Melanocytic naevi

Meet the patient

An anxious mother brings her seven-year-old to clinic over a pink, smooth, hairless papule that appeared on the girl's cheek six weeks ago and is "still growing". On the same morning, a 44-year-old man points to a dark mole on his back that "doesn't match any of the others" — his ugly duckling. Two worried patients, one question behind both: is this a benign naevus I can watch, or a melanoma I must biopsy today?[1]

Hold that question. Everything below — the maturation sequence, the dermoscopy-by-age rule, the ABCDE and ugly-duckling signs, the Spitzoid spectrum, the congenital-size table — exists to answer it at the bedside without either over-treating a cosmetic mole or, far worse, sending a melanoma home with reassurance.[4]

One cell, three addresses — junctional, compound, intradermal

Every common acquired naevus is the same cell at three addresses, and the address moves inward with age. A naevus begins as a clone of melanocytes parked at the dermo-epidermal junction — the junctional naevus, flat and dark, the regular pigment network on dermoscopy. Naevus cells then drop into the dermis, producing a compound naevus (raised, brown, combined reticular and globular). Eventually the junctional component retires and the cells lie wholly in the dermis — the intradermal naevus, the dome-shaped, flesh-coloured, soft papule that is the commonest naevus of adults.[4]

This is why a teenager's moles are flat and dark and a grandparent's are raised and fleshy — it is the same lesion ageing, not different diseases. The face, neck and upper trunk carry most of them.[1]

[4]

One-line discriminator: flat-and-dark means junctional, raised-and-brown means compound, dome-and-flesh-coloured means intradermal. Remember that the cell migrates inward and you have the whole sequence — and you will never be fooled by a benign fleshy facial papule the patient is sure is "new and growing".[1]

J-C-I — the dermoscopy-by-age rule

Benign naevus dermoscopy tracks the patient's age as reliably as a growth chart: globular in children, reticular in young adults, homogeneous in older adults. The three patterns mirror the three histological addresses.[4]

[1]

The melanoma-specific criteria that break the benign pattern — and that you are looking for, not against — are the dermoscopy red flags every registrar must name on sight:[4]

• Asymmetry of pattern and colour.
• Atypical, broadened, irregular pigment network.
• Blue-white veil — orthokeratosis over a pigmented vertical-growth-phase nodule; the ominous sign.
• Regression structures — scar-like white and blue-grey peppering.
• Atypical streaks and pseudopods at the periphery.
• Polymorphous vessels — mixed dot, linear and irregular.
• Multiple colours — brown, black, blue, grey, red and white within one lesion. [4]

The ABCDE and the ugly duckling — when to reach for the blade

The ABCDE rule is a screening sieve, and the "E" is the only letter that on its own justifies a biopsy. A stable ugly naevus can be photographed and watched; a naevus that is evolving — growing, changing colour, bleeding, itching, crusting — is the one that crosses the line.[1]

[1]

The ugly-duckling sign outperforms ABCDE in patients with many naevi, because each person's naevi resemble each other — the outlier is the suspect. A mole that looks unlike its siblings on the same back is your biopsy candidate even when it passes ABCDE on paper. In naevus-rich patients, most melanomas are caught by the ugly duckling, not by diameter.[2]

How common, who, and the gene you must not miss

The average adult carries 15 to 40 acquired naevi; the count peaks in the third and fourth decades and then falls as the immune system clears clones. Fair-skinned (Fitzpatrick I to II) individuals accumulate more, and the count is itself a melanoma risk marker independent of sun exposure — more than 50 common naevi, or more than 5 atypical, roughly doubles to triples melanoma risk.[2]

[2]

When melanoma clusters in a family, the gene to name is CDKN2A, encoding the p16/INK4a and p14/ARF tumour suppressors on chromosome 9p21 — the basis of familial atypical multiple mole melanoma (FAMMM) syndrome. NRAS mosaicism, not CDKN2A, drives the giant congenital naevi; the molecular split matters because it changes the surveillance, not just the mole count.[2]

