Breslow Thickness T Category Interpreter (AJCC 8th)
Breslow Thickness T Category Interpreter (AJCC 8th)
The AJCC eighth edition moved the T1a/T1b boundary to 0.8 mm and records thickness to one decimal place — so a melanoma measured at 0.75 mm is recorded as 0.8 mm and changes category. Ulceration is part of the answer, not an afterthought.
Breslow thickness to AJCC 8th edition T category
Thickness and ulceration to T categoryInvasive melanoma, Breslow thickness 1.6 mm, not ulcerated
The T category by thickness and ulceration, AJCC 8th edition
T1a — under 0.8 mm, NOT ulcerated
T1b — 0.8 to 1.0 mm with or without ulceration, OR under 0.8 mm with ulceration
T2 over 1.0 to 2.0 mm T3 over 2.0 to 4.0 mm T4 over 4.0 mm
a = not ulcerated, b = ulcerated, at T2, T3 and T4
Tis melanoma in situ TX thickness not assessable
- the rounding rule decides categories
- the eighth edition records thickness to the nearest 0.1 mm and not to the nearest 0.01 mm, rounding figures ending in 1 to 4 down and 5 to 9 up. The Panel’s own two examples: 0.75 mm is recorded as 0.8 mm and is T1b, and anything from 0.95 mm through 1.04 mm is recorded as 1.0 mm and is T1b. So the rule moves the category, and this page applies it
- why 0.8 mm
- it is new in the eighth edition. The Panel identified “a possible clinically important breakpoint in the region of 0.7 mm-0.8 mm” and chose 0.8 because it bears on sentinel node biopsy: melanomas under 0.8 mm are generally not recommended for it, while in the 0.8 to 1.0 mm group sentinel node metastases occur in roughly 5 to 12 per cent
- mitotic rate left T1
- the seventh edition used mitotic rate to split T1a from T1b. The eighth removed it: in the multivariable T1 analysis mitotic rate was not statistically significant (p = 0.57). The Panel nonetheless “strongly recommends that mitotic rate be assessed and recorded for all primary melanomas”, because it still carries information outside T1
- Clark level, and why it went
- Clark’s level of invasion graded melanoma by which anatomical layer of the dermis it reached. Breslow thickness measures the same thing as a distance and does it better: PathologyOutlines records Clark level as “superseded by Breslow depth but often still reported” and not used for staging. It survives in reporting datasets as an optional item and as one of the factors the ICCR guide lists for cases where no thickness can be given at all
- how the thickness is measured
- from the top of the granular layer of the epidermis to the deepest invasive cell, at a right angle to the adjacent epidermis, with a calibrated ocular micrometer. Where the surface is ulcerated the measurement starts at the BASE OF THE ULCER, which the ICCR guide notes “may lead to an underestimate of thickness”. Deep periadnexal extension is assumed periadnexal and excluded
- a T category is not a stage
- and not a prognosis. Staging needs the nodal and distant status as well, and the AJCC’s stage groupings and survival tables are in a copyrighted manual that this page does not reproduce. The melanoma laboratory markers are elsewhere: the S100B unit converter and the LDH unit converter
Worked example
Invasive melanoma, Breslow thickness 1.6 mm, not ulcerated
1.6 mm is already at one decimal place, so the recorded thickness is 1.6 mm
That is above 1.0 mm and at or below 2.0 mm, so the category is T2
Not ulcerated, so the subcategory is a: T2a
Mark it ulcerated and nothing else changes: T2b
Now the boundaries the eighth edition moved. Enter 0.74 mm, not ulcerated: recorded as 0.7 mm, T1a. Enter 0.75 mm: recorded as 0.8 mm, T1b. One hundredth of a millimetre of measurement, two categories, and the sentinel node discussion changes with it
Enter 1.04 mm: recorded as 1.0 mm, so still T1b. Enter 1.05 mm: recorded as 1.1 mm, so T2a
Enter 0.5 mm with ulceration: T1b, because under 0.8 mm ulceration is the only thing that separates T1b from T1a
T category by recorded thickness and ulceration
| Recorded thickness | Not ulcerated | Ulcerated |
|---|---|---|
| No invasion (in situ) | Tis | Tis |
| Under 0.8 mm | T1a | T1b |
| 0.8 to 1.0 mm | T1b | T1b |
| Over 1.0 to 2.0 mm | T2a | T2b |
| Over 2.0 to 4.0 mm | T3a | T3b |
| Over 4.0 mm | T4a | T4b |
| Cannot be assessed | TX | TX |
What the rounding rule does at each boundary
| Measured | Recorded | T category, not ulcerated |
|---|---|---|
| 0.70 to 0.74 mm | 0.7 mm | T1a |
| 0.75 to 0.79 mm | 0.8 mm | T1b |
| 0.80 to 0.84 mm | 0.8 mm | T1b |
| 0.95 to 1.04 mm | 1.0 mm | T1b |
| 1.05 to 1.14 mm | 1.1 mm | T2a |
| 1.95 to 2.04 mm | 2.0 mm | T2a |
| 2.05 to 2.14 mm | 2.1 mm | T3a |
| 3.95 to 4.04 mm | 4.0 mm | T3a |
| 4.05 to 4.14 mm | 4.1 mm | T4a |
A measurement with thresholds, and the thresholds moved
Breslow thickness is a distance, measured on a slide: from the top of the granular layer of the epidermis down to the deepest invasive cell, at a right angle to the adjacent epidermis, with a calibrated ocular micrometer. It has been the strongest single prognostic variable in cutaneous melanoma since Breslow described it in 1970, and in the AJCC eighth edition it and ulceration are the only two things the T category depends on.
