54 DERMATOLOGY

# 55 DERMATOLOGY

Detailed notes

Surgical dermatology is dominated by three malignancies — BCC, SCC and melanoma — plus a handful of benign mimics and pre-malignant lesions. The MRCS Part A examiner is interested in pattern recognition (which lesion is this?), prognostic factors (especially Breslow thickness) and management principles (margins, biopsy type, reconstruction).

Skin anatomy — the layers that matter

The skin has three layers: epidermis, dermis and subcutis.

The epidermis (avascular, stratified squamous epithelium) is built from deep to superficial:

➡ Basale — single layer of cuboidal stem cells sitting on the basement membrane. Melanocytes live here (neural crest origin) and donate melanin to keratinocytes. Origin of BCC.

➡ Spinosum — keratinocytes connected by desmosomes ("prickle cells"). Langerhans cells (antigen-presenting dendritic cells, bone-marrow derived) reside here. Origin of SCC.

➡ Granulosum — keratohyalin granules; cells begin to die.

➡ Lucidum — thin translucent layer present only in palms and soles (thick skin).

➡ Corneum — flattened anucleate keratinised cells.

The dermis contains collagen, elastin, vessels, nerves, sweat glands, sebaceous glands and hair follicles. The subcutis is fat and provides insulation and cushioning.

👩‍⚕️ Mnemonic for epidermal layers (deep → superficial): Behind School Girls Look Cute — Basale, Spinosum, Granulosum, Lucidum, Corneum.

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Basal cell carcinoma (BCC)

The commonest skin cancer. Slow-growing, locally invasive, virtually never metastasises. Arises from basal keratinocytes on chronically sun-exposed skin — face (especially nose, inner canthus, ear), bald scalp.

Classic appearance: pearly papule with rolled edge, surface telangiectasia and central ulceration ("rodent ulcer"). Other subtypes: superficial, morphoeic (scar-like, ill-defined), pigmented.

Risk factors: UV exposure, fair skin (Fitzpatrick I/II), age, immunosuppression, Gorlin syndrome (PTCH1 mutation — multiple BCCs at a young age).

Management: surgical excision with a 4 mm margin for low-risk lesions; Mohs micrographic surgery for high-risk sites (H-zone of face) or recurrence. Radiotherapy is an option in non-surgical candidates.

Prognosis depends on completeness of excision — incomplete excision is the main driver of local recurrence. Nodal and distant spread are vanishingly rare.

Squamous cell carcinoma (SCC)

Second commonest skin cancer. Arises from keratinocytes in the stratum spinosum. Unlike BCC, SCC can metastasise to regional lymph nodes (especially lesions on the lip, ear and immunosuppressed patients).

Risk factors:

➡ UV exposure (cumulative)

➡ Immunosuppression (transplant recipients have ~65× risk)

➡ HPV (genital and periungual SCC)

➡ Chronic wounds/scars/burns → Marjolin's ulcer

➡ Pre-malignant lesions: actinic keratosis, Bowen's disease

Appearance: keratinising nodule or ulcer with an everted (rolled outward) edge, often crusted with a keratin horn. Grows over weeks to months.

Management: excision with a 4–6 mm margin (depends on size and risk); sentinel lymph node biopsy is not routine but considered in high-risk lesions. Palpable nodes warrant FNA and block dissection if positive.

👩‍⚕️ Edge clue: rolled inward and pearly = BCC; everted/rolled outward and keratinised = SCC; pigmented and irregular = melanoma.

Pre-malignant and benign lookalikes

Lesion	Key features	Significance
Actinic (solar) keratosis	Rough, scaly erythematous patch on sun-exposed skin	Pre-malignant — small % progress to SCC
Bowen's disease	Well-demarcated erythematous scaly plaque, often lower leg	SCC in situ — full-thickness dysplasia, no basement membrane invasion
Keratoacanthoma	Dome-shaped lesion with central keratin plug; grows in weeks, regresses spontaneously	Benign but excised — indistinguishable from SCC clinically
Seborrhoeic keratosis	"Stuck-on", greasy, warty with keratin plugs; trunk of elderly	Benign; treat for cosmesis only
Dermatofibroma	Firm pigmented nodule, often after insect bite/trauma; positive dimple sign on lateral pressure	Benign
Pyogenic granuloma	Rapidly growing, friable red nodule after minor trauma; bleeds easily	Benign lobular capillary haemangioma

Melanoma

Malignancy of melanocytes at the dermo-epidermal junction. Most aggressive of the common skin cancers. Australia has the world's highest incidence — always think melanoma in a returning traveller with new pigmented lesion or inguinal node.

