Overview of Neuroendocrine Tumors

Classification and Diagnosis of Neuroendocrine Tumors

NETs represent cancers of neuroendocrine cells that can develop virtually anywhere in the body, though most are diagnosed in the gastrointestinal tract. Approximately 12,000 patients are newly diagnosed with NETs annually in the United States. Pancreatic and lung primaries are also common locations, but these tumors can arise from almost any organ system, making their presentation diverse and sometimes challenging to diagnose.

A critical distinction exists between functional and nonfunctional tumors, with functional tumors capable of releasing hormones that cause carcinoid syndrome. Patients may present with vague symptoms including unexplained abdominal pain, weight loss, diarrhea, and flushing. Due to the nonspecific nature of these symptoms, patients often visit health care providers 20 to 30 times before receiving an accurate diagnosis, sometimes experiencing delays lasting years.

The microscopic appearance forms the foundation of NET classification, distinguishing between well-differentiated and poorly differentiated tumors. Well-differentiated NETs are classified using Ki-67 grading: low grade (≤ 3%), intermediate grade (3%-20%), and high grade (≥ 20%). Poorly differentiated neuroendocrine carcinomas represent very aggressive biology requiring chemotherapy, while well-differentiated tumors form the focus of most targeted treatment strategies.