Permission to Collect Blood Over Time for Research

Inclusion Criteria: * Male or female, \>= 18 years old. There are no ethnic restrictions. * Active cancers of the GI tract (gastroesophageal, colorectal, or pancreatic-biliary) that have completed TNM staging by the American Joint Committee on Cancer; stage III disease if diagnosed within 2 months and all stage IV disease * Ability to understand and the willingness to sign a written informed consent document. * Existing staging CT imaging study Exclusion Criteria: * Life expectancy \< 6 months * History of deep vein thrombosis (DVT) or pulmonary embolism (PE) * Known pregnancy or positive urine pregnancy test in pre-menopausal women * On anticoagulant therapy (heparin, warfarin, direct thrombin inhibitors) * No CT imaging studies, or contraindications to undergoing CT imaging * Existing or anticipated need for a tunneled central venous catheter * Clinic visitation to Stanford Cancer center for secondary consultation purposes only * Inability to give informed consent