Oncogene-induced senescence — why a mutant clone stops growing

A benign naevus is a mutant clone that hit the brakes. Roughly 80 percent of acquired naevi carry BRAF V600E; most congenital naevi, and almost all giant ones, carry NRAS Q61. Either mutation fires the MAPK/ERK pathway and drives the melanocyte to proliferate — and then the clone stops, frozen in oncogene-induced senescence.[4]

The brake is p16/INK4a and p53-mediated senescence: proliferation triggers a cell-cycle arrest that caps the clone at a clinically visible naevus. This is why a benign naevus is stably growth-arrested for decades and why the vast majority never transform — progression to melanoma needs additional hits (CDKN2A loss, TERT promoter reactivation, further MAPK effectors) that dismantle the senescence barrier.[1]

The teaching point that earns marks: a naevus is not a melanoma-in-waiting, it is a senescent clone. Most melanomas arise de novo, not from a pre-existing naevus — the conspicuous exception being melanoma arising within a congenital naevus, especially a giant one.[3]

[4]

The special types — the naevi with a name

A handful of naevi earn a proper name because each carries a trap or a gene the examiner will test. Memorise the clinical picture, the one-line mechanism, and the single management verb for each.[1]

• Spitz naevus — a pink-red, firm, hairless, dome-shaped papule (usually under 10 mm) on the face or legs of a child, growing over weeks. Histology shows large spindled and epithelioid melanocytes, Kamino bodies, and clefts around nests. Because it can mimic Spitzoid melanoma, excisional biopsy is the default — the full spectrum is unpacked below.[1][6]
• Blue naevus — a blue-black papule from melanocytes arrested deep in the dermis; the depth (Tyndall effect) makes brown melanin look blue. Common on the dorsum of hands and feet, scalp, buttocks. Stable; excise only if it changes, because a changing blue lesion sits on the differential of nodular melanoma.[4]
• Halo naevus (Sutton naevus) — a central pigmented naevus ringed by a symmetrical depigmented halo. A CD8-positive T-cell response destroys the naevus cells and the surrounding normal melanocytes, producing the halo. Multiple, common in adolescents, associated with vitiligo. Self-resolving — the central naevus fades over years. No treatment; check the rest of the skin for vitiligo.[1]
• Meyerson naevus — an itchy halo of acute eczema around a benign naevus; a type-IV hypersensitivity reaction to naevus-cell antigen. It is not the halo of Sutton (which is depigmented, not eczematous). Settles with a topical steroid; the naevus stays.[1]
• Dysplastic (atypical, Clark) naevus — over 5 mm, asymmetric, irregular border, variable colour (pink, brown, red mixed), on a background of many naevi. Most are biologically stable and never progress; the clinical challenge is separating the stable atypical naevus from early melanoma, which is exactly why the "E" and the ugly duckling do the heavy lifting.[2]
• Giant congenital melanocytic naevus — a dark brown-black plaque over 20 cm projected adult diameter, with hypertrichosis and satellites; carries melanoma and neurocutaneous-melanosis risk. The full risk table and MRI logic follow.[5]

Congenital melanocytic naevi — size is destiny

For a congenital naevus, the projected adult size is the single number that sets both the melanoma risk and the neurocutaneous-melanosis risk. Small and medium CMN behave like acquired naevi; the giant ones are a different disease.[5]

CMN are present at birth or appear within the first year (tardive CMN). Risk is stratified by the projected adult diameter (PAD) — multiply the infant measurement by about 1.7 for the head and 3 for the rest of the body to forecast final size.[5]

[5]

Neurocutaneous melanosis — the satellite count that triggers the MRI

Neurocutaneous melanosis (NCM) is melanocyte proliferation in the leptomeninges, and it kills — symptomatic NCM carries roughly 50% mortality in early childhood. It is the reason a giant CMN is not a cosmetic problem but a multidisciplinary one.[3]

The risk factors that trigger screening are concrete: a giant CMN in an axial, head or neck distribution, and more than 20 satellite lesions. Leptomeningeal melanocytes track the dorsal midline neural crest, which is why the bathing-trunk and cape distributions are the dangerous ones.[5]

