Inclusion Criteria:

          -  Able to provide written informed consent by patient or legally acceptable
             representative

          -  Meets the criteria in the approved regulatory indication for first line treatment with
             osimertinib and agree to the restrictions, monitoring, and dose-adjustment criteria
             stipulated in the associated product label

          -  Epidermal growth factor receptor (EGFR) exon 19 deletions or exon 21 (L858R)
             substitution mutations (either alone or in combination with other EGFR mutations)

          -  No prior systemic therapy except neoadjuvant, adjuvant or concurrent chemotherapy
             given greater than 3 months prior to enrollment on study

          -  Asymptomatic or minimally symptomatic brain metastases (ie. Headache, nausea, or
             seizure responsive to dexamethasone/analgesic/antiepileptic on stable doses of
             medications for a minimum of 3 days)

          -  Brain metastases must meet the following criteria on a diagnostic MRI: at least one
             lesion can be classified as measurable disease per RANO-BM, ≤ 10 brain or brainstem
             metastases, ≤ 30 mm and brainstem metastases must be ≤ 5 mm, metastases > 5 mm from
             the optic nerve or chiasm

          -  ECOG performance status 0-2

          -  Life expectancy > 6 months

          -  Willing to abstain from sexual activity or willing to use double-barrier method during
             sexual intercourse

        Exclusion Criteria:

          -  Previous treatment with osimertinib, or any other EGFR TKI

          -  Patient with symptomatic brain metastases causing any neurologic deficit (not
             including headache, nausea, or medically controlled seizure)

          -  Multiple sclerosis

          -  Pacemaker or MRI-incompatible metal in the body

          -  Allergy to gadolinium MRI contrast

          -  Brain metastasis requiring surgery for decompression

          -  Leptomeningeal disease

          -  Previous cranial RT, or surgery for brain metastases

          -  Uncontrolled systemic lupus erythematosis, scleroderma or other connective tissue
             disorders considered a contraindication for radiotherapy

          -  Active cancer from another anatomical site within 5 years (non-melanomatous skin and
             cervical cancers permitted)

          -  Any medical or non-medical issue that would render patient unable to reliably complete
             regular QOL and neurocognitive assessments

          -  Judgment by the investigator that the patient should not participate in the study if
             the patient is unlikely to comply with study procedures, restrictions and requirements

          -  Treatment with an investigational drug within five half-lives of the compound or 3
             months, whichever is greater

          -  Patients with symptomatic CNS metastases who are neurologically unstable

          -  Patients currently receiving (or unable to stop use prior to receiving the first dose
             of study treatment) medications or herbal supplements known to be potent inducers of
             cytochrome P450 (CYP) 3A4

          -  Patients taking any drugs that are known to prolong QT interval that can't be
             withdrawn prior to Osimertinib

          -  Pregnant or breastfeeding