This clinical study is exploring a new way to treat patients with large digestive system neuroendocrine tumors that cannot be removed by surgery. The study involves a combination of two treatments. First, patients receive **Stereotactic Body Radiation Therapy (SBRT)**, which is a special kind of radiation that targets tumors precisely. Then, they receive **177Lu-DOTATATE Peptide Receptor Radionuclide Therapy (PRRT)**, a radioactive drug that binds to specific tumor cells and delivers radiation to kill them. The study will check if this combination treatment is safer and more effective than using PRRT alone.

• **Study Length**: The treatment phase lasts about 4 months, followed by 12 months of follow-up visits every 3 months.
• **Treatment Details**: Involves SBRT over 5 sessions and PRRT every 8 weeks for 4 doses.
• **Eligibility**: Open to adults with certain types of large, unresectable tumors who have previously tried other treatments.

Participants will be monitored through various imaging tests like **CT scans** and **MRI**. The study aims to determine if this combined approach can improve survival and reduce the risk of tumor growth without causing severe side effects.