Low-Grade (Grade 1) Neuroendocrine Tumors
Low-grade neuroendocrine tumors (Grade 1/G1) are well-differentiated neuroendocrine tumors characterized by a mitotic count of less than 2 per 10 high-power fields (HPF) and a Ki-67 proliferation index of less than 3% (or <2% by some classifications). 1
Classification by Site of Origin
Low-grade NETs can arise from multiple anatomical locations:
Gastroenteropancreatic (GEP) System
Carcinoid Tumors (approximately two-thirds of GEP-NETs):
- Small intestinal NETs (ileum, jejunum)
- Gastric carcinoids
- Duodenal/ampullary NETs
- Appendiceal carcinoids
- Rectal carcinoids
- Colonic NETs
These can present either with or without carcinoid syndrome (only 10% develop carcinoid syndrome) 2
Pancreatic Neuroendocrine Tumors (Pan-NETs) (approximately one-third of GEP-NETs):
Functioning tumors (70-85% of Pan-NETs):
- Insulinoma (producing excessive insulin)
- Gastrinoma (producing gastrin, causing Zollinger-Ellison syndrome)
- Glucagonoma (producing glucagon)
- VIPoma (producing vasoactive intestinal peptide)
- Somatostatinoma (producing somatostatin)
- PPoma (producing pancreatic polypeptide)
- Rare functioning tumors: CRHoma, GHRHoma, ACTHoma, calcitoninoma, neurotensinoma 2
Non-functioning tumors (15-30% of Pan-NETs) 2
Bronchopulmonary System
Typical Carcinoids (TC) - These are the low-grade pulmonary NETs, representing well-differentiated tumors with low malignant potential 3, 4
Other Sites
- Thymic carcinoids (may be associated with MEN-1 syndrome)
- Rare sites throughout the body where neuroendocrine cells exist 1
Key Histopathologic Features
All Grade 1 NETs share common characteristics:
- Well-differentiated morphology (organized growth pattern, uniform cells)
- Mitotic count: <2 per 10 HPF
- Ki-67 index: <3% (some classifications use <2%) 2, 1
- Positive immunohistochemistry for pan-neuroendocrine markers:
- Chromogranin A
- Synaptophysin
- Neuron-specific enolase (NSE)
- CD56 2
Clinical Significance
Important caveat: When mitotic count and Ki-67 index are discordant, always use the higher value to assign grade, as this better predicts clinical behavior 4. Grade 1 NETs generally have the most favorable prognosis among neuroendocrine neoplasms, with significantly better outcomes than intermediate-grade (G2) or high-grade (G3) tumors 1.
The distinction between Grade 1 and higher-grade tumors is critical for treatment planning, as most guideline recommendations for well-differentiated, low-to-intermediate grade tumors primarily apply to G1 and G2 NETs 1.