Testing the Safety of M6620 (VX-970) When Given With Standard Whole Brain Radiation Therapy for the Treatment of Brain Metastases From Non-small Cell Lung Cancer, Small Cell Lung Cancer, or Neuroendocrine Tumors

Inclusion Criteria: * Patients with a histologically confirmed diagnosis of non-small cell lung cancer (NSCLC), including neuroendocrine tumors or small cell lung cancer (SCLC) who are being evaluated for palliative WBRT (with or without neurosurgical resection or stereotactic radiosurgery \[SRS\]) for radiologically or histologically diagnosed brain metastases presumed to be from the lung cancer are eligible for this Phase I study. Group 2 will only include NSCLC patients. * Life expectance of greater than two months to allow completion of study treatment and assessment of dose-limiting toxicity * Group 2 patients should have archived or fresh tumor tissue available from the non-craniotomy site and will have fresh tumor tissue available from the planned craniotomy * Age \>= 18 years. Because no dosing or adverse event data are currently available on the use of M6620 (VX-970, berzosertib) in patients \< 18 years of age, children are excluded from this study, but will be eligible for future pediatric trials. * Eastern Cooperative Oncology Group (ECOG) performance status =\< 2 (Karnofsky \>= 60%) * Note: though patients with ECOG performance status of 3 due to neurological deficits who are otherwise fit to receive systemic therapy per clinician assessments will be allowed * Leukocytes \>= 3,000/mcL * Absolute neutrophil count (ANC) \>= 1,500/mcL * Platelets \>= 100,000/mcL * If no known liver metastases: total bilirubin \< 1.5 x institutional upper limit of normal (ULN); if…