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Breslow Depth — the Number That Drives Everything

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If a melanoma has been diagnosed, the pathology report will list a Breslow depth in millimeters — often a fraction of one. That small number carries enormous weight: it is the strongest predictor of how the melanoma will behave and the anchor for nearly every decision that follows, from the width of the excision to whether the lymph nodes are checked. Here is what the measurement means and why fractions of a millimeter matter.

Last updated: July 2026

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What Breslow thickness measures

Breslow thickness is a measurement of how deeply a melanoma has grown into the skin. A pathologist looks at the biopsy under a microscope and measures, in millimeters, from the top of the skin's surface layer straight down to the deepest melanoma cell 1. It is reported as a single number — often less than one millimeter — and it describes vertical depth, not the width of the spot on the surface.

That depth matters because melanoma becomes dangerous largely by growing downward, toward the blood vessels and lymph channels that could carry it elsewhere 2. Breslow thickness, or Breslow depth, is the vertical distance in millimeters from the skin surface to the deepest melanoma cell. A melanoma can look small from above and still measure as thick, which is exactly why the number is taken from under the microscope rather than from the naked eye.

Why one number drives so many decisions

Breslow thickness drives melanoma care because it is the strongest single predictor of how the tumor is likely to behave, and the staging system is built around it. Thickness sets the T category — the 'tumor' part of staging — with each threshold marking a step up in stage 1. From that flow the two biggest treatment decisions: how much skin to remove around the melanoma, and whether to check the lymph nodes.

Surgical margins widen as thickness grows, and a sentinel node biopsy is generally offered only once a melanoma reaches a certain depth 3. So a difference of a few tenths of a millimeter is not academic — it can be the line between a simple excision and a larger operation with node sampling. That is why pathologists measure it so carefully, and why it anchors the whole plan.

The thickness categories, in millimeters

Melanoma thickness is grouped into T categories, and knowing which one a tumor falls into explains much of what comes next. The cut points are set by the AJCC staging system, and the same thresholds are used everywhere 1. Broadly, they run from melanoma in situ — confined to the top layer, with no measurable depth — up through progressively thicker tumors, each step carrying a higher stage and closer follow-up.

T categoryBreslow thickness
In situ (Tis)Confined to the surface layer; no invasive depth
T11.0 mm or less
T2More than 1.0 up to 2.0 mm
T3More than 2.0 up to 4.0 mm
T4More than 4.0 mm

Within T1, the 0.8 mm mark is a further dividing line, and ulceration — a microscopic break in the skin over the tumor — pushes a melanoma into a higher-risk subgroup regardless of the exact thickness 1.

How thickness sets the surgical margins

The width of the operation to remove a melanoma is decided mostly by its Breslow thickness. After the diagnostic biopsy, a wide local excision removes the melanoma site along with a margin of surrounding normal skin, and that margin grows with the tumor's depth 3. The goal is to clear any microscopic tumor extending beyond what the eye can see, while removing no more healthy skin than necessary.

Breslow thicknessRecommended surgical margin
Melanoma in situ0.5–1.0 cm
1.0 mm or less1.0 cm
More than 1.0 up to 2.0 mm1.0–2.0 cm
More than 2.0 mm2.0 cm

These margins are measured out around the melanoma, so what to expect from a wide local excision — its size and the resulting scar — scales with depth. Wide local excision margins by Breslow are among the most standardized parts of melanoma surgery.

Ulceration and the other details on the report

Breslow thickness is the headline, but a melanoma report carries other findings that modify the picture. The most important is ulceration — whether the skin over the tumor has broken down, visible only under the microscope. Ulceration worsens the outlook at any given thickness and moves a melanoma into a higher-risk staging subgroup 1. It is one reason two melanomas of the same depth can be staged differently.

Reports may also note the mitotic rate — how actively the tumor cells are dividing — along with whether the biopsy margins were clear. None of these replaces thickness; they refine it. When a diagnosis or a measurement is borderline and would change the plan, a dermatopathology second review is a reasonable step, because where the depth falls relative to a threshold can genuinely alter the surgery.

Why the biopsy method affects the number

Breslow thickness can only be measured accurately if the biopsy captured the full depth of the tumor. A superficial shave that slices across the base of a melanoma can leave the deepest cells behind, so the measured thickness comes out lower than the truth — and an under-measured melanoma can be under-treated. For a lesion that could be melanoma, a narrow but full-thickness sample that reaches beneath the tumor is preferred 4.

This is why the choice of biopsy technique is not a minor detail when melanoma is on the table. If a first biopsy transected the tumor and the true depth is uncertain, the report will often say so, and a repeat or wider excision may be needed to establish the real Breslow thickness before staging can be trusted.

