Diagnosis
In many cases, gastrinoma is diagnosed based on the patients history which is typically characterized by recurrent episodes of peptic ulcer disease or by severe reflux esophagitis and/or diarrhea or by acid-related symptoms which fail to respond to standard treatment regimens. To confirm the diagnosis of gastrinoma a series of blood tests must be made. One of those tests is the serum gastrin level, which is the most reliable test for patients with gastrinoma. The normal levels of gastrin are 150 pg/mL ( > 72.15 pmol/L); therefore elevated levels of > 1000 pg/mL (> 480 pmol/L) would establish the diagnosis of gastrinoma. Another test that can be conducted is the secretin stimulated test, which is useful in patients who have the sign and symptoms of gastrinoma but the gastrin levels are below < 1000 pg/mL. Usually, an Intravenous bolus consisting of secretin 2mcg/kg and is measured in 10 minute intervals up to 30 minutes total. Secretin, which is a hormone released from the duodenal S cells that induce the release of pancreatic bicarbonate (HCO3) that would neutralize the acidic environment due to high gastrin levels. Therefore, if the patients level of gastrin remains consistently high indicating gastrin release due to tumor such as gastrinoma.Other commonly used tests to further confirm the diagnosis are

CT scan of the abdominal area
Somatostatin receptor scintigraphy which is used to identify the location of the tumor
PET scan
Endoscopic ultrasound if there are no signs of tumor metastases. Treatment
Surgery is first line treatment in gastrinomas; however it often fails to be curative.
Gastrinomas are neuroendocrine tumors (NETs), usually located in the duodenum or pancreas, that secrete gastrin and cause a clinical syndrome known as Zollinger–Ellison syndrome (ZES). A large number of gastrinomas develop in the pancreas or duodenum, with near-equal frequency, and approximately 10% arise as primary neoplasms in lymph nodes of the pancreaticoduodenal region (gastrinoma triangle).Most gastrinomas are sporadic (75–80%), whereas approximately 20–25% are associated with multiple endocrine neoplasia type 1 (MEN-1). Over 50% of gastrinomas are malignant and can metastasize to regional lymph nodes and liver. One fourth of gastrinomas are related to multiple endocrine neoplasia type 1, Zollinger–Ellison syndrome, peptic ulcer disease. Signs and symptoms
Gastrinoma in the early stages will have signs and symptoms of indigestion or similar to irritable bowel disease (IBD) such as:

Hypergastrinemia
Refractory or recurrent peptic ulcers involving duodenum
Chronic diarrhea
Generalized cancer symptoms
Abdominal pain
Gastrointestinal bleeding
Obstruction of intestine
Weight loss/poor appetite
Anemia (Due to vitamin B12 malabsorption, and bleeding)
Hematemesis
Gastroesophageal reflux disease
Esophageal complications (Barretts esophagus, esophagitis, stricture formation)
Vomiting
Steatorrhea

Pathophysiology
Gastrin is secreted by the G cells. G cells are primarily found in the pyloric antrum but can also be found in the duodenum and the pancreas. The primary function of gastrin is to induce the release of hydrochloric acid (HCl) from the parietal cells located in the fundus of the stomach. Parietal cells are responsible for hydrochloric (HCl) secretion along with intrinsic factor that binds to vitamin B12 and helps with its uptake in the terminal ileum. Other functions of gastrin include stimulating the growth of gastric mucosa and gastric motility and promoting gastric emptying.