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)

   - Eighteen years of age or older

   - CT or MRI order obtained and within 4 weeks of CgA measurement

   - BRAHMS CgA II KRYPTOR baseline measurement available

   - Patient has discontinued the following treatments for at least 3 weeks before study
   start: i) proton pump Inhibitors (PPI), ii) corticoids, iii) H2-receptor antagonists

   - Baseline Eastern Cooperative Oncology Group Performance Scale (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 1.5x upper limit of normal (ULN)

   - Severe liver dysfunction in the absence of liver metastasis defined by aspartate
   aminotransferase (AST), serum total bilirubin and/or alanine transaminase (ALT) 1.5x
   ULN; severe liver dysfunction in the presence of liver metastasis defined by AST and
   ALT over 5x ULN and total bilirubin over 1.5x 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: i) proton pump Inhibitors (PPI), ii)
   corticoids, iii) H2-receptor antagonists

   - Chronic alcohol and/or substance abuse

   - Known pregnancy