Ameloblastoma: High-yield Dental Notes

Classification of Ameloblastoma:

Based on clinical, radiographic & histopathology & behavioural & prognostic aspects, Ameloblastoma is classified as:

1. Classic- Solid/ Multicystic ameloblastoma (SMA)
2. Unicystic ameloblastoma (UA)
3. Peripheral ameloblastoma (PA)
4. Desmoplastic ameloblastoma (DA), including hybrid lesion.

 Solid/ Multicystic Ameloblastoma (SMA)

Clinical Features:

• Benign epithelial tumour, with almost no tendency to metastasize. Though it is locally invasive.
• High recurrence if improperly removed
• Located centrally (intraosseous).
• Few or no clinical signs in early stages.
• Later signs:

• Facial deformity, loose teeth
• Spontaneous fracture may be seen.
• Bony swelling may be seen
• Pain: Due to either pressure of growing tumour on nerves or secondary infection.

• Enlarging tumour makes the surrounding bone elicit Crepitation/ Egg shell crackling
• Perforation of bone may occur.

Radiographic features:

• Multilocular lesion- typical appearance
• Unilocular may also be seen.
• Unilocular:

• Well defined single radiolucency.
• If associated with unerupted tooth, may resemble Dentigerous cyst or OKC.

Epidemiology:

• Black> White
• Age– 35-37 yrs average age at time of diagnosis.
• According to Gardener, mean age at time of diagnosis:

• SMA– about 39 yrs
• Unicystic– about 22 yrs
• Peripheral– about 51 yrs

• Gender– almost equal or slight male (1.1:1) predilection.
• Location: Mandible> Maxilla (2.2:1)
• Posterior mandible> anterior mandible> posterior maxilla> anterior maxilla

Pathology:

Pathogenesis:

• Mostly arise from odontogenic epithelial remnants, specifically remnants of dental lamina.
• If these remnants lie outside bone in soft tissues, they form Peripheral Ameloblastoma.
• May arise as neoplastic change in lining or wall of cysts like Dentigerous or OKC called Mural ameloblastoma. (usually seen in posterior region).
• May originate from Epithelial rests of Malassez.

Microscopy:

WHO 1992 definition: A polymorphic neoplasm consisting of proliferating odontogenic epithelium which usually has a follicular or plexiform pattern lying in a fibrous stroma.

The various histologic patterns of SMA include:

Follicular pattern

• Epithelial Islands: Contain central mass of polyhedral cells, or loosely connected angular cells resembling stellate reticulum.
• Peripheral cells in the epithelial islands are cuboidal or columnar resembling inner enamel epithelium or pre-ameloblast.
• Cystic degeneration is common within epithelial islands

Plexiform pattern

• Tumour epithelium arranged as a network
• This network bound by cuboidal to columnar cells & includes Stellate Reticulum like cells.
• Cyst formation is due to stromal degeneration may be seen.
• Hyaline bodies like odontogenic cyst epithelium/wall may be seen

Acanthomatous SMA:

• Extensive squamous metaplasia
• Keratin may be formed sometimes (within tumour island)
• Generally, shows follicular pattern
• Third most common histologic type.

Granular cell SMA:

• Most often it shows Follicular pattern.
• Granular transformation of central stellate cells.
• Granular cells may be Cuboidal, Columnar or Round.
• Cytoplasm filled with Acidophilic granules.
• Granularity may be due to increased apoptosis & phagocytosis of cells by neighbouring neoplastic cells.

Desmoplastic SMA:

• Usually follicular pattern of SMA
• Connective tissue shows marked hyalinization (desmoplasia).

Basal cell SMA:

• Resemble basal & suprabasal spinosum cells.
• Rare variant
• SMA shows predominant basaloid pattern.
• Most actively proliferating type. It shows positive labelling for bothPCNA & KP-67. 
• This variant has most immature cells.

Clear cell SMA:

• Clear cells in stellate cell area of SMA follicles
• PAS positive
• May show malignant transformation.

Keratoameloblastoma(KA) & papilliferous KA:

• Simultaneous occurrence of areas of Ameloblastoma with pronounced keratinization & cystic areas of resembling OKC.
• Extremely rare.

Connective tissue of all histologic variants of SMA:

• Contains fibroblasts, collagen fibres & myofibroblasts.