NCM may be asymptomatic — found only on MRI as T1-bright leptomeningeal deposits, classically in the temporal lobes, cerebellum and amygdala — or symptomatic, presenting with seizures, cranial-nerve palsies, hydrocephalus, developmental delay, or cord compression. The symptomatic form is the lethal one. Screening MRI of the brain and spine with contrast in the first year of life is recommended for high-risk giant CMN, even when the child is neurologically normal.[3][5]

[1]

Two giant-CMN phenomena demand expert dermatopathology, never a shave. A proliferative nodule within a giant CMN in infancy grows fast and can mimic melanoma histologically — most are benign, but complete excision and expert review are mandatory. A halo congenital naevus developing a depigmented rim usually heralds benign immune regression, not melanoma.[3]

Reducing the giant CMN — what works and what only lightens

• Complete or serial excision (with tissue expansion) reduces melanoma risk and improves cosmesis, but is often infeasible for very extensive lesions; the risk-benefit is individualised.[5]
• Curettage or dermabrasion in the first 2 to 4 weeks of life strips the superficial pigmented component while naevus cells are still superficial — it lightens, but does not remove the deeper cells, so it does not eliminate melanoma risk.[5]
• Laser (Q-switched ruby or Nd:YAG, ablative CO2 or Er:YAG) lightens pigmentation but does not reduce melanoma risk and may obscure future dermoscopy — cosmetic only.[5]
• NRAS-targeted therapy (MEK inhibitors) is in trials for NRAS-mutant CMN and melanoma; not yet standard. [2]

The Spitzoid spectrum — Spitz naevus, atypical Spitz tumour, Spitzoid melanoma

This is the hardest corner of dermatopathology, and the one where a clinician most easily hurts a patient by under-calling it. A pink hairless papule in a child is usually a benign Spitz naevus; the same lesion in an adult is Spitzoid melanoma until proven otherwise.[6]

Etymology for the viva: Spitz is Sophie Spitz, the pathologist who described these spindled-and-epithelioid tumours in children in 1948. Naevus is the Latin for "birthmark" — a word that long outlived its meaning as the lesions proliferated far beyond the congenital.[1]

The modern WHO classification (2018) collapses the old Spitz-naevus / atypical-Spitz-tumour / Spitzoid-melanoma ladder into a continuum of Spitz melanocytic tumours, annotated by risk category — because the histological boundary between the ends is genuinely blurred and molecular profiling now does work the microscope alone cannot.[6]

The benign end — classic Spitz naevus

About 70 percent occur in patients under 20; the peak is age 3 to 10. A solitary, rapidly growing pink-red, dome-shaped, firm, hairless papule, usually under 10 mm, on the face or lower legs. The classic dermoscopy is a symmetric starburst — peripheral radiating streaks and pseudopods around a central pink-tan homogeneous area with dotted vessels. Histology shows spindled and epithelioid melanocytes in vertical nests with retraction clefts, eosinophilic cytoplasm, occasional Kamino bodies (PAS-positive eosinophilic globules at the junction), preserved maturation with depth, and rare superficial mitoses. The molecular signature is HRAS Q61 or kinase fusions of ALK, ROS1, NTRK1, NTRK3, BRAF, RET — none of which are the drivers of conventional melanoma.[1]

The middle — atypical Spitz tumour (AST) / Spitz melanocytoma

Lesions with one or more worrisome features — size over 10 mm, ulceration, deep mitoses, asymmetry, poor maturation, dense inflammation, or age over 10 — that fall between the benign and malignant ends. Molecular work-up is mandatory: FISH (RREB1, MYB, CCND1, CEP6) and next-generation sequencing for kinase fusions stratify risk, and BAP1 loss raises BAP1 tumour-predisposition syndrome — screen for uveal melanoma and renal cell carcinoma in the patient and family. Complete excisional biopsy with 1 to 3 mm margins; sentinel-lymph-node biopsy is considered in adults or atypical lesions, though its prognostic value is debated.[6]

