Mitotic Count and Risk Classification in Gastrointestinal Stromal Tumors (GIST): Fletcher Criteria

If Your Pathology Report Says GIST and “Mitotic Count”: How Is Recurrence Risk Determined?

Gastrointestinal stromal tumor (GIST) is a sarcoma that arises from special cells in the wall of the digestive tract, most often in the stomach and small intestine. One of the most important measures of recurrence risk after surgery is the mitotic count obtained by the pathologist; together with tumor size, location, and tumor rupture it defines the risk group. This article explains how the Fletcher (NIH) and AFIP tables are read, why it matters whether mitoses are counted per “50 high-power fields” or per “5 mm²”, and which evidence underlies the decision on adjuvant (post-operative) imatinib. For background, see our articles on stomach cancer symptoms, stages, and treatment and terms used in pathology reports (in Turkish). Key source: Joensuu et al., Lancet Oncol 2012 (doi:10.1016/S1470-2045(11)70299-6).

Why is the mitotic count so decisive?

GISTs arise from cells that resemble the interstitial cells of Cajal, which coordinate gut movements; most carry a mutation in the KIT or PDGFRA gene. They are most common in the stomach, then the small intestine, and can also occur in the esophagus, colon, and rectum. A mitosis is the stage of cell division seen under the microscope; the more mitoses are counted in a given area, the faster the tumor is growing.

In practice, risk is determined by four things together: tumor size, mitotic count, location, and tumor rupture. Even at the same size and mitotic count, small-intestinal tumors progress more often than gastric ones, so looking at a single number is misleading. The tumor is usually noticed as a bulge in the wall of the stomach or bowel on endoscopy or imaging (how stomach tumors look on endoscopy); symptoms can include gastrointestinal bleeding and anemia.

The real questions are: In which area and how were mitoses counted, which classification was used, and in which risk group does imatinib after surgery help? Because classifications can give different results, the same patient may receive different “risk” labels in different reports.

How do the Fletcher criteria and the mitotic count determine recurrence risk in GIST?

The Fletcher (NIH) criteria divide GIST into very low, low, intermediate, and high risk groups by tumor size and mitotic count; recurrence becomes more likely as both rise. Current practice counts mitoses per 5 mm² and also considers location and rupture. These are probabilities, not exact predictions; the team decides on adjuvant therapy together with mutation analysis.

How are the Fletcher (NIH) and AFIP tables read?

The table summarizes the risk groups of the Fletcher/NIH 2002 consensus. In the original table the mitotic count is given per 50 high-power fields (HPF); size is the largest diameter of the tumor.

*The original text says “fewer than 5”; AFIP and later systems apply the cut-off as “5 or fewer” / “more than 5”. The AFIP tables apply the same logic separately by location. The figure below shows which four inputs make up the risk group.

How much do mitotic count and location change the risk estimate?

AFIP data: same size, different mitoses

In the AFIP tables (Miettinen and Lasota 2006), tumors in the same size group show very different progression rates depending on mitotic count and location. For gastric tumors of 2–5 cm, progression on long follow-up was 1.9% with 5 or fewer mitoses and 16% with more than 5; for the jejunum/ileum at the same size, the figures were 4.3% and 73%. So when mitoses rose, the progression rate increased about 8-fold in gastric tumors and about 17-fold in small-intestinal tumors. “Progression” is the percentage of patients who developed metastasis or tumor-related loss of life.

How are mitoses counted? 50 HPF and 5 mm² are not the same

In older sources the mitotic count is given per “50 high-power fields”, and this area was considered roughly equal to 5 mm². Because the field of view of modern microscopes is wider, 50 HPF can correspond to a clearly larger surface than 5 mm². In a survey in Italy, 64.5% of 110 pathologists counted mitoses in 50 HPF and 35.5% in 5 mm². When the same 100 cases were re-evaluated, the risk class changed in 10% of cases at ocular field number 18, 28% at 20, and 41% at 22, when the 50 HPF count was compared with the 5 mm² count. The changes were seen in the very low, low, and intermediate risk groups; the authors note that this could lead to inappropriate adjuvant therapy.

Do the classifications agree with each other?

Five classification systems were compared in 558 patients (R0 resection, stomach and small intestine) from the Ulm GIST Registry (Schmieder 2016). All separated high-risk from lower-risk patients well; however, 5-year disease-free survival in the “high risk” group ranged from 56.1% to 74.6% depending on the system. Because the confidence intervals overlapped, the authors reported that no system was significantly superior to the others; the high-risk groups were also heterogeneous within themselves.

