What Is Breslow Thickness? Why Is 0.8 mm Critical, and How Does It Affect Melanoma Survival?

Melanoma is an aggressive skin cancer subtype accounting for approximately 5% of skin cancers and is often treatable when diagnosed early. Prognostic measures such as Breslow thickness, however, play a critical role in disease course and survival. A new Australian study found a significantly increased risk of melanoma-related death in patients with a Breslow thickness of 0.8-1.0 mm, drawing attention to the need to reconsider current cancer staging systems.

What Is Breslow Thickness?

Breslow thickness measures melanoma from its uppermost layer to the deepest point of invasion and is expressed in millimeters. It is critical to melanoma staging and prognosis.

Clark level is a classification system describing how deeply melanoma extends into the skin's layers. It grades invasion from the epidermis through the dermis to subcutaneous tissue on a scale of 1 to 5 and is used as a prognostic indicator, particularly in thin melanomas.

Study Methods

Diagnosed between 1982-2014, 144 thousand patients were included in this large-scale melanoma study conducted in Australia using cancer registries from the country's eight states and territories.

The research team was based at Sydney's Melanoma Institute Australia and University of Sydney and led by Dr. Serigne N. Lo. The results were published on December 11, 2024, in JAMA Dermatology .

Participants had primary invasive thin melanoma (T1) with a Breslow thickness ≤ 1 mm. Their median age was 56, and 54% were men. The researchers examined two thickness groups, < 0.8 mm and 0.8-1.0 mm, comparing melanoma-related and non-melanoma mortality.

• Primary Endpoint: Time to death from melanoma.
• Secondary Endpoint: Death from causes other than melanoma.

Findings

Melanoma Mortality Rates

• 20-Year Cumulative Melanoma Mortality: 6.3% for the entire cohort.
• Breslow Thickness 0.8-1.0 mm: 11%.
• Breslow Thickness < 0.8 mm: 5.6%.

Each increase of 0.1 mm was associated with a worse prognosis.

Survival Rates

• 20-Year Melanoma-Specific Survival for the Entire Cohort: 95.9%.
  • < 0.8 mm: 94.2%.
  • 0.8-1.0 mm: 87.8%.

Multivariable Analysis Findings

• Tumors 0.8-1.0 mm thick:
  • A 292% increase in the risk of melanoma-related death.
  • A 298% increase in the melanoma mortality rate.

What Does This Mean in Clinical Practice?

The research suggests that melanomas thicker than 0.8 mm cast doubt on the T1 threshold in the current AJCC (American Joint Committee on Cancer) staging system. The findings suggest considering lowering this threshold from 1.0 mm to 0.8 mm.

Limitations

• The study used registry data, which could not capture details such as tumor characteristics and treatment methods.
• Incomplete reporting of causes of death may have led to an underestimation of melanoma-related mortality and an overestimation of mortality from other causes.