The malignant end — Spitzoid melanoma

Most often an adult presentation, or a childhood lesion with frankly malignant architecture. The reassuring symmetric starburst is lost: dermoscopy shows an atypical, asymmetric starburst, homogeneous blue pigmentation, ulceration, or an atypical vascular pattern. Histology shows pagetoid spread, deep dermal mitoses, marked atypia, loss of maturation, necrosis, and expansile nodular growth. The molecular profile abandons the kinase fusions of benign Spitz and acquires conventional melanoma drivers — BRAF V600E, NRAS, TERT promoter mutations, CDKN2A loss, PTEN loss. Manage as melanoma: wide local excision with appropriate margins, sentinel-lymph-node biopsy, staging, and adjuvant therapy per melanoma protocols.[6]

The bedside rule — when a Spitzoid lesion gets excised

The classic trap: a pink hairless papule in an adult is Spitzoid melanoma until proven otherwise — not "just a Spitz naevus". The age of the patient flips the pre-test probability more than any dermoscopic feature.[6]

[1] [6] [1] [6]

The high-yield variants that turn up in the viva

A handful of named variants are pure viva currency — recognise the clinical picture and the gene.[4]

• Pigmented spindle-cell naevus of Reed — a dark brown-black, flat, well-defined papule on the thigh of a young woman; dermoscopy shows regular pigmented striae radiating from the centre (a regular starburst). Benign; often grouped with Spitz.[1]
• Deep penetrating naevus — a dark blue-black dermal naevus with wedge-shaped deep extension; benign but histologically mimics melanoma, so expert pathology review is mandatory.[4]
• Naevus spilus (speckled lentiginous naevus) — a tan background macule speckled with darker macules and papules; melanoma can arise within it, especially in giant variants — monitor.[4]
• BAP1-inactivated naevus — pink-tan epithelioid Spitzoid naevi, often multiple, in BAP1 tumour-predisposition syndrome (germline BAP1); the patient needs screening for uveal melanoma, mesothelioma, and renal cell carcinoma.[2]
• Combined naevus — two patterns in one lesion (common: blue plus compound); benign.[4]
• Recurrent (persistent) naevus / pseudomelanoma — pigmentation reappearing at an excision scar; not a true recurrence. Review the original histology before re-excising.[1]
• Desmoplastic naevus — a firm sclerotic papule in thick collagenous stroma; mimics a dermatofibroma. [4]

FAMMM, CDKN2A, and the cancer family

FAMMM syndrome is the kindred where melanoma is inherited, and CDKN2A is the gene you name. It encodes p16/INK4a (a cell-cycle brake) and p14/ARF (protecting p53) on chromosome 9p21; loss of either deregulates the very senescence mechanism that holds a benign naevus in check. CDKN2A accounts for roughly 20 to 40 percent of familial melanoma kindreds.[2]

The Amsterdam-style diagnostic criteria for a FAMMM kindred: one individual with melanoma and one first- or second-degree relative with melanoma and over 50 naevi with over 5 clinically atypical naevi in the proband or family.[2]

The cancer spectrum beyond the skin is what makes this a multisystem surveillance problem. CDKN2A carriers carry a 30 to 90 percent lifetime melanoma risk (younger onset, multiple primaries) and a 10 to 20-fold pancreatic-adenocarcinoma risk — annual MRI or endoscopic ultrasound from age 40 to 50 is now recommended. BAP1 tumour-predisposition syndrome adds uveal melanoma, mesothelioma, and renal cell carcinoma. MITF p.E318K confers moderate melanoma and RCC risk; CDK4 mutations are rare but phenocopy CDKN2A; MC1R red-hair variants nudge melanoma risk up.[2]

[2]

Management — stable naevus, watch; changing naevus, biopsy

The whole of naevus management collapses to one decision: is this lesion stable or changing? A stable benign naevus is observed; a changing or atypical naevus is biopsied — and the biopsy is excisional, never a shave, because Breslow depth cannot be recovered from a fragmented specimen.[1]

[5]