In the pooled population-based series of Joensuu and colleagues (2,560 patients, 10 series, no adjuvant therapy after surgery), 15-year recurrence-free survival was 59.9% and the number of recurrences declined after the first 10 years. Independent adverse factors were large tumor size, high mitotic count, non-gastric location, tumor rupture, and male sex. For 10-year recurrence-free survival the discrimination (AUC) was 0.79 for NIH, 0.78 for modified NIH, 0.82 for AFIP, and 0.88 for contour maps that model size and mitoses as continuous variables.

In conclusion, the modified NIH was judged the best scheme for identifying a single high-risk group to consider for adjuvant therapy, while contour maps were found suitable for individualized recurrence estimates. The mitotic count is a core input of all these models.

How does the risk group translate into treatment?

The mainstay of treatment for localized GIST is complete (R0) removal of the tumor. In patients with a clear risk of recurrence after surgery, imatinib (400 mg/day) is used as adjuvant therapy; the ESMO–EURACAN–GENTURIS guideline recommends continuing it for 3 years and performing mutation analysis before the decision. In low-risk patients, follow-up is usually sufficient.

Surgery: why does tumor integrity matter?

The aim of surgery is to remove the tumor with its capsule intact and clear margins. Tumor rupture (tearing or spillage into the abdomen) is an independent factor that markedly raises recurrence risk and enters the risk calculation, so the operative note should state whether rupture occurred. Because lymph node involvement is rare in GIST, routine lymph node dissection is generally not needed. PET-CT is used in some cases to assess spread; the liver is one of the most common sites of metastasis (liver tumors). For advanced disease see stage 4 and metastasis, and for the difference from adenocarcinoma in rectal location see colon and rectal cancer.

Adjuvant imatinib: three randomized trials

The three large trials testing imatinib after surgery cover different patient groups and different comparisons, so placing their results directly side by side would not be correct.

In Z9001, 1 year of imatinib delayed recurrence compared with placebo; in SSG XVIII, 3 years improved both recurrence-free and overall survival compared with 1 year. Absolute difference: 17.7 points in 5-year recurrence-free survival. In EORTC 62024, 2 years of imatinib showed no clear benefit over observation; this trial included intermediate- and high-risk patients together and the duration was shorter than 3 years, so it does not answer the same question as SSG XVIII.

In EORTC 62024, after a median follow-up of 9.1 years there was no significant difference between 2 years of imatinib and observation:

In the high-risk subgroup (526 patients, local pathology), 10-year imatinib-failure-free survival was 69% vs 61%; this difference was interpreted as a trend but was not statistically significant. No added benefit was seen in intermediate- and low-risk patients. ESMO’s summary of the long-term results also stresses that adjuvant imatinib is recommended in high-risk patients.

Why is mutation analysis as important as risk?

Most GISTs carry a mutation in KIT (most often exon 11) or PDGFRA, and imatinib suppresses the signal these mutations sustain. But not every mutation responds the same way: the PDGFRA D842V mutation is resistant to imatinib, and the benefit of imatinib is limited in SDH-deficient GISTs. Therefore, even when the risk group is high, mutation analysis is recommended before deciding on adjuvant imatinib. For the method, see next-generation sequencing (NGS) and why the location of a mutation matters.

Mutation results cannot be reduced to a single number like the risk tables; detailed results by mutation type are outside the scope of this article. Discuss what the mutation in your report means with the treating team. For access to new drugs see access map of FDA-approved new cancer drugs in Türkiye and, if suitable, clinical trials.

Intermediate risk: the gray zone

The “intermediate” group in the Fletcher table includes tumors under 5 cm with 6–10 mitoses, or tumors of 5–10 cm with a low mitotic count. Decisions are individual in this group: in EORTC 62024 no added benefit was seen in intermediate- and low-risk patients, whereas the modified NIH gathers the candidates for adjuvant therapy into a single high-risk group. Location (small intestine is riskier), rupture, or a mitotic count near the threshold can change the decision; in difficult cases a second opinion is reasonable.

Limits of mitotic counting

Mitoses are counted over an area of 5 mm² in the most active region of the tumor (the “hot spot”); different pathologists, microscopes, and sampling can give different results. Because the whole tumor is not seen on a needle biopsy or gastric biopsy, the mitotic count is only final on the surgical specimen. If imatinib was given before surgery, the mitotic count may be lower, so risk classification in such specimens is interpreted with care. In some small series, proliferation markers such as Ki-67 were also linked to risk groups; however, they are not part of the standard risk calculation.

