GIST Grading & Risk Assessment
Mitotic-rate-only histologic grade plus the full Miettinen & Lasota site-specific risk table.
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This calculator is a pathology decision-support and educational tool for mathematical normalization of mitotic counts and application of the Miettinen & Lasota GIST risk stratification table. It does not replace the current CAP protocol, institutional protocol, molecular testing, clinical correlation, or professional pathologist judgment. Data underlying this table are from the pre-imatinib era and estimate risk of progression without adjuvant therapy.
Reference
Histologic grade
Unlike soft tissue sarcoma grading, GIST grading uses mitotic rate only — GIST is generally less proliferative than many other soft tissue tumors, and the threshold separating low- from high-grade occurs at 5 mitotic figures per 5 mm². It does not incorporate differentiation or necrosis.
Mitotic count is initiated in the area of highest mitotic activity on screening magnification, and performed as consecutive high-power fields. Stringent criteria apply — pyknotic or apoptotic nuclei are not mitoses.
| Grade | Definition |
|---|---|
| GX | Grade cannot be assessed |
| G1 | Low grade — mitotic rate ≤5 per 5 mm² |
| G2 | High grade — mitotic rate >5 per 5 mm² |
Mitotic rate per 5 mm²
HPF area A = π(d/2)², where d is the measured field diameter at 40×.
Mitoses/5 mm² = (mitoses counted × 5) ÷ (A × HPFs examined). The grade and risk tier are tested against the unrounded rate.
Older microscopes: ≈50 HPF ≈ 5 mm². Modern wide-field microscopes: ≈20–25 HPF ≈ 5 mm². Measuring your actual field diameter gives the precise figure for your microscope rather than relying on these general figures.
Risk of progressive disease — mitotic rate ≤5 per 5 mm²
Guidelines for Risk Assessment of Primary GIST. Adapted with permission from Miettinen and Lasota. Copyright 2006 by Elsevier. Based on long-term follow-up of 1055 gastric, 629 small intestinal, 144 duodenal, and 111 rectal GIST from the pre-imatinib era.
| Size | Gastric | Duodenum | Jejunum/Ileum | Rectum |
|---|---|---|---|---|
| ≤2 cm | None (0%) | None (0%) | None (0%) | None (0%) |
| >2–≤5 cm | Very low (1.9%) | Low (8.3%) | Low (4.3%) | Low (8.5%) |
| >5–≤10 cm | Low (3.6%) | Insufficient data | Moderate (24%) | Insufficient data |
| >10 cm | Moderate (12%) | High (34%) | High (52%) | High (57%) |
Risk of progressive disease — mitotic rate >5 per 5 mm²
‡ For duodenal and rectal GIST, the >5–≤10 cm and >10 cm size groups are statistically combined due to small case numbers; the single combined risk value is shown at the >10 cm row. † Denotes a small number of cases. § No tumors of this category were included in the original study.
| Size | Gastric | Duodenum | Jejunum/Ileum | Rectum |
|---|---|---|---|---|
| ≤2 cm | None (0%) | Insufficient data | High (50%) | High (54%) |
| >2–≤5 cm | Moderate (16%) | High (50%) | High (73%) | High (52%) |
| >5–≤10 cm | High (55%) | Insufficient data | High (85%) | Insufficient data |
| >10 cm | High (86%) | High (86%) | High (90%) | High (71%) |
Notes
Site substitution. For anatomic sites not listed in the table (esophagus, mesentery, peritoneum) — or wherever "insufficient data" applies — it is best to use the risk criteria for jejunum/ileum.
Molecular correlation. This risk assessment applies best to KIT/PDGFRA-mutant GIST. SDH-deficient GIST is more unpredictable and may not follow this table’s risk estimates.
Biopsy limitations. Biopsies may not contain sufficient tissue for accurate mitotic counting and may miss mitotic hot spots. A high mitotic count/risk finding on biopsy is more likely to be reliable than a low one, which may underestimate the true rate found on resection.
Sources
Fletcher CD, Berman JJ, Corless C, et al. Diagnosis of gastrointestinal stromal tumors: a consensus approach. Hum Pathol. 2002;33(5):459-465. (NCI-sponsored consensus conference, 2002.)
Miettinen M, Lasota J. Gastrointestinal stromal tumors: pathology and prognosis at different sites. Semin Diagn Pathol. 2006;23(2):70-83. — primary source for the risk table.
National Comprehensive Cancer Network (NCCN) GIST Task Force report, adopting the Miettinen & Lasota criteria.
College of American Pathologists (CAP). Protocol for the Examination of Biopsy/Resection Specimens From Patients With Gastrointestinal Stromal Tumor (GIST) (document Stomach.GIST.Bx_4.3.0.0.REL_CAPCP).