Melanocytic pathology (naevus vs melanoma)

Meet the patient

A 54-year-old man's GP sends a pathology form with a shave biopsy of a "changing mole" from his back. The report reads "compound naevus with moderate atypia" — and the deep margin is positive. The registrar asks whether the patient can be discharged.[1]

He cannot. A transected base on a clinically changing lesion is an inadequate sample, and "moderate atypia" read on a specimen cut through its middle is a sampling problem, not a diagnosis. This single slide is where a melanoma gets understaged and a patient gets lost — and the question it poses is the one this whole page exists to answer: how do the pathologist and the clinician together avoid understaging a melanoma and overcalling a naevus?[1]

Definition & Scope

Melanocytic pathology is the histologic and molecular interpretation of lesions composed of melanocytes — from banal acquired naevi through intermediate/uncertain lesions to melanoma in situ (MIS) and invasive melanoma.[3][4] This leaf is the pattern and reporting companion to clinical topics on naevi, melanoma, and staging: it answers how the pathologist and clinician together avoid understaging and overtreatment.

Why Biopsy Technique Is Pathology

AJCC T category depends on Breslow thickness and ulceration measured on a full-thickness sample. Partial or superficial biopsies can understage invasive melanoma and distort subtype.[1][2][3]

Preferred for suspected melanoma: full-thickness excisional biopsy with narrow clinical margins (about 1–3 mm), oriented if helpful, submitted entirely.[2][3]

Acceptable with caution: deep saucerisation or punch when site/size preclude ellipse — report must state if the base is transected.[2]

Avoid as sole sample when melanoma is likely: incomplete superficial shave through the epidermis only. [1]

[1]

Architecture-First Criteria (Naevus vs Melanoma)

Features that favour a benign naevus

• Overall symmetry and sharp lateral circumscription
• Nested growth that becomes smaller and more disperse with depth (maturation)
• Limited pagetoid scatter (if any), usually central and in appropriate clinical context (e.g. traumatised or special-site naevus)
• Low dermal mitotic rate, especially deep [1]

Features that raise concern for melanoma

• Asymmetry, poor circumscription, consumption of epidermis
• Sheet-like dermal growth without maturation
• Pagetoid melanocytes at edges or throughout
• Cytologic atypia with dermal mitoses (especially deep)
• Ulceration, regression with residual atypical cells, satellites [1]

No single feature is absolute; constellation + clinical data decide.[2][3][12]

Essential Melanoma Report Elements

Pathology reports that feed AJCC staging should capture at least:[1][2]

Dysplastic (Atypical) Naevi

Dysplastic naevi show architectural disorder and cytologic atypia short of melanoma; history, classification debates, and epidemiology are reviewed in contemporary JAAD series.[6] Exam stance:

• Mild dysplasia completely excised → often observation per local protocol
• Moderate–severe dysplasia or positive margins → re-excision commonly advised
• Severe dysplasia vs MIS can be subjective — clear clinical photos, dermoscopy, and expert review reduce harm [1]

Never use “dysplastic naevus” as a dustbin label for incompletely sampled melanoma. [1]

Spitz Spectrum

Spitz tumours range from classic Spitz naevus through atypical Spitz tumour to Spitz melanoma, with distinctive molecular drivers (including kinase fusions / HRAS in subsets) that differ from conventional BRAF-driven naevi.[7][8] Age matters: childhood Spitz naevi are common; adult Spitzoid lesions deserve higher suspicion and complete excision.[7][12]

Criteria re-evaluations continue to stress combined clinical, histologic, and molecular integration rather than any single mitotic cutoff.[12][8]

Special-Site and Morphologic Traps

Immunohistochemistry & Ancillary Tests

Common panel thinking:[2][10]

• Melan-A/MART-1, SOX10, S100 — highlight melanocytes / map pagetoid spread
• HMB-45 — gradient with maturation in many naevi; diffuse deep labelling concerning in context
• Ki-67 — proliferation hotspots (interpret with care)
• PRAME — often diffuse in melanoma and limited in many naevi; ancillary only, with documented pitfalls and non-melanocytic positivity risks in broader practice[9][10][11]

Molecular tests (FISH, SNP array, NGS panels) are for selected ambiguous cases in specialist centres — they do not replace complete excision of concerning lesions.[7][3]

MPATH-Dx: Standardising the Grey Zone

The Melanocytic Pathology Assessment Tool and Hierarchy for Diagnosis (MPATH-Dx) maps diverse free-text diagnoses into classes linked to suggested management intensity, improving communication between pathologists and clinicians.[4][5] Version revisions refine class definitions for real-world reporting consistency.[4][5]

Teaching map (conceptual — follow institutional implementation): [1]

Management Algorithm (Pathology-Linked)

[1]

Special Populations

• Children: Spitz-rich differential; avoid overcalling melanoma; still fully excise atypical Spitz tumours when advised.[7]
• Skin of colour / acral sites: sample full thickness; do not dismiss amelanotic or hyperkeratotic acral lesions.[3]
• Pregnancy: biopsy timing is clinical; histologic criteria are not relaxed.
• Immunosuppression: melanoma biology may be aggressive — first-pass histology must still be adequate for staging.[3]

Pitfalls That Change Outcome

1. Superficial shave of obvious melanoma → wrong Breslow.[2]
2. Accepting “compound naevus” on a transected base when clinically irregular.
3. Treating PRAME as standalone melanoma proof.[11]
4. Ignoring special-site norms → overdiagnosis of acral naevi as MIS.
5. No clinical history on the request → wrong threshold for atypia.
6. Skipping expert review for Spitz melanoma candidates.[8]

Regional & Systems Notes

AJCC 8th pathology elements are the global language for trials and boards.[1][2] MPATH-Dx offers a US-origin framework increasingly discussed for standardising ambiguous reports.[4][5] Resource-limited settings: prioritise complete excision biopsy, measured Breslow, ulceration, and margin status even when PRAME/FISH are unavailable.

