Combination External Radiation and PRRT for Large GI Neuroendocrine Tumors.

Inclusion Criteria: * Male or female * Age ≥ 18 years * Patient must be able to provide study specific informed consent * Pathologically confirmed neuroendocrine tumor fulfilling all of the following criteria * Well-differentiated, grade 1-2 * Unresectable (prior resection is allowable), verified by tumor board or surgical oncology (surg onc) * Progression after one or two prior lines of systemic therapy * Somatostatin-receptor positive disease as determined by positive radiotracer-labeled DOTATATE PET/CT scan (modified Krenning score 3+) * One or more large lesions measuring 3 or more cm on contrast-enhanced CT or MRI * Eastern Cooperative Oncology Group (ECOG) performance status 0-2 * Estimated glomerular filtration rate (GFR) \> 30 mL/min (within 90 days prior to study registration) * Total bilirubin ≤ 3 x upper limit of normal (within 90 days prior to study registration) * Albumin \> 30 g/L (within 90 days prior to study registration) * White blood cell (WBC) ≥ 2,000 cells/mm\^3 (within 90 days prior to study registration) * Platelets ≥ 70000 cells/mm\^3 (within 90 days prior to study registration) * Hemoglobin ≥ 8.0 g/dl (Note: The use of transfusion or other intervention to achieve hemoglobin \[Hgb\] ≥ 8.0 g/dl is acceptable.) (within 90 days prior to study registration) Exclusion Criteria: * Any prior radiation therapy including prior PRRT, external radiation, or Yttrium-90 radioembolization to the same site/region * Contraindications to radiation therapy including …