INTRODUCTION
Neuroendocrine tumors comprise heterogeneous group of neoplasms which originate from endocrine cells, both within endocrine organs and within the cells of diffuse endocrine system. These tumors have vari- able clinical behavior ranging from well-differentiated, slow growing tumors to poorly-differentiated, highly invasive malignancies. These tumors generally fall into two broad categories;
- A group of neuroendocrine tumors that has the biology and natural history of a high grade malig- nancy and a characteristic small cell with undifferenti- ated or anaplastic appearance by light microscopy. WHO classifies this group of tumors as poorly dif- ferentiated neuroendocrine carcinomas. A typical ex- ample is the small cell lung cancer.
- The second group has variable but most often in- dolent biological behavior and characteristic well-dif- ferentiated histologic features. The majority of these tumors arise in the gastrointestinal tract and collectively, they are referred to as gastroenteropancreatic neuroen- docrine tumors (GEP-NETs)1,2.
Gastroenteropancreatic neuroendocrine tumors can also be classified as functioning or non-functioning tumors. The term “non-functioning” refers to the ab- sence of clinical syndromes of hormonal hypersecretion. The functioning tumors include insulinoma, glucagonoma, gastrinoma, VIPoma and somatostatinoma.