Breslow Thickness of a Melanoma

This tumor classification help determine one's prognosis

If your doctor sees a suspicious mole during an exam, the next step is a skin biopsy. If that reveals melanoma, a pathologist will perform tests to determine how advanced the disease is—a process called staging. In addition, the thickness of the tumor is measured and assigned what's called a Breslow number. This figure has been found to be significant when coming up with a patient's prognosis.

Close-up of mole on woman's body
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Once the stage and the Breslow number have been accurately determined, your doctor can effectively plan your treatment.

The TNM System

In 2016, the American Joint Committee on Cancer (AJCC) released new guidelines for staging melanoma, known as the TNM system. Each letter of this name stands for a different aspect of a tumor's characteristics:

  • T (tumor): The important aspects of the tumor itself include how big it is, whether it has ulcerated (there is a breakdown of the skin over the melanoma), and if it has spread beyond its own boundaries.
  • N (lymph nodes): Has the cancer spread to nearby lymph nodes?
  • M (metastasis): Has the cancer spread to distant lymph nodes or distant organs, such as the lungs, liver, brain, or bones?

Once TNM factors have been identified, the cancer is given a number from 0 to IV— and sometimes a letter, from A to C—with higher numbers meaning the cancer is more advanced.

Breslow Thickness

Besides the TNM number, you may also hear the doctor refer to a Breslow number, a measurement of thickness used for melanomas that have not spread. (Another measurement, called the Clark level, is no longer used as it has been deemed too subjective.)

In general, melanomas less than 1 millimeter (mm) thick—about 1/25 of an inch—have a very small chance of spreading. As the melanoma becomes thicker, it has a greater chance of spreading.

First reported by Alexander Breslow, MD, in 1970, the Breslow thickness is defined as the total vertical height of the melanoma, from the very top (called the granular layer) to the area of deepest penetration in the skin.

An instrument called an ocular micrometer is used to measure the thickness of the excised (removed) tumor. Due to its accuracy in predicting outcomes, the Breslow thickness has been incorporated into the standard TNM staging system for melanoma.

The Breslow thickness is an extremely important prognostic factor in melanoma, along with tumor (T) stage and the existence of skin ulceration (broken skin, bleeding, swelling). In general, the higher the Breslow thickness, the worse the prognosis—in other words, the thicker the melanoma, the greater chance it has of spreading.

Prognosis Based on Breslow Number

Five-year survival rates can be determined by Breslow thickness. Keep in mind that these survival rates are averages and may not reflect your individual case:

  • Less than 1 mm: Around 95%, meaning 95 people out of 100 people will be alive five years after being diagnosed with a melanoma that is less than 1 mm thick
  • 1.01 to 2 mm: Around 90%
  • 2.01 to 4 mm: Around 77%
  • Greater than 4 mm: 65%

A Word From Verywell

While melanoma can be a frightening diagnosis, more and more effective treatments are available, even for late-stage cancers. Accurate staging will result in receiving the most effective treatment possible.

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Article Sources
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  1. National Cancer Institute. SEER Training Modules. Melanoma staging schemes. Updated 2019.

  2. Melanoma Molecular Map Project. TNM staging. Updated 2015.

  3. Gawkrodger, D, Ardern-Jones, MR. Dermatology E-Book: An Illustrated Colour Text. New York, NY: Elsevier Health Science; 2016.

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