Breslow Thickness: The primary factor determining T-stage and surgical margins.
Melanoma Type
Stage 0 - Confined to epidermis
MM
Stage Derivation
Enter the Breslow thickness and ulceration status to derive the AJCC T-category and recommended excision margins.
Guidelines & Evidence
Verified
Last Review: 2026-07-17
When to Use
When to Use
Breslow thickness and Clark level are the two primary histopathological measurements obtained from melanoma biopsy specimens. Breslow thickness is the dominant prognostic variable in all modern staging systems and directly determines surgical margins and sentinel lymph node biopsy (SLNB) eligibility. Clark level is a secondary anatomical descriptor that retains limited utility in specific circumstances.
Indications
All confirmed primary cutaneous melanoma biopsy specimens — mandatory measurement
Determining T-category in AJCC 8th edition melanoma staging
Guiding excision margin width (thickness-based)
Determining eligibility and urgency for sentinel lymph node biopsy
Prognostic counseling following pathological diagnosis
When NOT to Use
Shave biopsies that transect the base — Breslow thickness will be artificially underestimated; re-excision biopsy required
Melanoma in situ (Tis) — no Breslow depth applicable; excision with 0.5–1 cm margins
Clark level alone for AJCC staging — replaced by Breslow thickness in 8th edition
Uveal or mucosal melanoma — different staging systems apply
How it Works
Breslow Thickness — Measurement
Measured in millimeters to the nearest 0.1 mm using an ocular micrometer. Measurement extends from the top of the granular layer of the overlying epidermis (or the base of ulceration if the tumor is ulcerated) vertically to the deepest invasive melanoma cell. First reported by Alexander Breslow in 1970 as the single strongest predictor of melanoma prognosis.
Breslow Thickness — AJCC 8th Edition T-Category Thresholds
| Tis | Melanoma in situ (no invasive component) |
| T1a | < 0.8 mm, no ulceration |
| T1b | 0.8–1.0 mm (any), OR < 0.8 mm with ulceration |
| T2a | > 1.0–2.0 mm, no ulceration |
| T2b | > 1.0–2.0 mm, with ulceration |
| T3a | > 2.0–4.0 mm, no ulceration |
| T3b | > 2.0–4.0 mm, with ulceration |
| T4a | > 4.0 mm, no ulceration |
| T4b | > 4.0 mm, with ulceration |
Clark Level — Anatomical Depth Classification
| Level I | Confined to epidermis — melanoma in situ |
| Level II | Invasion into (but not filling) the papillary dermis |
| Level III | Fills the papillary dermis to the junction with reticular dermis |
| Level IV | Invasion into the reticular dermis |
| Level V | Invasion into the subcutaneous fat |
Ulceration
Histological ulceration (absence of intact epidermis overlying the melanoma without prior trauma or instrumentation) is an independent adverse prognostic factor and upstages T-category in every thickness range. It reflects higher proliferative activity and angiogenic drive within the tumor.
Additional Reported Pathological Features
Mitotic rate (mitoses/mm²) — no longer used in AJCC 8th edition T1 staging but remains prognostically informative
Microsatellitosis — defined as nests of tumor cells > 0.05 mm in diameter separated from the main mass by normal dermis; upstages to N category
Lymphovascular invasion — adverse prognostic indicator, report as present/absent
Tumor-infiltrating lymphocytes (TILs) — brisk TILs associated with improved survival
Regression — diffuse regression (> 75% of tumor) raises concern for underestimated thickness
Clinical Pearls
Breslow vs. Clark: What to Report
Breslow thickness is the dominant prognostic variable — it is the mandatory primary measurement for all invasive melanomas.
Clark level is reported alongside Breslow but carries minimal independent prognostic information in tumors > 1 mm thick — Breslow dominates.
In thin melanomas (< 1 mm), Clark level IV or V was previously used (AJCC 6th edition) to classify T1b. The 8th edition removed this — Clark level no longer differentiates T1a from T1b.
AJCC 8th edition T1 staging depends exclusively on thickness (0.8 mm threshold) and ulceration.
Biopsy Technique Matters
Punch or shave biopsies that transect the base of a melanoma yield an artificially low Breslow measurement. If the surgical margin at the base is positive for melanoma, Breslow thickness cannot be accurately reported — the pathologist should note "at least X mm" and re-excision is required for definitive staging.
Limitations
Breslow thickness does not capture horizontal growth phase (radial vs. vertical growth) — both contribute to metastatic potential.
Inter-pathologist variability at the 0.8 mm threshold can affect T1 substaging — clinical teams should be aware of measurement uncertainty at boundary values.
Regression can reduce measurable thickness — a thick regressive zone with thin residual tumor may still represent a biologically high-risk lesion.
Next Steps
Excision Margins by Breslow Thickness (NCCN/BAD Guidelines)
| In situ (Tis) | 0.5–1.0 cm clinical margin |
| ≤ 1.0 mm (T1) | 1 cm margin |
| 1.01–2.0 mm (T2) | 1–2 cm margin (1 cm acceptable; 2 cm preferred) |
| > 2.0 mm (T3–T4) | 2 cm margin |
Sentinel Lymph Node Biopsy (SLNB) Eligibility
01
T1a (< 0.8 mm, no ulceration): SLNB generally NOT recommended — SLNB positivity rate < 5%.
02
T1b (0.8–1.0 mm OR < 0.8 mm with ulceration): SLNB discussion and shared decision-making — SLNB positivity ~5–10%.
03
T2 (> 1.0–2.0 mm): SLNB recommended — SLNB positivity ~15–20%.
04
T3 (> 2.0–4.0 mm): SLNB strongly recommended — SLNB positivity ~25–35%.
05
T4 (> 4.0 mm): SLNB recommended; baseline imaging (CT or PET-CT) also indicated.
Adjuvant Systemic Therapy Eligibility
Stage IIB (T3b N0) and IIC (T4b N0) patients are now eligible for adjuvant pembrolizumab or nivolumab per NCCN v2.2024. BRAF mutation testing recommended at diagnosis for all T2b and above to plan potential targeted therapy if node-positive disease is confirmed at SLNB.
The Evidence
Breslow Thickness — Original Paper
Thickness, cross-sectional areas and depth of invasion in the prognosis of cutaneous melanoma.
Breslow A. • Ann Surg.. 1970;172(5):902-8. First demonstration that vertical tumor thickness in mm is the strongest predictor of melanoma prognosis, surpassing Clark anatomical level.
View SourceClark Levels — Original Paper
The histogenesis and biologic behavior of primary human malignant melanomas of the skin.
Clark WH Jr et al. • Cancer Res.. 1969;29(3):705-27. Established the five-level anatomical classification of melanoma invasion depth that bears Clark's name.
View SourceExternal Links
Origins & History
The Clark classification was introduced in 1969 by Dr. Wallace H. Clark Jr. and colleagues at Harvard Medical School, defining five anatomical levels of melanoma invasion from the epidermis into the subcutaneous fat. One year later in 1970, Dr. Alexander Breslow — a pathologist at George Washington University — published the landmark observation that vertical tumor thickness in millimeters was a more reproducible and prognostically powerful predictor than Clark's anatomical levels. The Breslow thickness has since become the cornerstone of melanoma staging worldwide, embedded in all editions of the AJCC Cancer Staging Manual. Alexander Breslow died in 1980 before seeing the full global adoption of his discovery.
Last Comprehensive Review: 2026-07-17