Colorectal NET Grading
Mitotic rate + Ki-67 WHO grading for colorectal neuroendocrine tumors.
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This calculator is a pathology decision-support and educational tool for mathematical normalization of mitotic counts and application of established colorectal NET proliferation grading criteria. It does not replace the current WHO classification, AJCC staging, CAP protocol, institutional protocol, or professional pathologist judgment.
Reference
About this calculator
Grades well-differentiated colorectal NETs using mitotic rate per 2 mm² and Ki-67 index. The higher of the two grades determines the final result.
Cytologic atypia has no impact on clinical behavior of well-differentiated neuroendocrine tumors and is not part of this grading system. There is no atypia input in this calculator.
Grading thresholds
Exact boundaries, never rounded before comparison. 1.99 → G1; 2.00 → G2; 20.00 → G2; 20.01 → G3. Same convention for Ki-67 at 3% and 20%. Final grade = higher of the two — never averaged.
| Grade | Mitotic rate (per 2 mm²) | Ki-67 (%) |
|---|---|---|
| Well-differentiated NET, G1 | < 2 | < 3 |
| Well-differentiated NET, G2 | 2 – 20 | 3 – 20 |
| Well-differentiated NET, G3 | > 20 | > 20 |
Mitotic rate
Only clearly identifiable mitotic figures are counted; hyperchromatic, karyorrhectic, or apoptotic nuclei are excluded. Assess the most mitotically active area (mitotic hot spot), evaluating at least 10 mm².
Ki-67 proliferation index
Reported as percent positive tumor cells in the area of highest nuclear labeling (the Ki-67 hot spot — not necessarily the same region as the mitotic hot spot).
Methodology is not standardized. Automated counting and manual "eyeballing" are both used. Eyeballing is acceptable for most tumors. For a Ki-67 index close to a grade cutoff (near 3% or near 20%), a manual count on a printed or captured image of the hot spot is recommended rather than eyeballing alone. A minimum of 500 tumor cells is recommended; note if fewer were available.
Field diameter reference table
Generated dynamically from A = π(d/2)², not from stored values. Your current diameter is highlighted.
Values are computed from the formula; differences of about ±1 HPF from the published CAP table can occur due to rounding conventions in the source document. Calculations in this app always use the unrounded formula result, not the table.
| Field diameter (mm) | Area (mm²) | HPF for 10 mm² |
|---|---|---|
| 0.40 | 0.126 | ≈80 |
| 0.41 | 0.132 | ≈76 |
| 0.42 | 0.139 | ≈72 |
| 0.43 | 0.145 | ≈69 |
| 0.44 | 0.152 | ≈66 |
| 0.45 | 0.159 | ≈63 |
| 0.46 | 0.166 | ≈60 |
| 0.47 | 0.173 | ≈58 |
| 0.48 | 0.181 | ≈55 |
| 0.49 | 0.189 | ≈53 |
| 0.50 | 0.196 | ≈51 |
| 0.51 | 0.204 | ≈49 |
| 0.52 | 0.212 | ≈47 |
| 0.53 | 0.221 | ≈45 |
| 0.54 | 0.229 | ≈44 |
| 0.55 | 0.238 | ≈42 |
| 0.56 | 0.246 | ≈41 |
| 0.57 | 0.255 | ≈39 |
| 0.58 | 0.264 | ≈38 |
| 0.59 | 0.273 | ≈37 |
| 0.60 | 0.283 | ≈35 |
| 0.61 | 0.292 | ≈34 |
| 0.62 | 0.302 | ≈33 |
| 0.63 | 0.312 | ≈32 |
| 0.64 | 0.322 | ≈31 |
| 0.65 | 0.332 | ≈30 |
| 0.66 | 0.342 | ≈29 |
| 0.67 | 0.353 | ≈28 |
| 0.68 | 0.363 | ≈28 |
| 0.69 | 0.374 | ≈27 |
References
1. WHO Classification of Tumours Editorial Board. Digestive System Tumours. Lyon, France: IARC; 2019 (WHO Classification of Tumours series, 5th ed., vol. 1).
2. Rindi G, Kloppel G, Couvelard A, et al. TNM staging of midgut and hindgut (neuro)endocrine tumors: a consensus proposal including a grading system. Virchows Arch. 2007;451(4):757-762.
3. Tang LH, Gonen M, Hedvat C, Modlin I, Klimstra DS. Objective quantification of the Ki67 proliferative index in neuroendocrine tumors of gastroenteropancreatic system. Am J Surg Pathol. 2012;36(12):1761-1770.
4. Reid MD, Bagci P, Ohike N, et al. Calculation of the Ki67 index in pancreatic neuroendocrine tumors: a comparative analysis of four counting methodologies. Mod Pathol. 2016;29(1):93.
5. Kuiper T, van Oijen MGH, van Velthuysen MF, et al. Endoscopically removed rectal NETs: a nationwide cohort study. Int J Colorectal Dis. 2021;36(3):535-41.
6. Folkert IW, Sinnamon AJ, Concors SJ, et al. Grade is a Dominant Risk Factor for Metastasis in Patients with Rectal Neuroendocrine Tumors. Ann Surg Oncol. 2020;27(3):855-63.
7. Shi C, Klimstra DS. Pancreatic neuroendocrine tumors: pathologic and molecular characteristics. Semin Diagn Pathol. 2014;31(6):498-511.
8. Yachida S, Vakiani E, White CM, et al. Small cell and large cell neuroendocrine carcinomas of the pancreas are genetically similar and distinct from well-differentiated pancreatic neuroendocrine tumors. Am J Surg Pathol. 2012;36(2):173-84.
9. Sorbye H, Strosberg J, Baudin E, Klimstra DS, Yao JC. Gastroenteropancreatic high-grade neuroendocrine carcinoma. Cancer. 2014;120(18):2814-2823.
10. AJCC Version 9 Neuroendocrine Tumors of the Colon and Rectum Cancer Staging System. Copyright 2023 American College of Surgeons.
11. Yuan H, Yang Y, Wang W, Cheng Y. A case report of neuroendocrine tumor (G3) at lower rectum with liver metastasis. Medicine (Baltimore). 2018;97(38):e12423.
Source protocol: CAP Approved ColoRectal.NET_5.0.0.0.REL_CAPCP.
Educational and decision-support tool only. Does not replace the current WHO classification, AJCC staging, CAP protocol, institutional protocol, or professional pathologist judgment — particularly for distinguishing well-differentiated NET-G3 from poorly differentiated NEC, which requires direct morphologic evaluation.