Definition of a Multiparametric Prognostic and Predictive System of Classification of NET G3 Patients

Inclusion Criteria: * Histological diagnosis of well-differentiated neuroendocrine tumor G3 performed or reviewed by a NEN-dedicated pathologist. In case of "late-NET G3" (see Patients and Methods), a NEN-dedicated pathologist will perform a pathologic review of the previous NET G1/2, too, unless a NEN-dedicated pathologist has already done and except for diagnoses performed in a NEN-referral Center. * Age \> 18 years * Signed written informed consent * Available tumor tissue (formalin-fixed paraffin-embedded, FFPE) (preferably within 6 months). If the tumor contained in FFPE tissue block cannot be provided in total, sections from this block should be provided that are freshly cut. Preferably, 25 slides should be provided (minimum of 15 slides). If tumor tissue is not available, patients should be willing to undergone to a new biopsy. * late-NET G3 patients that will come to our Institute at the moment of NET G3 diagnosis will be enrolled only if they performed 68 Gallium-DOTATOC positron emission tomography (PET)/CT and 18 fluorodeoxyglucose (FDG)-PET/CT for the previous NET G1/2. The previous functional imaging performed during the NET G1/2 history shall be available for the review by our specialist in nuclear medicine. Exclusion Criteria: * Diagnosis of well-differentiated NET G1/2 or poorly-differentiated NEC * Diagnosis of mixed neuroendocrine non-neuroendocrine neoplasms (MiNENs) in which a NET G3 as neuroendocrine component * Cytological diagnosis of NET G3 or not availability of tumor tissue for pathological analysis * Concurrent neoplastic disease (e.g. advanced breast or prostatic cancer in hormonal treatment, hematologic diseases)