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Keratoacanthoma

Author: Andrew J. Richardson, MD

BASICS

DESCRIPTION

  • Solitary, rapidly proliferating, dome-shaped, erythematous or flesh-colored papule/nodule with central keratinous plug (1–2 cm).
  • Mimics squamous cell carcinoma (SCC) both clinically and microscopically.
  • Other forms: grouped, KA centrifugum marginatum, intraoral, subungual, regressing/nonregressing, generally eruptive.
  • Benign in most cases but can invade/metastasize; treatment required.
  • 3 stages:
  • Proliferative – rapid growth over weeks to months
  • Maturation – stabilization
  • Involution – spontaneous resolution with hypopigmented scar

System affected: Integumentary

EPIDEMIOLOGY

  • Most common age: >50 years
  • Seasonal: Summer and early fall
  • Sun-exposed, hair-bearing areas most common
  • Male > Female (2:1)
  • Highest in Fitzpatrick I–III
  • Incidence: 104/100,000

ETIOLOGY AND PATHOPHYSIOLOGY

  • Origin: Hyperkeratosis in follicular infundibulum
  • Cells: Squamous epithelial cell proliferation upward and downward
  • Regression may be immune-mediated or due to terminal differentiation
  • Triggers:
  • UV radiation
  • Procedures (e.g. surgery, cryotherapy, chemical peel, laser)
  • Viruses: HPV, Merkel cell polyomavirus
  • Genetics: Muir-Torre, Xeroderma Pigmentosum, Ferguson-Smith
  • Drugs: BRAF inhibitors
  • Chemical carcinogens

Genetic mutations: p53, H-ras

RISK FACTORS

  • UV exposure, tanning
  • Skin type I–III
  • Trauma
  • Tar, pitch, smoking
  • Immunosuppression
  • HPV
  • Discoid lupus erythematosus

PREVENTION

  • Sun protection measures

ASSOCIATED CONDITIONS

  • Sun-damaged skin, actinic keratosis, SCC
  • Muir-Torre syndrome: Associated GI/GU cancers

DIAGNOSIS

HISTORY

  • Starts as small pink macule, rapid growth to 1–2 cm
  • Can stabilize or regress
  • Usually asymptomatic or mildly tender
  • Family history, systemic malignancies if multiple lesions

PHYSICAL EXAM

  • Firm, dome-shaped, central keratin plug
  • Telangiectasia, atrophy, dyspigmentation
  • Subungual KAs: Painful
  • Look for lymphadenopathy

DERMOSCOPY

  • Central keratin = highest sensitivity
  • White circles = highest specificity
  • Cannot reliably distinguish from SCC

DIFFERENTIAL DIAGNOSIS

  • SCC
  • BCC (nodular/ulcerative)
  • Cutaneous horn
  • Hypertrophic actinic keratosis
  • Amelanotic melanoma
  • Merkel cell carcinoma
  • Molluscum contagiosum
  • Verruca vulgaris
  • Kaposi sarcoma
  • Sebaceous adenoma, others

DIAGNOSTIC TESTS & INTERPRETATION

  • Excisional biopsy (includes margin) = Gold standard
  • Deep shave biopsy if excision not possible
  • Avoid punch biopsy
  • Histopathology:
  • Keratin-filled crater
  • Mild atypia, glassy eosinophilic cytoplasm
  • Neutrophils/eosinophils, fibrosis in regressing KA

TREATMENT

SURGICAL

  • Excision + ED&C = preferred
  • Mohs surgery for aggressive/perineural lesions
  • ED&C for small extremity lesions
  • Immediate surgery in immunocompromised

MEDICAL

  • Intralesional methotrexate (12.5–25 mg, q2–3wk)
  • 5% imiquimod 3x/week Γ— 11–13 weeks
  • 5-FU cream daily
  • Intralesional 5-FU or IFN-Ξ±/Ξ²
  • Oral isotretinoin (0.5–1 mg/kg/day)

REFERRAL

  • Refer to Dermatology if:
  • Lesions >2 cm
  • Multiple/mucosal/subungual

ADDITIONAL THERAPIES

  • Photodynamic therapy
  • Cryotherapy, Laser, Radiation
  • Erlotinib (EGFR inhibitor)

FOLLOW-UP

  • Recheck every 6 months
  • Annual monitoring long-term
  • Teach skin self-exam
  • Colonoscopy if multiple lesions or Muir-Torre syndrome

PATIENT EDUCATION

  • Sunblock (SPF >30), protective clothing
  • Avoid indoor tanning, smoking, tar/pitch exposure
  • Arc welding risk

PROGNOSIS

  • May leave atrophic scars, hypopigmentation
  • 12% regress spontaneously, 88% with treatment
  • No metastases in review of 445 cases
  • 4–8% recurrence
  • Subungual/mucosal lesions do not regress spontaneously

ICD10 CODES

  • D23.9: Other benign neoplasm of skin, unspecified
  • D48.5: Neoplasm of uncertain behavior of skin
  • L85.8: Other specified epidermal thickening

CLINICAL PEARLS

  • Solitary, dome-shaped lesion with central plug = suspect KA
  • Rapid onset supports diagnosis
  • Excision is both diagnostic and therapeutic
  • Consider non-surgical options for non-candidates