How should the results be read?

Strengths

• Fletcher, AFIP, and contour maps point in the same direction: recurrence risk rises with size and mitotic count; non-gastric location and rupture raise it further.
• For adjuvant imatinib in high-risk patients there is randomized evidence (Z9001, SSG XVIII) and the difference is clinically large: about 18 points in 5-year recurrence-free survival.
• The contour maps were derived from a pooled series of 2,560 patients and tested in an independent cohort of 920 patients.
• The effect of the mitotic counting area was shown numerically in a separate study (Italy, 110 pathologists, 100 cases).

Points to keep in mind

• Risk tables are observational: The Fletcher and AFIP tables were derived from retrospective series; they give probabilities, not certain predictions.
• Variable counting method: 50 HPF and 5 mm² do not cover the same area; the risk class can change with the counting method.
• Differences between systems: The same patient can fall into different groups in different systems; in the Ulm registry no system was found significantly superior.
• Trials are not comparable: Z9001 (1 year, placebo), SSG XVIII (3 years, 1 year), and EORTC 62024 (2 years, observation) answer different questions; EORTC 62024 showed no significant benefit.
• Mutation distinction: Even when risk is high, no benefit is expected if the tumor carries an imatinib-resistant mutation.

Practical tips for patients and families

If your pathology report says GIST and gives a mitotic count, these steps may help:

• Find the tumor size, location, mitotic count, and the area in which counting was done (5 mm² or 50 HPF) in the report; if missing, ask the pathologist.
• Ask whether the operative note records tumor rupture and whether the surgical margin is clear (R0).
• Ask which risk classification was used (Fletcher/NIH, AFIP, modified NIH); you may see different labels in different reports.
• If adjuvant imatinib is recommended, ask for the mutation analysis result and discuss the duration of treatment and the follow-up plan in writing.
• Seek help early for imatinib side effects (swelling, diarrhea, nausea, fatigue): see our articles on fatigue, diarrhea, and easing side effects with nutrition; the guide to food and drug interactions is also useful.
• Do not skip follow-up; seek care immediately for new abdominal pain, black stools, bleeding, or signs of anemia.

Frequently asked questions

What is the Fletcher risk classification in GIST?

The Fletcher (NIH) classification divides GIST into very low, low, intermediate, and high risk groups by the largest diameter of the tumor and the mitotic count per 50 high-power fields. For example, a tumor larger than 5 cm with more than 5 mitoses is high risk. Later systems also consider location and tumor rupture.

Should mitoses be counted per 5 mm² or per 50 HPF?

Current practice is to count per 5 mm². Depending on the microscope’s field of view, 50 HPF can cover a larger area than 5 mm²; this can raise the mitotic count and change the risk class. In the Italian study that re-evaluated 100 cases, the class change was between 10–41%.

Should every GIST patient take imatinib after surgery?

No. Adjuvant imatinib is recommended for patients with a clear risk of recurrence; the ESMO guideline also recommends mutation analysis before the decision. In low-risk patients follow-up is usually sufficient; in EORTC 62024 no added benefit was seen in the intermediate- and low-risk group. No benefit is expected in imatinib-resistant mutations such as PDGFRA D842V.

For how many years is adjuvant imatinib taken?

In high-risk patients the recommended duration is 3 years: in SSG XVIII, 3 years of treatment raised 5-year recurrence-free survival from 47.9% to 65.6% compared with 1 year. In EORTC 62024, 2 years of treatment was not superior to observation. Duration and need are determined with the team according to risk group and mutation.

Glossary

GIST

Gastrointestinal stromal tumor; a sarcoma arising from cells resembling the interstitial cells of Cajal in the wall of the digestive tract, usually carrying a KIT or PDGFRA mutation.

Mitosis

The stage of cell division seen under the microscope; their number reflects how fast the tumor is growing.

HPF (high-power field)

The area seen under the microscope at high magnification; its real size depends on the microscope.

5 mm²

The standard surface area in which mitoses are counted; preferred to 50 HPF because it is independent of the microscope.

Fletcher (NIH) classification

The 2002 consensus that defines risk groups by tumor size and mitotic count.

AFIP (Miettinen) classification

A classification that gives progression rates by location in addition to size and mitoses.

Tumor rupture

Tearing of the tumor capsule or spillage of its contents before or during surgery; raises recurrence risk.

Adjuvant therapy

Additional treatment given after the tumor is removed to reduce recurrence (imatinib in GIST).

Contour map

A graph that models size and mitoses as continuous variables and gives an individual probability of recurrence.