**Study Overview**: This research is looking at how effective and safe a treatment called [177Lu]Lu-DOTA-TATE is for patients with certain types of tumors called gastroenteropancreatic neuroendocrine tumors (GEP-NETs). These tumors are in the digestive system and can be advanced, meaning they have spread a lot or can't be removed by surgery. The study is comparing [177Lu]Lu-DOTA-TATE with another treatment called octreotide LAR, which is already used for these tumors. *GEP-NETs* are a type of cancer that affects hormone-producing cells in the stomach, pancreas, and intestines. *Octreotide LAR* is a medication given as an injection to help control symptoms from these tumors. **Key Points**: 1. **Study Phases**: This study has three parts: screening, treatment, and follow-up. 2. **Eligibility**: Participants must be at least 12 years old, with specific tumor conditions, and have not received certain treatments before. 3. **Treatment Details**: Participants will receive either [177Lu]Lu-DOTA-TATE plus octreotide LAR or octreotide LAR alone. This study will help understand if adding [177Lu]Lu-DOTA-TATE offers better results compared to the standard treatment.