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 is said to be "neoplastic")
    • 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

Unilocular odontogenic keratocyst
Large multilocular odontogenic keratocyst
Intermediate magnification of an odontogenic keratocyst showing a folded cyst.
High magnification of an odontogenic keratocyst.


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

  • Biopsy is the diagnostic investigation of choice (OKCs have a consistent and unique appearance)
  • Features:
    1. Epithelium
      • Regular stratified squamous epithelium
      • 5-8 cells thick
      • Palisaded basal layer (cells are columnar in shape)
      • Lack rete ridges
      • Often have artifactual separation from basement membrane
      • Corrugated surface which can be parakeratinized (83%), orthokeratinized (10%) or both (7%)
    2. Thin fibrous capsule
    3. Satellite (daughter) cells
      • Particularly seen in those with NBCCS
    4. Cyst contents
      • Fluid has protein content <4g/ dL
    5. High mitotic activity
    6. Inflammatory changes
      • Inflamed cysts show hyperplastic epithelium which is no longer characteristic of OKCs and can have resemblance to radicular cysts instead
      • A larger biopsy is needed to confirm OKC if there is inflammation

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