Neuroendocrine tumors (NETs) are a wide group of neoplasms arising from the diffuse neuroendocrine cell system that features significant molecular and biological heterogeneity. They mainly derive from the enterochromaffin cells of the gastroenteropancreatic tract (GEP-NETs) and their incidence and prevalence are steadily rising, possibly as a consequence of improving diagnostic methods and earlier detection. A major feature of GEP-NETs is their somatostatin receptor (SSTR) immunogenicity, which is relevant both for diagnostic and therapeutic purposes.

For patients with unresectable or advanced disease, systemic treatment is the standard of care. In this setting, Somatostatin analogues (SSAs) are the standard first line therapy and, even if response rates are low, disease progression is halted in about two thirds of patient. Recently, targeted radionuclide therapy has claimed significant attention as a valuable treatment option for many solid neoplasms. This approach relies on the administration of a radionuclide linked to a carrier-molecule that selectively interacts with tumor associated antigens, being eventually internalized and releasing β-radiation emission and low-energy γ rays directly from the inside of the cancer cells. Peptide Receptor Radionuclide Therapy (PRRT) is strongly recommended in progressive metastatic/inoperable pretreated NETs that showed homogenous SSTRs expression by SSA positive PET-CT or single photon emission computed tomography (SPECT) imaging. Although PRRT is effective in the majority of cases, approximately 15-30% of patients will eventually progress during treatment. It is still challenging to distinguish potential responders versus non-responder patients. The identification of predictive biomarkers, apart from the required expression of somatostatin receptors, and of non-invasive diagnostic predictive exams, are an unmet need. Despite the promising clinical results, very little is known about the biological changes induced by PRRT on cancer tissue and tumor microenvironment and vascularization. The assessment of treatment' response therefore still relies on CT and PET-CT as markers of tumoral activity.

Among imaging modalities, ultrasound could play a key role in this setting. Indeed, contrast-enhanced ultrasound (CEUS) allows a thorough assessment of tumor perfusion through analysis of both contrast media flow pattern and time-intensity curves. This quantitative analysis, called dynamic contrast enhanced ultrasound (DCE-US) is a novel technique that estimates tissue perfusion based on phase-specific enhancement after the injection of microbubble contrast agents. The parameters derived from this analysis could be used for treatment monitoring in oncology, as they are easily comparable through time in each patient. In order to establish the bases for standardization of DCE-US, the European Federation of Societies for Ultrasound in Medicine and Biology (EFSUMB) recently published an update on this topic.

Inclusion Criteria:

• Age ≥ 18 years.
• Histologically confirmed diagnosis of unresectable, well-differentiated GEP-NET G2 or G3 according to WHO 2019 Classification with a Ki67 index > 2% up to 55%.
• Somatostatin receptor-positive (SSTR+) disease defined by a homogenous SSTRs expression by SSA positive PET-CT or SPECT imaging.
• At least one target lesion according to RECIST v1.1.
• Participants must have signed and dated an approved written informed consent and must be able to comply with any study specific procedure.

Exclusion Criteria:

• Known hypersensitivity to Lutetium 177Lu - radionuclides.
• Other known malignancies.
• Patients not able to declare meaningful informed consent on their own or any other vulnerable population to that.