The eighth edition changed that category in three ways worth knowing. It moved the T1a/T1b boundary to 0.8 mm, having identified a breakpoint somewhere in the region of 0.7 to 0.8 mm and choosing the figure that lines up with the sentinel node biopsy decision — melanomas under 0.8 mm are generally not offered it, and in the 0.8 to 1.0 mm band sentinel node metastases run at roughly 5 to 12 per cent. It removed mitotic rate from the T1 definition, where the seventh edition had used it, because in the Panel’s multivariable T1 analysis it was not significant at p = 0.57 — while still strongly recommending that mitotic rate be assessed and recorded on every primary melanoma. And it fixed the recording convention at one decimal place rather than two.
That last change is the one that catches people, because it moves categories. The rule is to round to the nearest 0.1 mm, with figures ending in 1 to 4 going down and 5 to 9 going up, and the Panel gives two worked examples: a melanoma measuring 0.75 mm is recorded as 0.8 mm and is therefore T1b, and anything from 0.95 mm through 1.04 mm is recorded as 1.0 mm and is also T1b. So a hundredth of a millimetre at 0.745 against 0.75 separates T1a from T1b, and the sentinel node conversation turns on it. Ulceration — a full-thickness epidermal defect, with the stratum corneum and the basement membrane gone — is the other input, and below 0.8 mm it is the only thing that separates the two T1 subcategories.
Clark’s level of invasion, which graded melanoma by the dermal layer reached, measured the same thing less well and has been superseded: it is not used for staging, although it is still often reported and the ICCR guide lists it among the factors to record where no thickness can be given at all. Two things this page cannot do: it cannot stage a patient, because staging needs the nodal and distant categories as well as the T, and it cannot offer a prognosis, because the AJCC’s survival data live in a copyrighted manual. For the laboratory markers that accompany melanoma follow-up, see the S100B unit converter and the LDH unit converter; for response assessment on treatment, the RECIST 1.1 response category interpreter. For the clinical assessment of a pigmented lesion before any of this, see the weighted 7-point checklist. A thin melanoma is still a melanoma: a low T category is a measurement, not a reassurance, and it is not a prognosis. This page computes the published number and names the guideline or trial that defines its thresholds. It renders no clinical decision.
Frequently asked questions
What are the AJCC 8th edition Breslow thickness thresholds?
T1 is a recorded thickness of 1.0 mm or less, T2 above 1.0 to 2.0 mm, T3 above 2.0 to 4.0 mm and T4 above 4.0 mm. Within T1, T1a is under 0.8 mm without ulceration and T1b is 0.8 to 1.0 mm with or without ulceration or under 0.8 mm with ulceration. At T2, T3 and T4 the a and b subcategories are simply not ulcerated and ulcerated.
Why does 0.75 mm matter?
Because the eighth edition records thickness to the nearest 0.1 mm and rounds figures ending in 5 to 9 upward. The Expert Panel’s own example is that a melanoma measuring 0.75 mm is recorded as 0.8 mm and is therefore T1b, while 0.74 mm is recorded as 0.7 mm and remains T1a. One hundredth of a millimetre changes the category.
Is a melanoma of exactly 1.0 mm T1 or T2?
T1. T1 is defined as a thickness of 1.0 mm or less and T2 begins above 1.0 mm. A recorded 1.0 mm is T1b — and because of the rounding rule, anything measured from 0.95 mm through 1.04 mm is recorded as 1.0 mm and is T1b as well.
Why was Clark level dropped from melanoma staging?
Because Breslow thickness measures invasion as a distance and predicts outcome better than the anatomical layer reached. PathologyOutlines describes Clark level as superseded by Breslow depth but often still reported, and it is not used for staging. It survives as an optional reporting item and as one of the factors recorded where no thickness can be measured.
Does this page stage a melanoma?
No. It gives the T category from the thickness and the ulceration status, which is one of the three components of a stage. The nodal and distant categories are needed as well, and the AJCC’s stage groupings and survival tables are in a copyrighted manual that is not reproduced here.
Related calculators
References
- Gershenwald JE, Scolyer RA, Hess KR, et al. Melanoma staging: evidence-based changes in the American Joint Committee on Cancer eighth edition cancer staging manual. CA Cancer J Clin. 2017;67(6):472–92. Read as the open-access author manuscript.
- Breslow A. Thickness, cross-sectional areas and depth of invasion in the prognosis of cutaneous melanoma. Ann Surg. 1970. The derivation publication for the measurement itself.
- International Collaboration on Cancer Reporting. Breslow thickness, melanoma of the skin dataset guide — measured from the top of the granular layer, or from the base of the ulcer where the surface is ulcerated, to the deepest invasive cell; periadnexal extension excluded; the 1-to-4-down, 5-to-9-up rounding rule. iccr-cancer.org
- PathologyOutlines. Breslow thickness — how the thickness is measured, the AJCC 8th edition pT thresholds, the rounding rule, and that Clark level is superseded by Breslow depth and not used for staging. pathologyoutlines.com/topic/skintumorbreslow.html
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/