Inclusion Criteria:

          -  Pathologic diagnosis of hormone receptor positive (estrogen receptor >= 1 percent or
             progesterone receptor >= 1 percent) with HER2 negative status, past treatment, or
             systemic disease status with current clinical diagnosis of up to 10 brain metastases
             based on contrast-enhanced magnetic resonance imaging (MRI) of the brain

          -  Plan to start or currently receiving an Food and Drug Administration (FDA)-approved
             CDK4/6 inhibitor (CDKi), which must be started no later than 2 weeks prior to planned
             radiosurgery with plan to continue CDKi following radiosurgery

          -  Up to 10 brain metastases =< 3 centimeters in greatest dimension, measured on
             radiation planning MRI

          -  Eastern Cooperative Oncology Group (ECOG)/Zubrod 0-1, or Karnofsky performance status
             70-100

          -  Contrast-enhanced MRI brain within 4 weeks of radiosurgical intervention (radiation
             planning MRI)

          -  Patients must be able to sign informed consent prior to study entry, including assent
             to standard of care post-treatment surveillance contrast-enhanced magnetic resonance
             imaging of the brain

          -  Patients who are enrolled in the study, and who continue to be prescribed CDK4/6
             inhibitor therapy and develop new brain metastases deemed treatable by radiosurgery,
             are specifically allowed to be re-treated while on study, and the new treated lesions
             will be separately counted by treatment category (1 to 3, 4 to 6, or 7 to 10 new
             treated lesions)

          -  Patients with prior treated brain metastases (radiosurgery, hypofractionated
             radiotherapy, or surgery) are eligible for the study regardless of prior history of
             asymptomatic or symptomatic radiation necrosis and may not be excluded from the study
             if that is the sole basis for exclusion

        Exclusion Criteria:

          -  Patients with current or prior invasive malignancy unless disease free for minimum of
             1 year

          -  Brain metastases > 3 cm

          -  Brain lesions causing midline shift or herniation > 1 cm

          -  Patients with unirradiated post-neurosurgical metastasectomy resection cavities,
             unless disease-free in the surgical bed for >= 6 months, are prohibited from pilot
             study enrollment

          -  No patients who require resection cavity radiation for treatment of a resected brain
             metastasis (i.e.: standard of care treatment) are eligible for enrollment

          -  Receipt of chemotherapeutic agents (other than CDK4/6 inhibitors or hormone
             receptor-related targeted agents) within 2 weeks of planned radiosurgery date

          -  Prior whole brain or craniospinal radiotherapy

          -  Fractionated radiation to unrelated central nervous system (CNS) tumor

          -  Concurrent malignant CNS tumor

          -  Recurrent or progressive brain metastasis necessitating surgical or medical
             intervention (i.e.: a non-radiotherapy intervention such as steroids)

          -  Recurrence or progressive brain metastasis from prior surgical resection necessitating
             surgical or medical intervention (i.e.: a non-radiotherapy intervention)

          -  Brain stem metastasis >= 1 cm

          -  Patients with scleroderma

          -  Severe acute co-morbidity, defined as follows:

               -  Unstable angina and/or congestive heart failure requiring hospitalization in the
                  last 3 months

               -  Transmural myocardial infarction within the last 6 months

               -  Acute bacterial or fungal infection requiring intravenous antibiotics at the time
                  of registration

               -  Chronic obstructive pulmonary disease exacerbation or other respiratory illness
                  requiring hospitalization in the last 6 months or precluding study therapy due to
                  inability to rest supine at the time of registration