Thyroid MTC Grading

IMTCGS two-tier high/low-grade medullary thyroid carcinoma calculator, with a decision tree.

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Thyroid MTC Grading Calculator — educational and documentation-support tool. Does not replace pathologic assessment. Histologic grade is not equivalent to overall clinical risk. Verify all criteria against the current editions before diagnostic use.

Reference

IMTCGS grading criteria

Applies the WHO 5th edition / IMTCGS two-tier grading scheme for medullary thyroid carcinoma (thyroid neuroendocrine neoplasm). The generic G1/G2/G3 system used for gastroenteropancreatic NETs is deliberately not used.

Thresholds are inclusive at 5: a value of exactly 5.0 mitoses/2 mm² or exactly 5.0% Ki-67 meets the high-grade criterion.

Grade	Criteria
High-grade MTC	Any one of: mitotic count ≥5/2 mm², tumor necrosis present, or Ki-67 ≥5%
Low-grade MTC	All three of: mitotic count <5/2 mm², tumor necrosis absent, and Ki-67 <5%

Decision tree

Mitotic rate ≥5/2 mm²? YES → high-grade MTC; NO → continue. Ki-67 ≥5%? YES → high-grade MTC; NO → continue. Tumor necrosis present? YES → high-grade MTC; NO → low-grade MTC.

High grade uses OR logic; low grade uses AND logic. Any single high-grade feature is sufficient for high-grade MTC. Low grade requires all three: mitoses <5/2 mm², Ki-67 <5%, and necrosis absent. Incomplete data is never defaulted to low grade.

Counting and assessment cautions

Field diameter must correspond to the actual microscope field used for counting. HPF area varies between microscopes — do not derive it from objective magnification alone.

Count only clearly identifiable mitotic figures. Exclude hyperchromatic nuclei, karyorrhectic nuclei, apoptotic nuclei, degenerative nuclear fragments, and ambiguous figures. Assess the most mitotically active tumor area/hotspot as appropriate.

Ki-67 should be assessed using an appropriate validated methodology and should not be based on subjective visual "eyeballing" alone.

Do not confuse tumor necrosis with apoptosis, karyorrhexis, crush artifact, or focal degenerative change.

Cytologic atypia alone should not be used as the determinant of MTC histologic grade. Grade is determined using the validated proliferative and necrosis criteria only — there is deliberately no atypia input that could alter the grade.

Mathematical field-area conversion

A = π(d/2)²

Total area = A × N

Mitoses/2 mm² = (M × 2) ÷ (A × N)

HPFs for 10 mm² = 10 ÷ A · HPFs ≈ 2 mm² = 2 ÷ A

The field-area conversion is arithmetic that standardizes your count to a fixed area. It is not itself a WHO grading criterion — it simply produces the mitoses/2 mm² value that the grading criterion is then applied to.

Precision handling

The HPF area is never rounded before being used in the total-area or rate calculations, and the ≥5 threshold is tested against the unrounded value. Only the displayed rate is rounded to one decimal place. Where a displayed value of 5.0 could disagree with the threshold test — for example an exact rate of 4.97 — the calculator says so explicitly rather than letting the rounded display mislead.

Field diameter reference table

Generated from the formula, not stored values.

Field diameter	HPF area (mm²)	HPFs for 10 mm²	HPFs ≈ 2 mm²
0.40 mm	0.1257	≈80	≈15.9
0.41 mm	0.1320	≈76	≈15.1
0.42 mm	0.1385	≈72	≈14.4
0.43 mm	0.1452	≈69	≈13.8
0.44 mm	0.1521	≈66	≈13.2
0.45 mm	0.1590	≈63	≈12.6
0.46 mm	0.1662	≈60	≈12.0
0.47 mm	0.1735	≈58	≈11.5
0.48 mm	0.1810	≈55	≈11.1
0.49 mm	0.1886	≈53	≈10.6
0.50 mm	0.1963	≈51	≈10.2
0.51 mm	0.2043	≈49	≈9.8
0.52 mm	0.2124	≈47	≈9.4
0.53 mm	0.2206	≈45	≈9.1
0.54 mm	0.2290	≈44	≈8.7
0.55 mm	0.2376	≈42	≈8.4
0.56 mm	0.2463	≈41	≈8.1
0.57 mm	0.2552	≈39	≈7.8
0.58 mm	0.2642	≈38	≈7.6
0.59 mm	0.2734	≈37	≈7.3
0.60 mm	0.2827	≈35	≈7.1
0.61 mm	0.2922	≈34	≈6.8
0.62 mm	0.3019	≈33	≈6.6
0.63 mm	0.3117	≈32	≈6.4
0.64 mm	0.3217	≈31	≈6.2
0.65 mm	0.3318	≈30	≈6.0
0.66 mm	0.3421	≈29	≈5.8
0.67 mm	0.3526	≈28	≈5.7
0.68 mm	0.3632	≈28	≈5.5
0.69 mm	0.3739	≈27	≈5.3

Grade ≠ complete clinical risk

Histologic grade is not equivalent to overall clinical risk. A patient's complete recurrence or metastatic risk cannot be calculated from grade alone — tumor size, extrathyroidal extension, lymphovascular invasion, nodal and distant metastasis, margin status and biochemical/molecular findings do not alter the IMTCGS histologic grade.

Sources

WHO Classification of Tumours Editorial Board. WHO Classification of Tumours: Endocrine and Neuroendocrine Tumours. 5th ed. IARC.

WHO 2022 thyroid tumor classification / standardized diagnostic approach, including the histopathologic grading scheme for MTC.

International Medullary Thyroid Carcinoma Grading System (IMTCGS).

College of American Pathologists. Protocol for the Examination of Specimens From Patients With Carcinomas of the Thyroid Gland — including mitotic-rate reporting per 2 mm² and Ki-67 reporting requirements for MTC.