Exam Pearls

• Maturation deep = friend; deep mitoses + sheets = foe.[2][12]
• MPATH-Dx reduces chaotic synonyms into management classes.[4][5]
• PRAME diffuse → supports melanoma in context; negative does not exclude it.[9][10]
• Link next steps to the skin-cancer-staging algorithm once invasive melanoma is confirmed.[1][3]

[1]

Etymology for viva gold: naevus is the Latin for "birthmark" — strictly a congenital mark, though most acquired naevi are anything but present at birth. Breslow thickness honours Alexander Breslow, whose 1970 work made depth the dominant prognostic factor in melanoma; the measurement that bears his name has outlived every rival staging dimension except ulceration.[1]

The mantra: sample deep enough to stage, read architecture before any stain, and send the ambiguous adult Spitzoid lesion to a second pair of eyes.[1]

Ward-round test — three stems

Stem 1 — the transected shave (answer)

The registrar wants to discharge a patient whose shave biopsy of a changing mole reads "compound naevus, moderate atypia, deep margin positive." What is the right call? Model: This is an inadequate sample, not a diagnosis. A transected base on a clinically changing lesion can neither stage nor exclude melanoma — plan a full-thickness excisional biopsy with 1–3 mm margins and restage. Never reassure on a shave that cut through the lesion.[1]

Stem 2 — the adult Spitzoid lesion (answer)

A 38-year-old has a pink, rapidly growing papule; biopsy shows a Spitzoid melanocytic proliferation. How does age change the threshold? Model: Childhood Spitz naevi are common and often managed conservatively, but an adult Spitzoid lesion carries real risk of Spitzoid melanoma. Complete excision, a low threshold for expert dermatopathology review, and molecular workup where the picture is ambiguous — age is the single variable that moves the needle most.[7]

Stem 3 — does diffuse PRAME mean melanoma? (answer)

A lesion stains diffusely for PRAME. Does that make it melanoma? Model: No. PRAME is ancillary only — diffuse staining supports melanoma in context but never diagnoses it alone, and a PRAME-negative result does not exclude melanoma either. Read it alongside architecture, cytology, and the clinical picture, never as a standalone verdict.[9]

References

1. [1]Gershenwald JE, Scolyer RA. Melanoma Staging: American Joint Committee on Cancer (AJCC) 8th Edition and Beyond Ann Surg Oncol, 2018.PMID 29850954
2. [2]Scolyer RA, Rawson RV, Gershenwald JE, et al. Melanoma pathology reporting and staging Mod Pathol, 2020.PMID 31758078
3. [3]Tasdogan A, Sullivan RJ, Katalinic A, et al. Cutaneous melanoma Nat Rev Dis Primers, 2025.PMID 40180935
4. [4]Barnhill RL, Elder DE, Piepkorn MW, et al. Revision of the Melanocytic Pathology Assessment Tool and Hierarchy for Diagnosis Classification Schema for Melanocytic Lesions: A Consensus Statement JAMA Netw Open, 2023.PMID 36630138
5. [5]Barnhill RL, Piepkorn MW, Duncan LM, et al. MPATH-Dx version 2.0 schema for melanocytic lesions: A robust tool for standardized diagnostic reporting Clin Dermatol, 2025.PMID 39260462
6. [6]Drozdowski R, Spaccarelli N, Peters MS, et al. Dysplastic nevus part I: Historical perspective, classification, and epidemiology J Am Acad Dermatol, 2023.PMID 36038073
7. [7]Yeh I, Busam KJ. Spitz melanocytic tumours - a review Histopathology, 2022.PMID 34958498
8. [8]LeBoit PE. Spitz melanoma Clin Dermatol, 2025.PMID 39265841
9. [9]Lezcano C, Jungbluth AA, Nehal KS, et al. PRAME Expression in Melanocytic Tumors Am J Surg Pathol, 2018.PMID 30045064
10. [10]Lezcano C, Jungbluth AA, Busam KJ. PRAME Immunohistochemistry as an Ancillary Test for the Assessment of Melanocytic Lesions Surg Pathol Clin, 2021.PMID 34023098
11. [11]Lezcano C, Jungbluth AA, Busam KJ. Immunohistochemistry for PRAME in Dermatopathology Am J Dermatopathol, 2023.PMID 37856737
12. [12]Ritter A, Tronnier M, Vaske B, et al. Reevaluation of established and new criteria in differential diagnosis of Spitz nevus and melanoma Arch Dermatol Res, 2018.PMID 29417221