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Reading your melanoma report

Breslow Thickness: — The Number That Guides Your Treatment

When your melanoma pathology report comes back, one number matters more than any other: Breslow thickness. It is the depth of the melanoma measured in millimetres, and your surgical team uses it to plan what happens next.

Medically reviewed by Dr. Bharati Devi Gorantla, Medical Oncologist, MBBS · MD · DM (Adyar, Chennai) · ECMO · MRCP SCE (UK) · Last reviewed September 2026

  • Depth, not width — Breslow thickness measures how deep into the skin the melanoma has grown, not how wide it appeared on the surface.
  • Measured under a microscope — The pathologist uses a calibrated eyepiece to measure from the skin surface to the deepest melanoma cell.
  • Drives surgical planning — Your surgeon uses this number to decide how wide the excision margin needs to be and whether to discuss lymph node assessment.
  • Clark level is older — Clark level (I–V) describes which skin layer was reached. Breslow has largely replaced it for treatment decisions, but both often appear on the same report.
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Breslow thickness is the depth of your melanoma measured in millimetres, from the skin surface down to its deepest cells. It is the single most important number on a melanoma pathology report. Your surgical team uses it to decide how wide the next excision needs to be and whether to discuss testing nearby lymph nodes.

What do the terms on your melanoma report mean?

Breslow thickness
The depth of the melanoma from the top of the skin surface to the deepest tumour cell, measured in millimetres. This is the single most important measurement on a melanoma pathology report and the one most likely to shape your next surgical conversation.
Clark level
An older staging system using Roman numerals I to V, describing which anatomical layer of skin the melanoma has reached — from the outermost layer (I) to the fat beneath the skin (V). Current AJCC guidelines rely primarily on Breslow thickness, but Clark level still commonly appears on reports.
Melanoma in situ
Melanoma cells confined to the very top layer of skin, with no downward growth. Because it has not invaded deeper tissue, there is no Breslow thickness to record. It is managed differently from invasive melanoma.
Pathological ulceration
Breakdown of the skin surface over the melanoma, identified under the microscope rather than to the naked eye. Recorded as present or absent. AJCC guidelines consider ulceration alongside Breslow thickness when assigning a T-stage.
Mitotic rate
The number of cancer cells actively dividing, counted under the microscope per unit area of tumour. A higher mitotic rate is noted on the report and considered alongside Breslow thickness in overall staging.

How does Breslow thickness affect what happens next?

Breslow thickness is the main measurement your surgeon uses to decide how wide to cut around the melanoma site at the next operation. Shallower melanomas generally call for a narrower margin; deeper ones call for a wider margin.

It also helps your team decide whether to discuss sentinel lymph node biopsy — a procedure that checks whether melanoma cells have reached the nearest lymph node. Your surgeon will explain whether this applies in your case and why.

The decision about re-excision margins and lymph node assessment is not made from Breslow thickness alone. Ulceration, mitotic rate, your overall health, and other factors all form part of the conversation. Your treating surgeon will weigh all of them together.

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Breslow thickness versus Clark level: what is the difference?

FeatureBreslow thicknessClark level
What it measuresActual depth of tumour in millimetresWhich anatomical skin layer was reached
How it is determinedCalibrated microscope measurementMicroscopic assessment of skin layer involvement
ScaleContinuous measurement in millimetresRoman numerals I (superficial) to V (deepest)
Role in current AJCC stagingPrimary measurement for T-stage classificationNo longer a required staging factor in AJCC 8th edition
Still reported?Yes — core part of every melanoma reportCommonly included, particularly for thinner melanomas

Did you know?

Breslow thickness was first described in 1970 by pathologist Alexander Breslow, who showed that measuring the vertical depth of a melanoma predicted its behaviour far better than any surface feature visible to the naked eye.

The AJCC has since built its entire melanoma T-staging system around this one measurement — a finding that emerged from a small series of cases and changed how melanoma is assessed worldwide.

Source: AJCC Cancer Staging Manual, 8th Edition; Breslow A, Thickness, cross-sectional areas and depth of invasion in the prognosis of cutaneous melanoma, Ann Surg 1970

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Common questions

Frequently asked questions

What exactly does Breslow thickness measure?

It measures the depth of the melanoma — how far it has grown downward into the skin — in millimetres. The pathologist measures from the granular layer of the epidermis, or from the base of any ulceration if the surface has broken down, to the deepest melanoma cell visible under the microscope. It does not describe how wide the melanoma looked on your skin, how long it had been there, or whether it has spread beyond the skin.

My report shows a Clark level. Is that different from Breslow thickness?

Yes, they measure different things. Breslow thickness gives an actual depth in millimetres. Clark level uses a Roman numeral (I to V) to describe which layer of skin the melanoma reached — from the surface layer down to the fatty tissue below. Current AJCC guidelines use Breslow thickness as the primary staging measurement; Clark level is no longer a required staging factor, though many reports still include both. If your report has both, Breslow thickness is the number your surgeon will focus on.

My Breslow thickness is very small. Does that mean the melanoma has not spread?

A smaller Breslow thickness describes a shallower melanoma and influences T-stage classification. It does not, on its own, confirm that melanoma cells have not reached the lymph nodes or other sites — that requires examination and, where indicated, further assessment. What your result means for your overall situation is a question for your treating team, who will consider it alongside the rest of your pathology report, examination findings, and any imaging.

What does Breslow thickness mean for my surgery?

Your surgeon uses Breslow thickness to plan two things: how wide to cut around the melanoma site, and whether to discuss sentinel lymph node biopsy. Shallower melanomas generally need narrower margins; deeper ones need wider ones. Whether sentinel lymph node biopsy applies to your case depends on Breslow thickness alongside other features of the report. Your surgeon will explain both decisions and the reasoning behind them at your next appointment.

My report mentions ulceration. How does that relate to Breslow thickness?

Pathological ulceration — breakdown of the skin surface over the melanoma, identified under the microscope — is recorded separately from Breslow thickness but considered alongside it. AJCC guidelines combine both when assigning a T-stage, so a melanoma with ulceration present is staged differently from one of the same depth without it. Your oncologist will explain exactly how these two findings together affect your T-stage and what that means for the treatment plan.

What should I ask my surgeon when we discuss the Breslow thickness result?

Ask what your Breslow thickness means for the excision margin at the next operation. Ask whether sentinel lymph node biopsy is being considered and, if so, what the procedure involves. If ulceration or a raised mitotic rate is noted on the report, ask how those findings change the plan. Bringing the pathology report itself to the appointment helps, so that every term can be explained in the context of your specific result.

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