The differential — what a mole is not

Six mimics account for almost every "is this a mole or something worse?" consultation. The discriminator is usually dermoscopy plus one bedside sign.[1]

[1] [1]

Investigations — clinical plus dermoscopy, then biopsy the changers

Clinical examination and dermoscopy settle the vast majority of naevi without a biopsy. Reserve histology for the lesions that fail the sieve: ABCDE-positive, ugly-duckling, changing, atypical on dermoscopy, symptomatic, or Spitzoid.[4]

• Excisional biopsy with 1 to 3 mm margins is the procedure of choice for any suspicious lesion — full thickness, so Breslow depth can be measured. Never shave-biopsy a lesion that might be melanoma; a fragmented specimen ruins staging.[1]
• Total-body photography plus sequential digital dermoscopy is the surveillance standard for high-naevus, atypical-naevus, or FAMMM patients — it catches the changer among the many.[2]
• MRI brain and spine with contrast screens for neurocutaneous melanosis in high-risk giant CMN.[5]
• Expert dermatopathology and molecular testing (FISH, CGH, fusion panels, BAP1 IHC) for Spitzoid and borderline lesions. [6]

How patients come to harm — the preventable list

• Sending a melanoma home as "just a mole" because it was stable on a single visit — the commonest medicolegal pitfall. A changing naevus, an ugly duckling, or dermoscopy chaos demands biopsy.[1]
• Shave-biopsying a pigmented lesion that turns out to be melanoma, fragmenting the specimen and destroying the Breslow depth — staging is then irrecoverable. Excisional biopsy, full thickness, every time.[1]
• Under-calling an adult Spitzoid lesion as a benign Spitz naevus — the recurring trainee error. A pink hairless papule in an adult is melanoma until proven otherwise.[6]
• Missing neurocutaneous melanosis in a giant CMN with new neurological symptoms — symptomatic NCM carries roughly 50% mortality; the MRI should already have been done in infancy.[3]
• Failing to screen a CDKN2A kindred for pancreatic cancer, or a BAP1 carrier for uveal melanoma — the naevus is the door to a cancer family.[2]
• Cosmetic procedures on an unexamined pigmented lesion — laser hair removal or tattooing over an unrecognised melanoma masks the diagnosis. Examine before any cosmetic intervention.[1]
• Unnecessary excision of a stable benign facial naevus, trading a mole for a scar. Observe and photograph; the morbidity of routine excision exceeds any prevention benefit. [4]

Pitfalls and special populations

• Pregnancy — naevi commonly darken and enlarge under hormonal influence; this is usually physiological. Biopsy only for ABCDE or ugly-duckling features, not for pregnancy alone.[4]
• Immunosuppression — higher naevus counts and higher melanoma risk; surveillance is essential and thresholds for biopsy are lower.[2]
• Children — naevus counts rise through childhood; the tasks are CMN, Spitz naevi, and genetic counselling if there is a familial melanoma kindred.[5]
• Cosmetic mole anxiety — the patient who wants "them all off". Counsel that high naevus count is a risk marker, not a precursor, and that most individual naevi are benign; photograph and watch rather than excise a stable backful. [1]

Prognosis — benign, with the giant-CMN and FAMMM exceptions

The prognosis of a benign naevus is excellent — the vast majority never transform. A high naevus count is a melanoma risk marker, not a precursor; most melanomas arise de novo. The two exceptions that change the conversation are the giant CMN (5 to 10% lifetime melanoma risk, concentrated in the first two decades, plus neurocutaneous melanosis) and the FAMMM/CDKN2A kindred (30 to 90% lifetime melanoma risk plus pancreatic cancer).[3][5]

Guidelines and the global picture

The framework — ABCDE, dermoscopy, total-body photography, excisional biopsy of changers — is globally consistent. NCCN and European guidelines converge on baseline full-skin examination from the early twenties for FAMMM/CDKN2A carriers, sequential digital dermoscopy for high-naevus patients, and excisional biopsy preferred over shave. Sun-protection campaigns (Australia's "Slip Slop Slap") and the IARC group-1 classification of tanning beds as melanoma carcinogens shape the public-health layer; artificial UV is banned for minors in Australia. In cost-constrained settings the principles are unchanged — observe the stable, biopsy the changer, and never shave a lesion that might be melanoma.[2]