ABCDE warning signs:

- Asymmetry

- Border irregular

- Colour variation

- Diameter >6 mm

- Evolution / Elevation

Subtypes:

Subtype	Frequency / Site	Notes
Superficial spreading	~70% — trunk (men), legs (women)	Commonest; long radial growth phase
Nodular	~15% — anywhere	Worst prognosis — vertical growth from the start
Lentigo maligna	Face, elderly, chronic sun-damaged skin	Long in-situ phase; best prognosis if caught early
Acral lentiginous	Palms, soles, subungual	Commonest melanoma in dark-skinned and East Asian patients; not sun-related; Hutchinson's sign = pigment spreading onto nail fold

Prognostic factors — the most important is Breslow thickness. Breslow is measured from the granular layer of the epidermis to the deepest tumour cell (in mm). It outperforms Clark's level (which measures depth by anatomical layer) and is what guides excision margins and sentinel node decisions.

Other adverse features: ulceration, high mitotic rate, lymphovascular invasion, satellite/in-transit metastases, nodal involvement.

Diagnostic biopsy: excisional biopsy with a 2 mm margin for any suspicious pigmented lesion. Never shave or incisional biopsy a suspected melanoma — you'll lose the Breslow measurement. Wide local excision is then planned based on histology.

Wide local excision margins (UK guidance):

Breslow thickness	WLE margin
In situ	0.5 cm
≤ 1 mm	1 cm
1–2 mm	1–2 cm
2–4 mm	2–3 cm
> 4 mm	3 cm

Sentinel lymph node biopsy is offered for Breslow >0.8 mm (or thinner if ulcerated). It is staging, not therapeutic.

👩‍⚕️ Easy mark: when an exam asks for the single best prognostic factor in melanoma — answer Breslow thickness every time.

Reconstruction after skin cancer excision

After excising a facial skin cancer, reconstructive choice depends on defect size, site and cosmesis:

- Primary closure — small defects, lax skin

- Local flap (e.g. bilobed, rhomboid, nasolabial) — preferred on the face for moderate defects; matches colour and texture

- Full-thickness skin graft (FTSG) — taken from postauricular or supraclavicular skin; good colour match for face

- Split-thickness skin graft (STSG) — large defects, poor cosmesis; donor-site morbidity

- Free flap — large composite defects

Other surgically relevant lesions

Rheumatoid nodules — firm, non-tender, on extensor surfaces (especially elbow). Histology shows central fibrinoid necrosis with palisading histiocytes (necrobiotic granulomatous inflammation). Strongly associated with seropositive RA.

Keloid scars — pathological scarring extending beyond the original wound margin (hypertrophic stays within). Common in darker skin, sternum, earlobes, deltoid. First-line treatment: intralesional triamcinolone. Surgical excision alone has a high recurrence rate and is usually combined with steroid or radiotherapy.

Oral lesions worth knowing:

- Leukoplakia — white plaque, cannot be scraped off, pre-malignant

- Erythroplakia — red velvety patch, higher malignant potential than leukoplakia

- Oral hairy leukoplakia — corrugated white plaque on lateral tongue; EBV-driven; pathognomonic of HIV/AIDS

- Lichen planus — white reticular Wickham's striae

- Oral candidiasis — white curd-like plaques that scrape off

Dermatomes for herpes zoster

Shingles follows a dermatomal pattern and is a favourite SBA topic. Memorise the landmarks:

➡ T4 — nipple

➡ T7 — xiphisternum

➡ T10 — umbilicus

➡ L1 — inguinal ligament

➡ S2–S4 — perineum

T8–T10 lesions therefore appear over the costal margin and upper abdomen.

[Image: MCQs banner]

Test yourself

A woman returns to the UK after 10 years in Australia and presents with an enlarged lymph node in the upper femoral triangle. Diagnosis?

