According to current guidelines, patients have to withhalt long-acting somatostatin analogues (LA-SSA) for 4-6 weeks prior to every 177Lutetium-DOTATATE administration. The primary objective of the study is to investigate the effect of LA-SSA on the absorbed dose in tumor lesions during PRRT.

The study population includes all patients aged over 18 with a neuroendocrine tumor grade I and II and a clinical indication for PRRT. Patients treated for at least three months with LA-SSA are randomized over two interventional arms: one arm where patients discontinue LA-SSA 4-6 weeks before the first PRRT treatment and one arm where patients continue LA-SSA treatment and receive the first PRRT administration within one week after the most recent LA-SSA injection. Patients in the control arm who have not been treated with LA-SSA in the last three months will start PRRT according to standard local protocol.

Inclusion Criteria:

• Age ≥ 18 years;
• Able to provide spoken and written informed consent for the trial;
• Histopathological confirmed neuroendocrine tumor;
• Fulfill the clinical criteria for PRRT;
• At least one soft tissue lesion > 2 cm;
• Aimed administered activity of 7400 MBq;
• ECOG score (performance status) 0-2.

Exclusion Criteria:

• Not possible to discontinue LA-SSA for 4-6 weeks;
• Use of short-acting SSAs;
• Pregnancy and lactating female patients;
• Inability to comply to the study procedures;
• Factors that might affect the biodistribution (for example, indication for furosemide directly after PRRT infusion, limited fluid intake, any renal catheters, etc.).