The mantra, and the viva honesty line

The mantra: Stable naevus, watch; changing naevus, biopsy; Spitzoid in an adult, melanoma until proven.[1]

Ward-round test — four stems, thirty seconds each

Stem 1 — the pink papule on the cheek (answer)

A seven-year-old has a six-week-old pink, smooth, hairless 6 mm papule on the cheek. Dermoscopy shows a symmetric starburst. What do you do? Model: This is the classic Spitz naevus — child, pink, hairless, symmetric starburst. The triad (child plus starburst plus pink papule) permits observation with sequential dermoscopy at 3 to 6 months rather than immediate excision, provided the lesion is classic and the parents are reliable. If any feature is atypical — pigmented, over 10 mm, ulcerated, asymmetric, or changing — convert to full excisional biopsy.[1]

Stem 2 — the same papule in a 44-year-old (answer)

A 44-year-old man has the identical pink hairless papule, six weeks old, on his forearm. What changes? Model: Everything. A pink hairless Spitzoid papule in an adult is Spitzoid melanoma until proven otherwise — the classic trap. Excise fully (excisional biopsy with 1 to 3 mm margins), send for expert dermatopathology and molecular profiling (look for BRAF/NRAS/TERT rather than the HRAS or kinase fusions of benign Spitz), and proceed to wide local excision and sentinel-lymph-node biopsy if confirmed. Age flips the pre-test probability more than any dermoscopic feature.[6]

Stem 3 — the giant bathing-trunk naevus and the new seizure (answer)

A two-year-old with a giant bathing-trunk congenital naevus and over 20 satellites presents with a first seizure. What is the diagnosis, and what was missed? Model: This is symptomatic neurocutaneous melanosis (NCM) — leptomeningeal melanocyte proliferation producing seizures, cranial-nerve palsies, hydrocephalus, or developmental delay. Symptomatic NCM carries roughly 50% mortality. What was missed is the screening MRI of brain and spine with contrast in the first year of life, recommended for all high-risk giant CMN (axial/head-and-neck distribution, over 20 satellites) even when the child is neurologically normal. Image urgently now and involve neurosurgery and paediatric oncology.[3][5]

Stem 4 — the shave biopsy that ruined the staging (answer)

A registrar shave-biopsies a 9 mm pigmented lesion on a calf to "save a scar". Histology returns melanoma, but the base is fragmented and Breslow depth cannot be measured. What went wrong? Model: A shave biopsy of a suspected melanoma fragments the specimen and destroys the Breslow depth — the single most important staging variable, now irrecoverable. Any lesion that might be melanoma gets a full-thickness excisional biopsy with 1 to 3 mm margins. If a shave was already done, re-excise the site fully and proceed to staging and sentinel-lymph-node biopsy on the assumption of at least the deepest measurable component, acknowledging the uncertainty the shave created. [1]

References

1. [1]Brown A, Sawyer JD, Neumeister MW. Spitz Nevus: Review and Update Clin Plast Surg, 2021.PMID 34503728
2. [2]Toussi A, Mans N, Welborn J, et al. Germline mutations predisposing to melanoma J Cutan Pathol, 2020.PMID 32249949
3. [3]Alos L, Carrasco A, Teixidó C, et al. Melanoma on congenital melanocytic nevi Pathol Res Pract, 2024.PMID 38518732
4. [4]Yeh I. Melanocytic naevi, melanocytomas and emerging concepts Pathology, 2023.PMID 36642570
5. [5]Mologousis MA, Tsai SY, Tissera KA, et al. Updates in the Management of Congenital Melanocytic Nevi Children (Basel), 2024.PMID 38255375
6. [6]Yeh I, Busam KJ. Spitz melanocytic tumours - a review Histopathology, 2022.PMID 34958498