Decoding Your Pathology Report
At a Glance
Your pancreatic neuroendocrine tumor (pNET) pathology report uses the Ki-67 index and mitotic rate to measure tumor growth and assign a WHO Grade (G1 to G3). These markers, along with cell differentiation, help your doctor understand how aggressive the tumor is and guide treatment decisions.
Your pathology report is the most important document in your medical folder. It is the “source of truth” that tells your care team exactly what kind of cells they are dealing with and how those cells are likely to behave. Understanding a few key terms will empower you to discuss your diagnosis more clearly with your specialist.
Measuring Growth: The Proliferation Markers
Pathologists use two main measurements to see how fast your tumor is growing. Think of these as a “speedometer” for your cancer:
- Mitotic Rate: A pathologist looks under a microscope and physically counts how many cells are in the actual act of splitting (mitosis) within a specific area, measured as per 2 mm² (historically measured as 10 high-power fields) [1][2].
- Ki-67 Index: This is a more sensitive test that uses a special stain to identify a protein (Ki-67) present in any cell that is preparing to divide [1][3]. It is expressed as a percentage—the higher the percentage, the more “active” the tumor [4].
The WHO Grading System
The World Health Organization (WHO) uses these “speed” measurements to assign a Grade (G1, G2, or G3). This grade helps predict how aggressive the tumor is [2].
| Grade | Ki-67 Index | Mitotic Rate (per 2 mm²) | Growth Description |
|---|---|---|---|
| Grade 1 (G1) | Less than 3% | Less than 2 | Slow-growing (Indolent) [2] |
| Grade 2 (G2) | 3% to 20% | 2 to 20 | Intermediate growth [2] |
| Grade 3 (G3) | Greater than 20% | Greater than 20 | Fast-growing (Aggressive) [2] |
The Critical Distinction: pNET G3 vs. pNEC
If your report mentions “Grade 3,” it is vital to know which category it falls into. These two types are treated very differently:
- Well-Differentiated pNET G3: The cells still look somewhat like healthy pancreatic cells, even though they are dividing quickly [5]. These often have a better outlook and respond to different treatments than the alternative [5][6].
- Poorly Differentiated pNEC (Carcinoma): The cells look very abnormal and messy under the microscope [5]. This is a much more aggressive form that typically requires intensive chemotherapy [7][8].
Confirming the Diagnosis: The “Stains”
To make sure the tumor is actually neuroendocrine, pathologists use immunohistochemistry (IHC)—stains that “light up” when they hit specific proteins found in neuroendocrine cells. Look for these names on your report:
- Synaptophysin and Chromogranin A: These are the “gold standard” markers used to confirm the tumor is neuroendocrine [2][9].
- INSM1: A newer, very reliable marker used to double-check the diagnosis [9].
Pathology Report Checklist
Check your report for these essential items. If they are missing, ask your doctor for a “supplemental” or “addendum” report:
- [ ] Differentiation: Is it called “well-differentiated” or “poorly differentiated”? [5]
- [ ] Ki-67 Index: Is there a specific percentage listed? [10]
- [ ] Mitotic Rate: Is there a count of mitoses per 2 mm² (or 10 high-power fields)? [2]
- [ ] IHC Markers: Are Synaptophysin or Chromogranin A mentioned as positive? [2]
- [ ] Grade: Is it clearly labeled G1, G2, or G3? [2]
Common questions in this guide
What does the Ki-67 index mean on my pNET pathology report?
How does the WHO grading system work for pancreatic neuroendocrine tumors?
What is the difference between a Grade 3 pNET and a pNEC?
What do positive Synaptophysin and Chromogranin A results mean?
Questions to Ask Your Doctor
Curated prompts to bring to your next appointment.
- 1.What is my specific Ki-67 percentage and mitotic count?
- 2.Is my tumor classified as a Grade 3 pNET (well-differentiated) or a pNEC (poorly differentiated)?
- 3.Which neuroendocrine markers, like Chromogranin A or INSM1, were used to confirm the diagnosis?
- 4.Was there any evidence of lymphovascular invasion or positive margins in the sample?
- 5.Was the biopsy sample large enough to give a definitive grade, or do we need more tissue?
Questions For You
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References
References (10)
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PMID: 35227293 - 8
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PMID: 34647171 - 10
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PMID: 26774266
This page explains pancreatic neuroendocrine tumor pathology terminology for educational purposes only. Your pathologist and oncologist are the best sources for interpreting your specific report.
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