• ((Malignant melanoma::☑️ Australia has the highest melanoma incidence; lower-limb melanoma drains to inguinal nodes.))
• ((Anal cancer::Drains to inguinal nodes but no skin history given; less likely in this clinical picture.))
• ((Cervical cancer::Drains primarily to pelvic, not inguinal, nodes.))

👩‍⚕️ Always ask about sun exposure and prior skin lesions in any patient with isolated inguinal lymphadenopathy.

A patient presents with a bluish lesion on the toe and inguinal lymphadenopathy. Diagnosis?

• ((Melanoma::☑️ Acral lentiginous subtype — palms, soles, subungual; not sun-related.))
• ((Squamous cell carcinoma::Keratinising ulcer with everted edge, not bluish pigmented nodule.))
• ((Basal cell carcinoma::Pearly with telangiectasia; doesn't metastasise to nodes.))
• ((Ischaemia::Painful, cold, no lymphadenopathy.))

A 53-year-old farmer presents with a groin mass and a discoloured growth under the big toenail. Diagnosis?

• ((Subungual melanoma::☑️ Acral lentiginous melanoma — pigmented band, Hutchinson's sign, inguinal spread.))
• ((Subungual haematoma::Grows out with the nail; no nodal disease.))
• ((Onychomycosis::Fungal — yellow, crumbly nail; no lymphadenopathy.))
• ((Glomus tumour::Painful, small, vascular subungual lesion; benign.))

👩‍⚕️ A pigmented subungual lesion with Hutchinson's sign is melanoma until proven otherwise — never treat as trauma.

Which feature is associated with a good prognosis in malignant melanoma?

• ((Breslow thickness of 0.6 mm::☑️ Thin melanoma (<1 mm) has excellent survival.))
• ((Clark's level 5::Invasion into subcutaneous fat — poor prognosis.))
• ((Lymphatic invasion::Adverse prognostic feature.))
• ((Satellite nodules::Local recurrence pattern — stage III disease.))
• ((Ulceration::Independent adverse prognostic factor; upstages tumour.))

What is the single most important prognostic factor in melanoma?

• ((Breslow thickness::☑️ Depth in mm from granular layer to deepest tumour cell — drives staging and margins.))
• ((Nodal status::Important but Breslow predicts node positivity itself.))
• ((Completeness of excision::Important for cure but not the dominant prognostic variable.))
• ((Clark's level::Older system, superseded by Breslow.))
• ((Mitotic rate::Adverse feature but secondary to Breslow.))

A 1 cm pigmented lesion on the back is itchy and bleeding. Next step?

• ((Excisional biopsy with 2 mm margin::☑️ Diagnostic step — preserves Breslow measurement.))
• ((Wide local excision with 2 cm margin::Only after histology confirms melanoma and depth.))
• ((Incisional biopsy::Risks sampling error and disrupts Breslow.))
• ((Shave biopsy::Truncates depth — Breslow is lost.))
• ((Core biopsy::Not appropriate for cutaneous lesions.))

👩‍⚕️ Never shave a suspected melanoma — definitive margins are decided AFTER Breslow is known.

An ulcerated swelling with keratinised skin, calcium deposits and bone formation is most likely:

• ((Squamous cell carcinoma::☑️ Keratin pearls, dystrophic calcification and metaplastic bone are classic.))
• ((Basal cell carcinoma::Pearly with telangiectasia; doesn't keratinise heavily.))
• ((Malignant melanoma::Pigmented, not keratinised.))
• ((Actinic keratosis::Pre-malignant scaly patch; no invasion or bone formation.))
• ((Lipoma::Soft, subcutaneous, no keratin or ulceration.))

A 70-year-old ex-sailor has a 6-month crusty exophytic ulcer on the cheek with everted edge and palpable cervical nodes. Diagnosis?

• ((Squamous cell carcinoma::☑️ Everted keratinising ulcer with nodal spread on sun-exposed skin.))
• ((Basal cell carcinoma::Pearly with rolled inward edge; rarely metastasises.))
• ((Keratoacanthoma::Rapid growth over weeks with central keratin plug; regresses.))
• ((Malignant melanoma::Pigmented and irregular, not crusted/keratinised.))
• ((Merkel cell tumour::Rare; rapidly growing red-purple nodule on head/neck.))

