Odontogenic Keratocyst

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  • Benign but aggressive intraosseous lesions of odontogenic origin
  • Reclassified back to odontogenic keratocyst in the WHO 2017 classification (previously classified as keratocystic odontogenic tumour [KCOT] from 2005 to 2017)[1]

Epidemiology

  • Account for 5-10% of jaw cysts
  • Peak incidence 20-30yrs
  • ♂ > ♀ (slightly)
  • Commonest site — angle of the mandible
    • 70-80% occur in the mandible
    • 50% at the angle of the mandible

Clinical Features

  • Usually asymptomatic (commonly incidental findings)
    • When large/infected can cause pain/swelling/discharge/pathological fracture/tooth displacement/buccal expansion
  • Characteristic insidious pattern of growth
    • Unlike other cysts, OKCs do not have a high internal pressure ∴ they preferentially expand along the medullary cavity (the path of least resistance)
    • A cyst in the mandible may extend through much of the ramus and body without significant expansion of the jaw
    • Clinical signs often fail to appear until the cyst is well advanced
  • Usually solitary cysts (consider Nevoid basal-cell carcinoma syndrome (Gorlin-Goltz Syndrome))
  • High recurrence rate

Differential Diagnosis

Memory Aid - Multilocular lesions of the mandible (MACHO)
Mxyoma
Amelobastoma
Central giant cell tumour
Haemangioma/vascular malformation
Odontogenic Keratocyst

Aetiology and Pathogenesis

Aetiology

Pathogenesis

  • Mutation of PTCH
    • PTCH is a tumour suppressor gene that encodes the PTCH protein
    • PTCH protein is a receptor for sonic hedgehog (SHH)
    • In adult tissue, SHH plays a role in cell cycle regulation (SHH dysfunction is implicated in various cancer types)
    • ↓ PTCH gene activity → release of the break on cell cycle (mediated by SHH) → ↑ proliferative activity in epithelial lining of keratocysts
    • This increase in proliferative activity causes enlargement of the cyst by mural growth (as opposed to osmotic growth seen in other cysts)
    • Increase in proliferative activity may also contribute to recurrence rates
  • Mural growth of cysts
    • Growth is by extension of finger-like processes into marrow spaces rather than by expansion
    • Growth of the wall is faster than the expansion of cyst cavity ∴ the lining becomes folded
    • Cyst enlarges slowly along the pathway of least resistance

Investigations

Laboratory Investigations

Aspiration of cyst contents may be helpful for analysis of protein content (biochemistry) and keratinization (cytology)

Imaging

Plain film

  • Well defined radiolucent area with a sharply demarcated and corticated bony wall
  • Radiographically usually multilocular
    • Unilocular lesions tend to have a scalloped margin
    • When multilocular, can mimic ameloblastoma if many locules exist
  • Can mimic other cysts
    • 40% in a ‘dentigerous’ position
  • Adjacent roots/teeth may become displaced by large cysts, but usually the cyst will extend around the roots and inferior alveolar nerve without displacing them or causing significant expansion

Computed Tomography

  • Can facilitate diagnosis, and 3D characterisation for surgical planning

Histopathology

Lined by ortho- (10%) or parakeratinized (83%) epithelium (7% have both)— fluid has protein content <4g/ dL Histologically high mitotic activity: growth is neoplastic with invasion of the medulla and not by bony expansion

Management

  • Management

Treatment = enucleation ± Carnoy’s solution (decreases recurrence)

Prognosis and Complications

  • Prognosis and Complications

Follow-up

Satellite cysts or daughter cysts increase the likelihood of recurrence, as does an association with Gorlin– Goltz syndrome Orthokeratinizing keratocysts are much less aggressive. Review for recurrence (up to 60%) particularly in Gorlin– Goltz syndrome

References

  1. El-Naggar AK, Chan JK, Grandis JR. WHO classification of head and neck tumours. 2017. ISBN: 9789283224389