What is a Neuroendocrine Tumor (NET)?
Neuroendocrine tumors (NETs) are well-differentiated malignancies that arise from specialized neuroendocrine cells throughout the body's diffuse endocrine system, most commonly occurring in the gastrointestinal tract, pancreas, and lungs. 1
Origin and Cellular Characteristics
NETs develop from neuroendocrine cells that possess both neuronal and endocrine properties, with the capacity to secrete hormones and amines. 1 These cells share similarities with neuroendocrine cells of the embryological gut and are distributed throughout multiple organ systems. 2
Anatomic Distribution
The most common sites include:
- Gastrointestinal tract: stomach, small intestine, appendix, rectum
- Pancreas (pancreatic NETs or Pan-NETs)
- Lungs and bronchopulmonary system
- Thymus
- Less commonly: other organs throughout the body 2, 1
Key Distinction: NETs vs NECs
It is critical to understand that the broader term "neuroendocrine neoplasm" (NEN) encompasses two distinct entities:
- Well-differentiated NETs: Typically slow-growing tumors with organoid architecture, often hormone-producing
- Poorly differentiated neuroendocrine carcinomas (NECs): Highly malignant, rapidly proliferating tumors that represent only 10-20% of all neuroendocrine neoplasms 1, 3
This distinction has profound implications for prognosis and treatment, as NECs behave much more aggressively than NETs.
Functional vs Non-Functional Classification
NETs are further categorized as:
- Functional NETs: Secrete hormones causing clinical symptoms (e.g., intermittent flushing, diarrhea in gastrointestinal NETs) 2
- Non-functional NETs: Do not produce hormonal symptoms and often present later with mass effect 4
Epidemiology
The incidence of NETs has increased dramatically—more than six-fold between 1997 and 2012. 1 Current estimates indicate:
- US incidence: 3.56-6.98 per 100,000 people per year 2, 1
- 20-year prevalence: 48 per 100,000 1
- US prevalence may exceed 170,000 individuals 2
Men are affected slightly more frequently than women and demonstrate worse outcomes. 1
Hereditary Associations
While most NETs are sporadic, approximately 5-10% occur in hereditary syndromes 1:
- Multiple Endocrine Neoplasia Type 1 (MEN1): Associated with pancreatic, duodenal, stomach, thymus, and lung NETs
- Von Hippel-Lindau (VHL) disease: Associated with pancreatic NETs
- Tuberous sclerosis (TSC) and neurofibromatosis: Also linked to pancreatic NETs
Important caveat: Recent genomic sequencing revealed that an additional 17% of apparently sporadic pancreatic NETs carry germline mutations in DNA repair genes (MUTYH, CHEK2, BRCA2), suggesting the hereditary component may be underrecognized. 1, 3
Diagnostic Markers
NETs are characterized by immunohistochemical expression of:
- Synaptophysin and/or Chromogranin A (CgA): Defining neuroendocrine markers 1
- These markers confirm the neuroendocrine phenotype on tissue diagnosis
Clinical Heterogeneity
NETs are highly heterogeneous tumors, and all elements must be considered when determining management:
- Tumor burden and extent of disease
- Proliferative activity (grading: G1, G2, G3)
- Somatostatin receptor (SSTR) expression status
- Rate of tumor growth
- Presence or absence of hormonal symptoms 2, 1
This heterogeneity explains why treatment must be individualized based on these specific tumor characteristics rather than site of origin alone.