Breslow Thickness: Definition

Breslow thickness (also called Breslow depth) is a measurement used by pathologists to describe how far cancer cells in invasive melanoma — a type of skin cancer that starts in pigment-producing cells called melanocytes — have grown into the skin. The measurement tells doctors the distance, in millimeters, from the skin surface to the deepest point where melanoma cells are found. Breslow thickness is one of the most important factors used to determine the pathologic tumor stage (pT) of melanoma and to predict how the cancer is likely to behave. Thicker melanomas are more likely to spread (a process called metastasis) and generally require more extensive treatment than thinner ones.

How do pathologists measure Breslow thickness?

Pathologists measure Breslow thickness using a microscope and a special ruler called a calibrated eyepiece. The measurement starts at the top of the skin — specifically from the surface of the granular layer of the epidermis (the thin outermost layer of skin) — and goes down to the deepest point where melanoma cells are found. If an ulcer (an area where the skin’s surface has been lost) is present, the measurement starts from the base of the ulcer. The result is reported in millimeters (mm) and included in the pathology report alongside other tumor features.

Why is Breslow thickness important?

The deeper a melanoma has grown into the skin, the more likely it is to have reached blood vessels and lymphatic vessels — the channels through which cancer cells can spread to lymph nodes and distant organs. Breslow thickness directly affects:

Tumor stage — Breslow thickness is the primary feature used to assign the pathologic T stage (pT) for melanoma. Higher pT stages reflect deeper tumors and a higher risk.
Prognosis — Thinner melanomas generally have a much better outlook than thicker ones. The risk of recurrence and spread increases with Breslow thickness.
Treatment decisions — The Breslow thickness helps guide decisions about how widely the surrounding skin should be removed and whether to perform a sentinel lymph node biopsy (a procedure to check the first lymph node where cancer is most likely to spread). Thicker melanomas may also be evaluated for additional treatment such as immunotherapy or targeted therapy.

What do the Breslow thickness measurements mean for staging?

For invasive melanoma, Breslow thickness determines the pathologic T stage (pT) using the following categories:

pT1 — 1.0 mm or less. The thinnest category. Subdivided into pT1a (less than 0.8 mm without ulceration) and pT1b (0.8 to 1.0 mm, or any tumor with ulceration). The 0.8 mm threshold is the point at which sentinel lymph node biopsy is often considered.
pT2 — more than 1.0 mm and up to 2.0 mm. Subdivided into pT2a (without ulceration) and pT2b (with ulceration).
pT3 — more than 2.0 mm and up to 4.0 mm. Subdivided into pT3a (without ulceration) and pT3b (with ulceration).
pT4 — more than 4.0 mm. The thickest category, subdivided into pT4a (without ulceration) and pT4b (with ulceration).

In each category, the presence of ulceration (loss of the skin surface over the tumor) raises the substage and is associated with a worse prognosis. For example, a 1.5 mm melanoma without ulceration is pT2a, but a 1.5 mm melanoma with ulceration is pT2b.

When is Breslow thickness reported?

If only a small piece of the tumor is removed during the first procedure — called a biopsy — a preliminary Breslow thickness may be reported. However, the most accurate measurement is made after the entire melanoma has been removed in a procedure called an excision. This final measurement is used to assign the definitive pT stage and to guide further treatment.

Breslow thickness applies only to invasive melanoma — melanoma that has grown beyond the epidermis into the deeper layers of the skin. It is not reported for melanoma in situ, which by definition is confined to the epidermis and has no depth of invasion to measure.

What does my Breslow thickness mean?

The significance of your Breslow thickness depends on the exact measurement and other tumor features, particularly whether ulceration is present and whether the melanoma has spread to lymph nodes. In general:

Thin melanomas (pT1) — usually have an excellent outlook, with high cure rates after complete surgical removal.
Intermediate melanomas (pT2–pT3) — require careful staging, often including sentinel lymph node biopsy, and may benefit from additional treatment depending on lymph node findings.
Thick melanomas (pT4) — carry a higher risk of recurrence and spread and may be treated with a combination of surgery, sentinel lymph node biopsy, and modern systemic therapies such as immunotherapy or targeted therapy.

Your oncologist or dermatologist will explain what your specific Breslow thickness means in the context of your full pathology report and overall health.

Questions to ask your doctor

What is my Breslow thickness, and what pT stage does this correspond to?
Was ulceration present, and how does this affect my staging and prognosis?
Do I need a sentinel lymph node biopsy or any additional treatment based on this measurement?