Breslow thickness, stage, and outlook

Because thickness drives stage, it also tracks closely with prognosis: the thinner a melanoma when it is removed, the better the odds. Melanoma found early — thin and confined to the skin — has a high five-year survival, and survival falls as the disease reaches the lymph nodes or beyond 5. This is the whole argument for early detection: depth is largely destiny, and depth is a function of how soon the melanoma is caught.

A Breslow number is best read alongside the full melanoma stages and, if you want it, a plain explanation of the melanoma survival rate by stage rather than in isolation. It describes this tumor at this moment; it does not predict an individual's future with certainty. What it does do is give you and your team a shared, precise starting point for every decision that follows.

Common questions

Generally, yes. A thinner melanoma is caught at an earlier stage, needs a smaller surgical margin, and carries a better outlook. Melanoma in situ, confined to the top layer, has no invasive depth to measure at all. Because the thresholds fall at fractions of a millimeter, even a small difference in depth can change the stage and the plan, which is why the measurement is taken so carefully.

Broadly, melanoma in situ and tumors 1.0 mm or less are considered thin, while those thicker than 2.0 mm are considered thick, with an intermediate band in between. Ulceration and other features shift the risk within those groups. Each step up in thickness raises the stage and generally means wider surgery and closer follow-up, so the exact number on the report matters.

Two common reasons. Melanoma in situ sits entirely in the surface layer, so there is no invasive depth to measure and no Breslow number is given. Alternatively, a shallow first biopsy may have cut across the tumor so its true depth could not be determined; in that case the definitive thickness comes from a repeat or wider excision. Your team can explain which applies.

It is the main factor, along with features like ulceration. A sentinel node biopsy is generally not recommended for very thin melanomas and is typically discussed once a tumor reaches about 0.8 mm, or is thinner but higher-risk. Because the decision hinges on depth, an accurate Breslow measurement matters before the node question is settled. It remains a shared decision with your surgeon.

The definitive thickness comes from a sample that captured the full depth of the tumor. If the first biopsy was too shallow and transected the melanoma, the initial number can underestimate the truth, and a wider excision may reveal a greater depth. The deepest accurate measurement is the one used for staging. This is why full-thickness sampling is preferred for suspected melanoma.

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Breslow depth and why timing matters

  • —A mole or spot that is growing, thickening, or becoming raised, since a melanoma tends to grow deeper the longer it goes unremoved
  • —A pigmented spot that bleeds, ulcerates, or forms a sore that does not heal
  • —A pigmented lesion changing in color, border, or size over weeks to a few months
  • —A new lump near a known melanoma site, or a lymph node that enlarges

This article explains what Breslow thickness means; it cannot interpret your own report or assess any spot on your skin. The pathology findings, stage, and plan are interpreted by the dermatologist, surgeon, and pathology team caring for you. Bring your report and questions to them.

References

  1. 1.Gershenwald JE, Scolyer RA, Hess KR, et al. (2017). Melanoma staging: Evidence-based changes in the American Joint Committee on Cancer eighth edition cancer staging manual. CA: A Cancer Journal for Clinicians. doi:10.3322/caac.21409Melanoma thickness (Breslow depth) is measured to the deepest tumor cell and sets the T category; ulceration modifies staging within a thickness group.
  2. 2.National Cancer Institute (PDQ Adult Treatment Editorial Board) (2025). Melanoma Treatment (PDQ®)–Health Professional Version. NIH / National Cancer Institute. link ✓Melanoma is a melanocyte cancer that becomes dangerous by growing downward and spreading through blood and lymphatic channels.
  3. 3.Swetter SM, Tsao H, Bichakjian CK, et al. (2019). Guidelines of care for the management of primary cutaneous melanoma. Journal of the American Academy of Dermatology. doi:10.1016/j.jaad.2018.08.055Wide local excision margins, and whether a sentinel node biopsy is offered, scale with the melanoma's Breslow thickness.
  4. 4.American Family Physician (2011). Shave and Punch Biopsy for Skin Lesions. American Family Physician. link ✓An accurate Breslow thickness requires a full-thickness biopsy; a superficial shave that transects the tumor can underestimate depth.
  5. 5.National Cancer Institute, Surveillance, Epidemiology, and End Results (SEER) Program (2025). Cancer Stat Facts: Melanoma of the Skin. NIH / National Cancer Institute (SEER). linkMelanoma five-year relative survival is high for early, thin disease and falls as the disease reaches the nodes or distant sites (survival by stage).

5 sources, numbered by first appearance. General health information, not medical advice. AI-assisted editorial content — citations link their sources. Editorial policy

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