Benign Epithelial Tumors

Seborrheic Keratosis - Waxy Old Barnacles

• Common, benign epidermal tumor; often in middle-aged/elderly.
• Appearance: Well-demarcated, "stuck-on", waxy papules/plaques; variable pigmentation (tan to black).
  • Often called "barnacles of old age".
• Sites: Trunk, face, scalp, extremities; spares palms, soles, mucous membranes.
• Histopathology: Acanthosis, papillomatosis, hyperkeratosis, keratin-filled invaginations (horn cysts, pseudohorn cysts).
• Variants:
  • Dermatosis Papulosa Nigra (DPN): Multiple small, pigmented papules on face/neck; common in darker skin types.
  • Stucco Keratoses: Small, dry, gray-white papules on lower extremities.
• 📌 Mnemonic: Stuck-on Keratosis.

⭐ Sudden eruption of multiple seborrheic keratoses (Leser-Trélat sign) can be a paraneoplastic sign, often associated with internal malignancy, especially gastric adenocarcinoma.

• Treatment: Usually not required; cryotherapy, curettage, or shave excision for cosmetic reasons or irritation.

Epidermal & Pilar Cysts - Common Skin Pockets

• Epidermal Cyst (Infundibular):
  • Origin: Hair follicle infundibulum.
  • Lining: Epidermal-like (has granular layer).
  • Contents: Cheesy keratin; central punctum common.
  • Sites: Face, neck, trunk.
  • 📌 Gardner syndrome association.
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• Pilar Cyst (Trichilemmal):
  • Origin: Hair follicle outer root sheath (isthmus).
  • Lining: No granular layer (trichilemmal keratinization).
  • Contents: Homogenous, firm keratin; punctum rare.
  • Sites: 90% on scalp.
  • Often multiple; AD inheritance possible.
  • 

⭐ Pilar cysts show abrupt keratinization without a granular layer (trichilemmal keratinization).

Adnexal Neoplasms - Appendage Adventures

• Syringoma:
  • Benign eccrine duct tumor.
  • Small, skin-colored to yellowish papules; typically periorbital.
• Trichoepithelioma:
  • Benign hair follicle (trichogenic) tumor.
  • Skin-colored papules/nodules, often facial.
  • Multiple familial type linked to Brooke-Spiegler syndrome (CYLD gene).
• Pilomatricoma (Calcifying Epithelioma of Malherbe):
  • Hair matrix origin.
  • Firm, deep-seated, often solitary nodule; common on head, neck, upper extremities.
  • Histology: Basaloid cells & anucleated "ghost cells" (shadow cells) with frequent calcification.

  ⭐ Pilomatricomas classically exhibit "ghost cells" (anucleated squamous cells) on histology, a pathognomonic finding.

• Cylindroma (Dermal Eccrine Cylindroma):
  • Tumor with apocrine or eccrine differentiation.
  • Solitary or multiple (scalp involvement: "turban tumor").
  • Histology: Nests of cells fitting like "jigsaw puzzle" pieces.
• Sebaceous Hyperplasia:
  • Benign enlargement of sebaceous glands.
  • Yellowish, soft, slightly umbilicated papules; common on the face of older individuals.
  • Must differentiate from Basal Cell Carcinoma (BCC).

Keratoacanthoma - Volcano Mimic Tumor

• Benign epithelial tumor; rapid growth; mimics Squamous Cell Carcinoma (SCC).
• Etiology: UV radiation, HPV, immunosuppression, chemical carcinogens (tar).
• Clinical Features:
  • Firm, dome-shaped nodule; central keratin-filled crater ("volcano lesion").
  • Sun-exposed skin (face, dorsum of hands).
  • Growth cycle: Rapid proliferation (2-8 weeks) → Stable phase → Spontaneous regression (2-12 months) → Scar.
• Histopathology: Exo-endophytic crater; central keratin plug; well-differentiated squamous cells; glassy eosinophilic cytoplasm.
• Management: Excisional biopsy standard to rule out SCC.

⭐ Hallmark: Rapid initial growth followed by spontaneous involution; however, differentiation from SCC is critical.

High‑Yield Points - ⚡ Biggest Takeaways

• Seborrheic keratosis: "Stuck-on" look; Leser-Trélat sign suggests internal malignancy.
• Acanthosis nigricans: Velvety skin; linked to insulin resistance or gastric cancer.
• Keratoacanthoma: Rapid growth, keratin plug; often treated as SCC due to aggressive potential.
• Trichoepithelioma: Facial papules; Brooke-Spiegler syndrome association.
• Syringoma: Periocular papules from eccrine ducts; common in women.
• Pilomatricoma: Hard nodule in children (head/neck); "ghost cells" histologically.
• Epidermal nevus: Congenital, follows Lines of Blaschko; risk of Epidermal Nevus Syndrome.