The short answer
Breslow depth, also called Breslow thickness, is the thickness of a melanoma in millimeters. The pathologist measures it under a microscope. Along with ulceration, it sets the T category in melanoma staging. This page explains the number and what to ask your care team.
Breslow depth is the thickness of the melanoma in millimeters, measured by the pathologist under a microscope.
It is one of the main things that set the T category, together with whether the tumor is ulcerated.
NCI lists tumor thickness among the factors that affect outlook, but one number does not predict what will happen to one person.
Your care team puts the depth together with the rest of the report to plan next steps.
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This page is for anyone whose melanoma pathology report lists a Breslow depth. It explains what the number measures, how it fits into staging, and what to ask your care team.
Breslow depth is a thickness in millimeters
After a melanoma is removed or sampled, a pathologist looks at thin slices of it under a microscope. One of the first things they record is how thick it is.
NCI describes tumor thickness as the distance "from the surface of the skin to the deepest part of the tumor." The number is given in millimeters. Your report may call it Breslow depth, Breslow thickness, or just tumor thickness. They all mean the same thing.
How the number is used in staging
In melanoma, the T category of TNM staging is set by two things: thickness and ulceration. NCI lists these thickness groups:
- T1: 1.0 mm or less. T1a is under 0.8 mm without ulceration. T1b is under 0.8 mm with ulceration, or 0.8 to 1.0 mm with or without it.
- T2: more than 1.0 mm up to 2.0 mm.
- T3: more than 2.0 mm up to 4.0 mm.
- T4: more than 4.0 mm.
For T2 through T4, the letter "a" means no ulceration and "b" means ulceration is present.
The T category is only part of the stage. The N category (lymph nodes) and M category (spread to distant sites) also matter. So a depth alone does not tell you your stage.
Why thickness matters to your team
NCI lists tumor thickness among the factors that affect outlook and treatment choices. Ulceration, lymph node findings, and general health are on the same list.
In general, thicker melanomas are staged higher. But staging describes groups of people, not you. Your care team is the right source for what your depth means in your situation.
Breslow depth versus Clark level
Older reports, and some current ones, also list a Clark level. Clark level names the skin layer the melanoma reaches, from I to V. It is not a measurement.
NCI states that Breslow thickness "is more reproducible" and "should always be reported." If your report has both, the Breslow depth is the one used in current T categories. See What Does Clark Level Mean? for more.
When the number may not be final
A biopsy sometimes removes only part of a melanoma. If the base of the sample still has tumor cells, the true depth could be greater than what was measured. Reports sometimes say this with words like "at least" or "transected at the base."
If you see that wording, ask whether the depth might change after a wider excision.
Ask these before you leave
- What is my Breslow depth, and is it a complete measurement?
- Is there ulceration?
- What T category does that give me?
- What other parts of the report affect my stage?
- What are the next steps, and why?
Related pages
See Melanoma Stages, Sentinel Lymph Node Biopsy for Melanoma, What Does Ulceration Mean on a Melanoma Report?, and Understanding Your Pathology Report.
Sources
Common questions
What is Breslow depth?
It is the thickness of a melanoma, measured in millimeters. NCI describes it as the distance from the surface of the skin to the deepest part of the tumor.
Is Breslow depth the same as Clark level?
No. Clark level describes which skin layer the melanoma reaches. Breslow depth is a measurement in millimeters. NCI notes that thickness is more reproducible and should always be reported.
Does a thin melanoma mean I do not need more treatment?
Not always. The plan depends on the whole report, including ulceration and other findings. Ask your care team what your depth means for your next steps.
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Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes, and this is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and then compared with the sources listed on it, looking for statements those sources do not back up. That check is not a sentence-by-sentence record and can miss errors. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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