What is the likely etiology and pathology of a dome‑shaped lesion with a central keratin plug?

Keratoacanthoma

A dome-shaped lesion with a central keratin plug is most likely a keratoacanthoma (KA), a rapidly growing cutaneous neoplasm that arises from the hair follicle infundibulum and presents the critical diagnostic challenge of distinguishing it from squamous cell carcinoma (SCC).

Etiology

The pathogenesis of keratoacanthoma involves multiple contributing factors 1, 2, 3:

• UV radiation exposure is the primary etiologic factor, with lesions predominantly occurring on sun-exposed skin
• Trauma may act as a "second insult" following UV damage, triggering koebnerization 4
• Age-related factors, with mean presentation around 49-57 years old 1, 3
• Male predominance is consistently observed 1
• Other risk factors include immunosuppression and, less commonly, HPV infection 4

Pathology

Macroscopic Features

Keratoacanthoma presents as a dome-shaped nodule with a central keratin-filled crater 5, 2, 3. The lesion characteristically grows rapidly over 2-6 weeks, reaching sizes typically under 2 cm 1, 6, 7.

Microscopic Features

The histopathology reveals 5, 2:

• Crateriform architecture with a central keratin-filled crater
• Proliferation of enlarged pale pink cells with ground glass-like cytoplasm
• Absence of significant nuclear atypia (in benign KA)
• Buttressing of the crater margins
• Keratin pearls may be present
• Inflammatory infiltrate in the underlying connective tissue
• Four recognized stages: early/proliferative, well-developed, regressing, and regressed

Critical Diagnostic Pitfall

The most important clinical consideration is that KA is histologically and clinically indistinguishable from well-differentiated SCC 8, 5, 6, 7. The guidelines explicitly exclude keratoacanthoma from SCC management protocols 8, yet research demonstrates that:

• KA can undergo malignant transformation within the lesion 5
• Keratoacanthoma-type crateriform SCC exists as a distinct malignant entity 7
• Even with complete histologic examination, differentiation can be challenging 5, 6

Management Algorithm

When KA is Clinically Suspected:

1. Complete excision is mandatory, especially when:

   • Located on sun-exposed areas 5
   • Patient is elderly 5, 3
   • Rapid growth is documented 3, 6, 7

2. If complete excision is impossible:

   • Perform partial excision with sufficient specimen showing intact architecture 5
   • Implement careful post-biopsy surveillance even if histology confirms KA 5
   • Recognize that conventional SCC may remain in residual tissue 5

3. Surgical margins:

   • Use 4 mm margins for excision 7
   • Ensure primary closure is feasible
   • Submit entire specimen for histopathologic examination

Post-Treatment Surveillance

• Monitor for recurrence (reported rate: 6.89%) 1
• Follow-up at 3 months minimum 7
• Continued surveillance for new lesions given UV damage field effect

Special Considerations for Skin of Color

Patients with darker skin tones face higher risk of post-operative complications including post-inflammatory hyperpigmentation, hypertrophic scars, and keloids 6. However, the malignant potential of these lesions necessitates complete excision regardless of these risks, as conservative management cannot be justified without definitive histologic diagnosis.

The key principle: treat every dome-shaped lesion with a central keratin plug as potentially malignant until proven otherwise through complete excision and histopathologic examination 5, 6, 7.