In recent decades there has been a steady increase in the reported incidence of neuroendocrine tumors (NETs) around the world. The most frequent localizations of neuroendocrine neoplasms are the respiratory system, pancreas, and gastrointestinal tract. Pancreatic neuroendocrine tumors are up to 10% of all significant pancreatic neoplasms. Gastrinoma or Zollinger—Ellison syndrome is a rare functionally active neoplasm with a high malignancy potential. Patients with torpid erosive reflux esophagitis and with frequent recurrent ulcers or erosions of the stomach and duodenum are the candidates for differential diagnosis procedures. Tumor visualization has a fundamental importance in determining the location of the primary lesion, the stage of the disease, the choice of treatment method and evaluating its effectiveness. However, most often, verification of diagnosis is complicated by the small size of tumors. However, in view of high malignancy potential, only an early diagnosis and timely treatment can increase the survival rate of this category of patients.