A 68-year-old farmer has a 2 cm ulcerated lesion on the ear pinna with pearly white raised edges. Diagnosis?

• ((Basal cell carcinoma::☑️ Pearly rolled edge + telangiectasia on sun-exposed skin.))
• ((Squamous cell carcinoma::Everted, keratinising, faster-growing.))
• ((Bowen's disease::Erythematous scaly plaque — SCC in situ.))
• ((Actinic keratosis::Rough scaly patch, not ulcerated nodule.))
• ((Keratoacanthoma::Dome-shaped with central keratin plug; rapid growth.))

A farmer has BCC resected from the face. What is the most important prognostic indicator?

• ((Completeness of excision::☑️ BCC almost never metastasises — local recurrence depends on clear margins.))
• ((Pathological subtype::Morphoeic carries higher recurrence risk but margins still dominate.))
• ((Lymphatic infiltration::Vanishingly rare in BCC.))
• ((Lymphovascular invasion::Not a meaningful BCC variable.))
• ((Mitotic index::Not used in BCC prognostication.))

A 65-year-old man has a 1 cm facial defect after BCC excision. Best reconstruction?

• ((Local flap::☑️ Best colour/texture match on the face; avoids tension.))
• ((Split-thickness skin graft::Poor cosmesis and contracts; donor-site morbidity.))
• ((Full-thickness skin graft::Good colour match but inferior to flap for contour.))
• ((Primary closure::Risks tension and distortion of facial landmarks at 1 cm.))
• ((Conservative dressing::Healing by secondary intention — poor cosmesis on face.))

A 34-year-old gardener has a raised ulcerated lesion that bleeds easily on the dorsum of his hand, 10 days after trauma. Diagnosis?

• ((Pyogenic granuloma::☑️ Rapidly growing friable lobular capillary haemangioma after minor trauma.))
• ((Squamous cell carcinoma::Slower growth over months; keratinising.))
• ((Malignant melanoma::Pigmented and irregular; not post-traumatic.))
• ((Amelanotic melanoma::Possible mimic but rare; biopsy if doubt.))
• ((Glomus tumour::Painful subungual lesion.))

A 21-year-old has a small pigmented nodule on her calf at the site of an old insect bite that feels larger than it looks. Diagnosis?

• ((Dermatofibroma::☑️ Firm, deeper than it appears, positive dimple sign on lateral compression.))
• ((Malignant melanoma::Irregular border, colour variation, evolution — clinical context differs.))
• ((Pyogenic granuloma::Friable, bleeding red nodule, not deep firm pigmented one.))
• ((Dercum's disease::Multiple painful lipomas; not pigmented or post-bite.))

A 70-year-old has multiple raised greasy lesions with keratin plugs on her trunk. Diagnosis?

• ((Seborrhoeic keratosis::☑️ "Stuck-on" greasy warty lesions of older adults; benign.))
• ((Actinic keratosis::Rough scaly, sun-exposed sites, not greasy.))
• ((Melanoma::Single irregular pigmented lesion, not multiple greasy ones.))
• ((Bowen's disease::Solitary erythematous scaly plaque.))
• ((Viral warts::Verrucous but not greasy or stuck-on.))

A 53-year-old has a dome-shaped lesion on the chin with a central keratin plug that grew in 1 week. Diagnosis?

• ((Keratoacanthoma::☑️ Rapid growth over weeks, central keratin crater, spontaneous regression.))
• ((Squamous cell carcinoma::Slower growth; clinically indistinguishable so usually excised.))
• ((Basal cell carcinoma::Pearly rolled edge, not keratin crater.))
• ((Sebaceous cyst::Smooth dome with central punctum, no keratin horn, slow growth.))
• ((Molluscum contagiosum::Small umbilicated papules in children.))

👩‍⚕️ Keratoacanthoma vs SCC is a classic trap — rapid growth in weeks with a central keratin plug favours keratoacanthoma, but excise to be safe.

A 58-year-old has a 3 cm nodule on the extensor elbow. Biopsy shows necrobiotic granulomatous inflammation. Diagnosis?

• ((Rheumatoid nodule::☑️ Central fibrinoid necrosis with palisading histiocytes — seropositive RA.))
• ((Gouty tophus::Urate crystals with surrounding granulomatous reaction; chalky.))
• ((Lipoma::Soft, no inflammation on histology.))
• ((Sebaceous cyst::Lined by squamous epithelium; not granulomatous.))
• ((Sarcoid nodule::Non-caseating granulomas without necrobiosis.))

