What diagnosis is most likely on histopathologic examination of a 1‑cm erythematous dome‑shaped lesion with a central keratin plug on the right lower leg of a 78‑year‑old woman with a three‑week history of rapid growth?

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Keratoacanthoma

The histopathologic diagnosis will most likely be keratoacanthoma, a rapidly growing benign epithelial tumor characterized by a central keratin-filled crater surrounded by proliferating squamous epithelium.

Clinical Reasoning

The clinical presentation is pathognomonic for keratoacanthoma:

  • Rapid growth over 3 weeks - This explosive growth pattern is the hallmark feature that distinguishes keratoacanthoma from other cutaneous neoplasms
  • Dome-shaped morphology with central keratin plug - This "volcanic crater" appearance is virtually diagnostic
  • 1 cm size - Typical for keratoacanthoma, which usually reaches 1-2 cm before stabilizing
  • Elderly patient (78 years) - Peak incidence is in older adults
  • Sun-exposed site (lower leg) - Common location for UV-related epithelial proliferations

Histopathologic Features Expected

On pathologic examination, the lesion will demonstrate:

  • Cup-shaped or crater-like architecture with central keratin-filled core
  • Well-differentiated squamous epithelium with minimal atypia
  • Proliferating epithelial cells at the periphery pushing into the dermis
  • Abundant eosinophilic cytoplasm (glassy appearance)
  • Inflammatory infiltrate at the base

Critical Distinction from Squamous Cell Carcinoma

While keratoacanthoma is classified as a keratoacanthomatous SCC variant in modern pathology 1, it represents a prognostically favorable histologic subtype that may be clinically useful to identify 1. The distinction matters because:

  • Keratoacanthomas often spontaneously regress (though not reliably)
  • They have minimal metastatic potential
  • Treatment approach may be less aggressive than for conventional SCC

Common Pitfall

The main diagnostic challenge is distinguishing keratoacanthoma from well-differentiated squamous cell carcinoma, as they share overlapping histologic features. The rapid growth history and symmetric architecture with central keratinization are key clinical and pathologic clues. When submitting the biopsy, providing the pathologist with the clinical history of rapid 3-week growth and the dome-shaped morphology with central plug is essential for accurate clinicopathologic correlation 1.

Management Implications

Despite its benign behavior, complete excision is recommended because:

  • Spontaneous regression is unpredictable
  • Distinction from invasive SCC can be challenging
  • Rare cases may behave aggressively
  • Cosmetic outcome is better with early definitive treatment

References

1
Guideline

guidelines of care for the management of cutaneous squamous cell carcinoma.

Journal of the American Academy of Dermatology, 2018

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This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.