Inclusion Criteria:

• Primary well-differentiated G1 and G2 neuroendocrine tumor located in jejunum, ileum, colon, rectum, duodenum, appendix, stomach, or pancreas.
• Measurable disease according to RECIST criteria (version 1.1).
• 18 years of age or older.
• CT or MRI order obtained and within 4 weeks of CgA measurement.
• B·R·A·H·M·S CgA II KRYPTOR baseline measurement available.
• Patient has discontinued the following treatments for a least 3 weeks before study start:
  • Proton pump inhibitors (PPI);
  • Corticoids;
  • H2-receptor antagonists.
• Baseline ECOG PS < 2.
• Written informed consent signed.

Exclusion Criteria:

• Other active malignancy with the exclusion of melanoma or other cancers that occurred more than 5 years ago.
• Participation in another clinical trial involving an investigational therapeutic (exception: diagnostic studies and studies evaluating known therapies).
• No measurable disease by RECIST criteria (version 1.1).
• Severe renal dysfunction defined as creatinine of 1.5 x ULN.
• Severe liver dysfunction in the absence of liver metastasis defined by aspartate aminotransferase (AST), serum total bilirubin and/or alanine transaminase (ALT) 1.5 x ULN; Severe liver dysfunction in the presence of liver metastasis defined by AST and ALT over 5 x ULN and total bilirubin over 1.5 x ULN).
• Severe gastrointestinal disorders (chronic atrophic gastritis, pancreatitis, inflammatory bowel disease, irritable bowel syndrome).
• Severe cardiovascular disease (severe symptomatic congestive heart failure, pulmonary artery hypertension, acute coronary syndrome).
• Patients receiving active treatment with the following medications and samples were collected less than 3 weeks after discontinuing:
  • Proton pump inhibitors (PPI);
  • Corticoids;
  • H2-receptor antagonists.
• Chronic alcohol and/or substance abuse.
• Known pregnancy.