Breslow thickness
Tumour thickness; depth of invasion; Breslow depth; vertical thickness
Breslow thickness — the vertical depth of melanoma invasion measured from the top of the granular cell layer (or the base of an ulcer) to the deepest identifiable tumour cell — is the single most important prognostic parameter in primary melanoma and the foundation of AJCC 8 T-classification. Measured in millimetres on a properly oriented vertical section, it determines pT category (Tis / T1a / T1b / T2a / T2b / T3a / T3b / T4a / T4b), wide-excision margin (NICE NG14), eligibility for SLNB, baseline imaging and adjuvant therapy. Accurate measurement requires complete excisional biopsy of a suspicious pigmented lesion — partial shave biopsies risk under-staging.
How Breslow is measured
- Vertical distance from the top of the granular cell layer (or the base of an ulcer where the granular layer is absent) to the deepest identifiable tumour cell.
- Measured to 0.1 mm with a calibrated micrometer eyepiece on a vertically-oriented haematoxylin-and-eosin section.
- Reported on the pathology record alongside subtype, ulceration, mitotic rate, lymphovascular invasion, perineural invasion and clearance.
- If the lesion ulcerates, the ulcer base — not the original granular layer — is the reference; this protects against underestimation.
- Subcutaneous nests of tumour separated by fibrosis or scar are included in the measurement; satellite metastases (separated by > 0.3 mm of normal dermis) are recorded separately as microsatellite (N1c equivalent).
AJCC 8 T-category cut-offs
- Tis — melanoma in situ (no invasion).
- T1a — < 0.8 mm without ulceration.
- T1b — < 0.8 mm WITH ulceration, OR 0.8–1.0 mm with or without 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.
- Note the AJCC 8 inclusive cut-offs (e.g. 1.0 mm exactly is T1b, not T2a).
Clinical implications
- WLE margin (NICE NG14): stage-based rather than Breslow-band-only — stage 0 at least 0.5 cm; stage I (including T1 and T2a N0) 1 cm; stage II (including T2b/T3/T4 N0) 2 cm, with 1 cm only if 2 cm would cause unacceptable disfigurement or morbidity.
- SLNB — consider after discussion for Breslow 0.8–1.0 mm with ulceration, mitotic index ≥ 2 or LVI, and for Breslow > 1.0 mm; see SLNB.
- Baseline imaging — consider whole-body and brain CE-CT from stage IIB and offer from stage IIC+ per NG14 §1.4.6–1.4.10.
- Adjuvant therapy — pembrolizumab TA837 for resected stage IIB/IIC; pembrolizumab TA766 or nivolumab TA684 for resected stage III; dabrafenib + trametinib TA544 for resected stage III BRAF V600-mutant disease.
Pitfalls and limitations
- Partial shave biopsy — transection at the deep margin gives "at least X mm" — true thickness unknown; always full excisional biopsy for suspected melanoma.
- Ulcerated tumour — measure from ulcer base, not granular layer.
- Polypoid or pedunculated melanoma — Breslow measured along the stalk gives a different prognosis to comparable Breslow non-polypoid lesions; document polypoid morphology.
- Regression — Breslow may underestimate true historical depth where extensive regression is present; record regression on the report.
- Lymph-node involvement — Breslow remains the primary prognostic determinant for the primary, but nodal disease (N1+) shifts the patient to stage III regardless of Breslow.
- Inter-observer variability — modest at extremes, larger at the 0.8 mm / 1.0 mm / 2.0 mm boundaries; specialist dermatopathology review when staging decision pivots on a boundary.
References
- Breslow A. Thickness, cross-sectional areas and depth of invasion in the prognosis of cutaneous melanoma. Ann Surg; 1970.
- Gershenwald JE et al. Melanoma staging — evidence-based changes in the AJCC 8th edition. CA Cancer J Clin; 2017;67:472–92.
- NICE NG14. Melanoma: assessment and management. London: NICE; 2015 (updated 27 July 2022), recommendations 1.3-1.5.
- Royal College of Pathologists. Dataset for histopathological reporting of primary cutaneous malignant melanoma and regional lymph nodes (G125). London: RCPath; February 2019; current RCPath hub marks the dataset on hold and lists later TNM 9 appendices.
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