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Clark Level and Breslow Thickness: What Do These Measures Mean?

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Updated June 03, 2014

Woman checking mole for cancer
Peter Dazeley, Photographer's Choice/Getty Images
Your doctor says that your melanoma is stage IIA (T2bN0M0), Clark level III and Breslow depth 2 mm. You wonder what foreign language she's speaking! Indeed, melanoma jargon can be confusing and exasperating. To help you understand why a particular treatment was chosen for you, here is a brief explanation of what these terms mean.

If your doctor sees a suspicious mole or other lesion during a skin exam, the next step is a skin biopsy. If the biopsy reveals a melanoma, the pathologist will examine the biopsy to determine the stage (extent) of the disease in order to effectively plan your treatment. Besides the stage and TNM number, you may also hear the doctor use a Breslow or Clark number to describe your prognosis (outlook). Here is how to interpret them:

Breslow Thickness

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.

In general, the higher the Breslow thickness, the worse the prognosis (keep in mind that these survival rates are averages and may not reflect your individual case):

  • less than 1 mm: 5-year survival is 95% to 100%
  • 1 to 2 mm: 5-year survival is 80% to 96%
  • 2.1 to 4 mm: 5-year survival is 60% to 75%
  • greater than 4 mm: 5-year survival is 37% to 50%
Due to its accuracy in predicting outcomes, the Breslow thickness has been incorporated into the standard TNM staging system for melanoma. In 2001, a large study confirmed the importance of Breslow thickness as one of the three most important prognostic factors in melanoma, along with tumor (T) stage and the existence of ulceration (broken skin, bleeding, swelling).

Clark Level

The Clark level refers to how deep the tumor has penetrated into the layers of the skin. This system was originally developed by W. H. Clark, MD, back in 1966. Clark levels are officially defined as follows:

  • Level I: confined to the epidermis (top-most layer of skin); called "in situ" melanoma; 100% cure rate at this stage
  • Level II: invasion of the papillary (upper) dermis
  • Level III: filling of the papillary dermis, but no extension in to the reticular (lower) dermis
  • Level IV: invasion of the reticular dermis
  • Level V: invasion of the deep, subcutaneous tissue

Since 2002, Clark's levels have been used less and less for calculating prognosis, since research has shown them to be less predictive of outcome, less reproducible and more subjective than the Breslow depth. Other disadvantages of this system are that it is often very difficult to differentiate between Clark Level II and Level III, and it can't be used on melanomas of the palms and soles.

There is one instance in which Clark’s levels continue to be used to predict prognosis: in patients with thin (less than 1.0 mm) melanoma, a Clark’s level IV or V lesion portends a worse prognosis. For this reason, pathologists continue to include the Clark’s level along with Breslow thickness and existence of ulceration in their reports.

More on How to Interpret Your Melanoma Pathology Report

Sources:

Balch CM, et al. "Prognostic factors analysis of 17,600 melanoma patients: validation of the American Joint Committee on Cancer melanoma staging system." Journal of Clinical Oncology 2001 19(16):3622-34. 20 November 2008.

"Melanoma Staging Schemes." SEER's Training Web Site. National Cancer Institute. 20 November 2008.

Roberts DLL, et al. "U.K. guidelines for the management of cutaneous melanoma." British Journal of Dermatology 2002 146:7–17. 20 November 2008.

"How is Melanoma Staged?" American Cancer Society. 20 November 2008.

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