchymal brain lesion(s) with maximum tumor diameter ≤3.0 cm. If multiple lesions are present, the other(s) must not exceed 3.0 cm in maximum diameter. At least one lesion must be ≥1.0 cm in maximum diameter and ≥0.5 cm in a perpendicular diameter to be considered measurable disease.
• Negative urine or serum pregnancy test done ≤ 14 days prior to CT simulation, for women of child-bearing potential only.
• Ability to understand and willingness to sign a written informed consent document.

Exclusion Criteria:

• Diagnosis of germ cell tumor, small cell carcinoma or hematologic malignancy.
• Metastases in the brain stem, midbrain, pons, medulla, or within 7 mm of the optic apparatus (optic nerves, chiasm and optic tracts).
• Diagnosis of leptomeningeal disease.
• Prior cranial radiotherapy within 90 days of trial enrollment or prior SRS at any time to any lesion to be treated on protocol
• Chemotherapy (including oral agents and targeted agents) or immunotherapy given within 14 days of SRS. Hormonal therapy is permitted. For Her2+ breast cancer patients, anti-Her2 therapy cannot be given within 14 days of SRS. Patients who are scheduled to receive trastuzumab emtansine after SRS cannot be enrolled.
• Contraindications to gadolinium contrast-enhanced MRI (eg, non-compatible pacemaker, eGFR<30, gadolinium allergy).