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Pathology and Grading (Ki-67, NET vs NEC)

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Neuroendocrine neoplasms (NENs) are evaluated by how their cells look (differentiation) and how fast they grow (grade). Well-differentiated tumors are called NETs, while aggressive, poorly differentiated ones are NECs. The Ki-67 index measures this growth rate to guide treatment.

Key Takeaways

  • • Neuroendocrine tumors are classified by their differentiation (how normal the cells look) and their grade (how fast they grow).
  • • Well-differentiated tumors are known as NETs, while poorly differentiated, highly aggressive tumors are called NECs.
  • • The Ki-67 index and mitotic count are essential measurements that reveal the percentage of actively dividing cells.
  • • Pathologists use immunohistochemistry (IHC) markers like Synaptophysin and Chromogranin A to definitively confirm a neuroendocrine diagnosis.
  • • Knowing the specific grade and Ki-67 percentage helps doctors determine the most effective targeted treatments or chemotherapy.

When you receive a pathology report for an endocrine tumor—specifically a neuroendocrine neoplasm (NEN)—it may feel like reading a foreign language. However, two main concepts determine your diagnosis and treatment plan: differentiation (how the cells look) and grade (how fast they are growing) [1][2][3].

Differentiation: The “Identity” of the Cell

Differentiation describes how much the tumor cells still look and act like healthy, normal endocrine cells [4][5].

  • Well-differentiated (NETs): These cells look very similar to normal cells. They often grow slowly and may still produce hormones or have receptors (like somatostatin receptors) that doctors can target with specific therapies [6][7].
  • Poorly differentiated (NECs): These cells look very abnormal and disorganized. They have lost their “identity” as endocrine cells, grow very rapidly, and are much more aggressive [8][1][9].

Grading: The “Speedometer” of Growth

To determine the grade, pathologists look at how quickly the cells are dividing. They use two main measurements [10][11]:

  1. Mitotic Count: The number of cells caught in the act of dividing (mitosis) when viewed under a microscope.
  2. Ki-67 Index: A special stain that colors a protein found only in dividing cells. The result is given as a percentage; for example, a Ki-67 of 10% means 10 out of every 100 cells are actively dividing [12][13].

The WHO Grading System

For most neuroendocrine tumors (like those in the pancreas or gut), the World Health Organization (WHO) uses these tiers [12][13][14]:

Grade Ki-67 Index Mitotic Count Growth Speed
Grade 1 (G1) < 3% < 2 Very Slow
Grade 2 (G2) 3% – 20% 2 – 20 Intermediate
Grade 3 (G3) > 20% > 20 Rapid

Important Note: A Grade 3 NET (well-differentiated) is biologically different from a Neuroendocrine Carcinoma (NEC) (poorly differentiated), even though both have high Ki-67 numbers. NECs often have specific genetic mutations (like TP53 or RB1) that NETs do not have [15][16][9].

Immunohistochemistry: Confirming the Diagnosis

To “prove” a tumor is neuroendocrine, pathologists use immunohistochemistry (IHC)—a process of using special dyes to look for specific proteins [17][18].

  • Synaptophysin and Chromogranin A: These are the “gold standard” markers. If a tumor “stains positive” for these, it confirms the cells are neuroendocrine in nature [14][18].
  • INSM1: A newer, very sensitive marker that helps identify neuroendocrine cells, especially in aggressive or poorly differentiated tumors [17][18].
  • SSTR2: Staining for somatostatin receptors helps your doctor decide if you are a candidate for certain types of specialized scans or “cold” and “hot” (radiolabeled) hormone therapies [7].

Always ask your doctor for the exact Ki-67 percentage, as a “Grade 2” tumor with a 4% index may be managed differently than one with an 18% index [12].

Frequently Asked Questions

What is the difference between a NET and a NEC?
A neuroendocrine tumor (NET) is well-differentiated, meaning its cells look somewhat like normal cells and usually grow more slowly. A neuroendocrine carcinoma (NEC) is poorly differentiated, meaning the cells look highly abnormal, disorganized, and tend to grow very rapidly.
What does the Ki-67 index mean on my pathology report?
The Ki-67 index is a percentage that shows how many of your tumor cells are actively dividing. A higher Ki-67 percentage means the tumor is growing more quickly. This number is a key factor in determining your tumor's grade.
What does it mean if my tumor is positive for Synaptophysin and Chromogranin A?
These are specific proteins found in neuroendocrine cells. If your tumor tissue stains positive for these markers during a lab test called immunohistochemistry, it confirms that your tumor is a neuroendocrine neoplasm.
How is a Grade 3 NET different from a NEC?
Even though both have a high growth rate (high Ki-67 index), a Grade 3 NET is still well-differentiated and biologically distinct. A NEC is poorly differentiated, much more aggressive, and often contains specific high-risk genetic mutations like TP53 or RB1 that NETs do not have.
Why is the exact Ki-67 percentage important for a Grade 2 tumor?
The exact percentage helps your oncologist tailor your treatment plan. For instance, a Grade 2 tumor covers a wide range (3% to 20%). A tumor with a 4% Ki-67 index may be managed very differently than one with an 18% index because they grow at different speeds.

Questions for Your Doctor

  • • What is my exact Ki-67 index percentage and mitotic count?
  • • Is my tumor classified as a 'well-differentiated NET' or a 'poorly differentiated NEC'?
  • • Based on my grade (G1, G2, or G3), how aggressive do you expect this tumor to be?
  • • Did my pathology report show positive staining for Chromogranin A and Synaptophysin?
  • • Does my tumor show any high-risk genetic changes, such as TP53 or RB1 mutations?
  • • How does the 'differentiation' of my tumor affect which treatments, like somatostatin analogs or platinum-based chemotherapy, will work best?

Questions for You

  • • Did my pathology report use terms like 'typical carcinoid' or 'atypical carcinoid' (for lung tumors) instead of G1 or G2?
  • • Has my doctor mentioned whether my tumor cells look mostly 'normal' (well-differentiated) or very 'abnormal' (poorly differentiated) under the microscope?
  • • Do I have a copy of my full pathology report to bring to my appointments for a second opinion?

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This page explains neuroendocrine pathology terminology for educational purposes. Your oncologist and pathologist are the best sources for interpreting your specific report and making treatment decisions.

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