A patient develops a keloid scar after surgery. Most appropriate first-line treatment?

• ((Intralesional steroid injection::☑️ Triamcinolone reduces fibroblast proliferation and collagen synthesis.))
• ((Surgical excision alone::High recurrence — must combine with steroid or radiotherapy.))
• ((Radiotherapy::Adjunct after excision in resistant keloids; not first-line alone.))
• ((Pressure therapy::Useful for hypertrophic scars and burns; less effective for keloid.))
• ((Topical antibiotics::No role — keloid is not infective.))

An 80-year-old smoker has a smooth, red, velvety patch on her tongue. Diagnosis?

• ((Erythroplakia::☑️ Red velvety lesion — high malignant potential, higher than leukoplakia.))
• ((Leukoplakia::White thickened plaque that cannot be scraped off.))
• ((Lichen planus::White reticular Wickham's striae.))
• ((Oral candidiasis::White curd-like plaques that scrape off.))
• ((Geographic tongue::Migratory benign patches, asymptomatic.))

A patient with weight loss, cough and lymphadenopathy has hair-like lesions on the lateral aspect of the tongue. Diagnosis?

• ((AIDS::☑️ Oral hairy leukoplakia (EBV-driven) on lateral tongue is pathognomonic of HIV/AIDS.))
• ((Lichen planus::Wickham's striae — reticular white lacy lesions.))
• ((Candidiasis::White curd-like plaques that scrape off.))
• ((Leukoplakia::Smooth white plaque, not corrugated or hair-like.))
• ((Tongue carcinoma::Ulcerated lesion, not hair-like; risk factors smoking/alcohol.))

A 76-year-old has herpes zoster of T8–T10 on the left. Where will the rash appear?

• ((Costal margin::☑️ T8–T10 dermatomes cover the abdominal wall just below the ribs.))
• ((Pectoral region::T2–T4.))
• ((Left inguinal region::T12–L1.))
• ((Nipple::T4.))
• ((Suprapubic area::T12.))

👩‍⚕️ Anchor dermatomes: nipple T4, xiphisternum T7, umbilicus T10, inguinal ligament L1.

Revision summary

➡ Epidermis (deep → superficial): Basale, Spinosum, Granulosum, Lucidum (palms/soles only), Corneum. Melanocytes in basale; Langerhans in spinosum.

➡ BCC — commonest skin cancer; pearly rolled edge + telangiectasia; sun-exposed; rarely metastasises; 4 mm excision margin; completeness of excision is the key prognostic factor.

➡ SCC — second commonest; everted keratinising ulcer; risks include immunosuppression, HPV, chronic wounds (Marjolin's ulcer); 4–6 mm margin; can spread to regional nodes.

➡ Melanoma — ABCDE; subtypes: superficial spreading (commonest), nodular (worst), lentigo maligna (elderly face), acral lentiginous (dark skin, subungual, Hutchinson's sign).

➡ Breslow thickness is THE prognostic factor. WLE margins: in situ 0.5 cm, ≤1 mm → 1 cm, 1–2 mm → 1–2 cm, 2–4 mm → 2–3 cm, >4 mm → 3 cm. SLNB if >0.8 mm.

➡ Suspected melanoma → excisional biopsy with 2 mm margin first. Never shave biopsy.

➡ Pre-malignant: actinic keratosis (→ SCC), Bowen's disease (SCC in situ), leukoplakia, erythroplakia (higher risk).

➡ Benign mimics: keratoacanthoma (rapid growth + central keratin, regresses), seborrhoeic keratosis (stuck-on greasy, elderly trunk), dermatofibroma (firm, dimple sign), pyogenic granuloma (post-trauma, bleeds).

➡ Keloid → intralesional triamcinolone first-line; excision alone recurs.

➡ Oral hairy leukoplakia (lateral tongue, corrugated) → HIV/AIDS (EBV).

➡ Facial reconstruction: local flap > FTSG > STSG for cosmesis.

➡ Dermatome anchors: T4 nipple, T7 xiphisternum, T10 umbilicus, L1 inguinal ligament.