The diagnosis of melanoma often brings immediate questions about the severity and potential outcome of the disease. Breslow thickness is a measurement that provides objective insight into the cancer’s nature. This single number, recorded in a pathology report, represents the vertical depth of the tumor and is considered the most important factor for predicting the course of localized melanoma. Understanding this measurement helps determine the overall risk profile and subsequent medical decisions.
Defining Breslow Thickness and How It Is Measured
Breslow thickness is the vertical depth of invasion of the melanoma into the skin layers. It measures how far cancer cells have grown into the dermis, the layer beneath the epidermis, rather than the width or surface area of the lesion. The measurement is taken in millimeters (mm) and is determined only after the lesion has been surgically removed via an excisional biopsy and sent to the laboratory.
A specialized doctor called a pathologist examines the tissue sample under a high-powered microscope to determine this depth. The measurement begins at a specific point on the epidermis, which is the top of the granular layer, or the base of any ulceration if the surface skin is broken. From this starting point, the pathologist uses a calibrated micrometer within the microscope to measure the distance straight down to the deepest point where invasive melanoma cells are found.
The resulting number is reported to the nearest tenth of a millimeter (e.g., 0.7 mm or 1.5 mm). This depth directly indicates risk: the deeper the tumor grows, the greater the chance it has reached blood vessels or lymph channels, allowing cancer cells to travel to distant parts of the body. A lower Breslow thickness indicates a shallower tumor and a more favorable outlook.
Using Thickness to Determine Melanoma Staging and Prognosis
Breslow thickness is foundational to the American Joint Committee on Cancer (AJCC) staging system, serving as the basis for assigning the “T” (Tumor) stage. The T stage categorizes the primary tumor based on its depth, which directly reflects the patient’s risk profile. Thinner melanomas are grouped into lower T categories, while thicker melanomas are assigned higher T categories.
The staging thresholds are clearly defined: T1 melanomas are \(\le\) 1.0 mm thick, T2 between 1.1 mm and 2.0 mm, T3 between 2.1 mm and 4.0 mm, and T4 exceeding 4.0 mm. These categories provide a standardized way for doctors worldwide to communicate the tumor’s extent and predict the likelihood of the cancer spreading. As the Breslow thickness increases through these T categories, the associated risk of the cancer spreading to lymph nodes or other organs also increases.
A particularly important clinical distinction exists within the T1 category, separated by a thickness of 0.8 mm. Melanomas thinner than 0.8 mm without adverse features are considered very low-risk, while those between 0.8 mm and 1.0 mm carry a measurably higher risk of melanoma-related death. This 0.8 mm threshold often serves as a dividing line for more aggressive monitoring. The prognosis is closely tied to this number; five-year survival rates for tumors under 1.0 mm often exceed 95%, dropping significantly for tumors thicker than 4.0 mm.
Breslow Thickness and Subsequent Treatment Planning
The numerical value of the Breslow thickness immediately dictates subsequent clinical management, primarily determining the necessary surgical margin for definitive treatment. Following the initial biopsy, the patient undergoes a Wide Local Excision (WLE), which involves removing a wider area of healthy tissue surrounding the original tumor site to ensure all remaining cancer cells are cleared. The width of this required margin is directly linked to the Breslow thickness.
For very thin melanomas (\(\le\) 1.0 mm thick), a surgical margin of 1 centimeter (cm) is typically recommended for the WLE. As the thickness increases to the intermediate range of 1.01 mm to 2.0 mm, the recommended margin widens to 1 cm or 2 cm, depending on the specific case. Thicker melanomas, specifically those greater than 2.0 mm, require the largest margin of 2 cm to minimize the risk of local recurrence.
Breslow thickness also determines the necessity of a Sentinel Lymph Node Biopsy (SLNB), a procedure used to check if cancer cells have traveled to the nearest lymph node basin. Routine SLNB is not performed for the thinnest melanomas (under 0.8 mm without ulceration) due to the very low risk of spread. However, if the Breslow thickness exceeds 0.8 mm to 1.0 mm, or if the tumor is thinner but shows signs of ulceration, an SLNB is typically recommended. This procedure provides crucial information for planning further observation or therapy.