Page 9 of 16

PA33.4 | Common Skin Tumors & Morphology — SDL Guide (Part 4)

Master Recognition Table — Skin Tumors

Use this table as your final revision checklist before the practical exam.

Tumor	Clinical Clue	Key Histology	Behavior
Seborrheic keratosis	Stuck-on, warty, dark plaque	Basaloid cells + horn cysts + flat lower border	Benign
Acrochordon	Soft, pedunculated, flesh-coloured neck papule	Fibrovascular core + normal squamous epithelium	Benign
Epidermal inclusion cyst	Smooth cyst with punctum; rupture → inflammation	Squamous epithelium with granular layer + laminated keratin	Benign
Acanthosis nigricans	Velvety, dark flexural skin	Papillomatosis + mild hyperkeratosis	Marker; rule out internal malignancy
Cylindroma	Scalp ('turban tumor' if multiple)	Jigsaw-puzzle basaloid nests with hyaline sheaths	Benign
Syringoma	Multiple eyelid papules	Tadpole (comma-shaped) ducts in fibrous stroma	Benign
Pilomatricoma	Firm, calcified nodule in children	Basaloid + ghost cells + calcification	Benign
Junctional nevus	Flat, uniformly pigmented macule	Nests at DEJ only	Benign
Compound nevus	Slightly raised pigmented papule	Nests at DEJ + dermis; maturation present	Benign
Intradermal nevus	Dome-shaped, flesh-coloured papule	Nests in dermis only; maturation present	Benign
Dysplastic nevus	>6 mm, variegated, irregular border	Bridging nests, lentiginous growth, lamellar fibroplasia	Potential precursor
Actinic keratosis	Sandpaper plaque, sun-exposed skin	Basal dysplasia only; solar elastosis	Premalignant
Bowen disease	Scaly erythematous plaque	Full-thickness atypia; intact BM	Premalignant (SCC in situ)
BCC	Pearly nodule, rolled border, telangiectasia	Basaloid nests + peripheral palisading + retraction cleft	Locally destructive; rare mets
SCC	Firm ulcerated plaque, sun-exposed	Keratin pearls + intercellular bridges + dermal invasion	Malignant; 5–10% mets
Melanoma	ABCDE; variegated color	Pagetoid spread + atypical melanocytes + loss of maturation	Highly malignant
Dermatofibroma	Firm brown nodule, lower leg; dimple sign	Storiform spindle cells + epidermal hyperplasia + trapped collagen	Benign
Cutaneous metastasis	Firm dermal nodule(s), no epidermal connection	Primary histotype in dermis, epidermis spared	Malignant (secondary)

Three-panel medium-power histology comparison: Panel A shows BCC with peripheral palisading of basaloid cells and retraction artifact; Panel B shows SCC with a central keratin pearl and intercellular bridges; Panel C shows melanoma with pagetoid spread of atypical melanocytes through the epidermis. — **Panel A:** BCC — basaloid cell nests with peripheral palisading (single-file cells at nest periphery), retraction cleft between tumor and fibromyxoid stroma. **Panel B:** SCC — polygonal squamous cells with eosinophilic cytoplasm, centrally located keratin pearl (concentric laminated whorls), intercellular bridges, individual cell keratinization. **Panel C:** Melanoma — atypical melanocytes at dermoepidermal junction forming junctional nests, upward pagetoid migration of single melanocytes into suprabasal epidermis, pleomorphic nuclei with prominent nucleoli.

SELF-CHECK

A 45-year-old woman has a 1 cm firm brown nodule on her right leg. Lateral pressure on the nodule causes it to dimple inward. Biopsy shows spindle cells in a storiform pattern with overlying epidermal hyperplasia. The BEST diagnosis is:

A. Nodular melanoma

B. Dermatofibroma

C. Squamous cell carcinoma

D. Epidermal inclusion cyst

Reveal Answer

Answer: B. Dermatofibroma

The 'dimple sign' (Fitzpatrick's sign) on lateral compression is the classic clinical feature of dermatofibroma. Histologically, storiform spindle cells + overlying epidermal hyperplasia + trapped collagen confirms the diagnosis. Melanoma shows atypical melanocytes with pagetoid spread; SCC shows keratin pearls; epidermal cyst shows a cystic space lined by squamous epithelium.