Treatment:

• Treatment ranges from simple enucleation and curettage to en bloc resection.
• Curettage only, often leave small islands of tumor within the bone, this results in recurrences (50-90%).
• Marginal resection: Most widely used treatment, but recurrence of up to 15% may be seen.
• Removal of the tumor, followed by peripheral ostectomy, often reduces the need for extensive reconstructive surgery.
• Ameloblastomas of the posterior maxilla are particularly dangerous because of the difficulty of obtaining an adequate surgical margin around the tumor.
• Though ameloblastoma is radiosensitive, Radiation therapy is contraindicated because of a possibility of secondary radiation-induced malignancy.

Unicystic Ameloblastoma

• Well defined, often large monocystic cavity with a lining.
• Locally but rarely entirely lined by odontogenic epithelium.
• On the Luminal surface of cyst, one or several polypoid or papillomatous, pedunculated exophytic masses may be seen: These are called Intracystic, Luminal or Intraluminal ameloblastoma.
• Epithelial nodules may grow within the connective tissue: Called Mural or Intramural.
• May be associated with an unerupted tooth.

Clinical & Radiographic findings:

• If there is secondary infection:

• Local swelling
• Occasional pain
• Lip numbness
• Discharge or drainage

• May be unilocular or multilocular; Unilocular more common
• Root resorption is common.
• If crown of unerupted tooth is involved, it is displaced by cystic tumour rather than project into cystic lumen – Differentiation from Dentigerous cyst.

Epidemiology:

• Tooth associated lesion occurs in younger patients compared to those not associated with tooth. 
• Males: Tooth associated lesion more common; Females: Lesion without tooth are common.
• Mandible > Maxilla; Posterior mandible & ascending ramus is most commonly involved.

Pathology:

Pathogenesis:

• May arise from pre-existing odontogenic cyst (commonly Dentigerous cyst) or it may arise de novo.
• According to Leider et al.: They may arise from:

1. From REE.
2. From Dentigerous or another odontogenic cyst.
3. From Solid ameloblastoma undergoing cystic degeneration.

Microscopy: 

• Minimum criteria for diagnosing a lesion a unicystic ameloblastoma:

• Single cystic sac.
• Lined by odontogenic epithelium (usually present locally).
• Epithelium may be variable, mimicking lining of Dentigerous or Radicular cyst.

Sub-groups of unicystic ameloblastoma:

Luminal type:

• Epithelial lining may show transformation to cuboidal or columnar basal cells.
• Nuclear palisading with polarization.
• Cytoplasmic vacuolization.
• Intercellular spacing.
• Sub-epithelial hyalinization.

Intramural tissue:

• Infiltration from cyst lining or as free islands of follicles (SMA) often with central cystic degeneration.
• About 2/3 of both tooth-associated & non-tooth-associated show intramural invasion.
• Though slightly more common in non-tooth type.
• Intra luminal proliferation more common in tooth associated type.
• Tumours with intramural invasion have higher recurrence.

Treatment:

• May be treated conservatively by enucleation or it may need aggressive treatment as classic SMA, depending on the histopathologic presentation.

Peripheral Ameloblastoma:

• Peripheral ameloblastoma is a benign neoplasm or hamartomatous lesion confined to soft tissue overlying tooth bearing area of jaws.
• Several characteristic similar to SMA.
• Do not invade underlying bone.

Clinical & Radiographic features:

• Painless, sessile, firm & exophytic growth.
• Surface is relatively smooth, but may be granular or pebbly. Sometimes Papillary or Warty.
• Surface colour:  Normal to red to dark red.
• Size– 0.3 to 4.5 cm (1.3 cm)
• Superficial bone erosion may be seen, due to pressure of lesion.

Epidemiology:

• 2-10% of all Ameloblastomas.
• Age– 9-92 yrs. (Average age: 52 yrs.). More common in 5^th to 7^th decade.
• Gender: more common in males.
• Location: more common in mandible. Mandibular premolar region most common site.

Pathology:

Pathogenesis:

• Probably arises from dental lamina remnants. (cell rests of Serre).
• May also arise from surface epithelium. Continuity between tumour & surface epithelium has been seen.

Microscopy:

Epithelium: 

• Epithelial islands show palisaded columnar basal cells.
• Stellate reticulum like cells are few in number (negligible).
• Lesions exhibiting Acanthomatous areas are difficult to distinguise from Basal cell carcinoma.
• Ghost cells in Acanthomatous area may be seen. May be confused with Calcifying ghost cell odontogenic cyst.
• Vacuolated or clear cells may be seen in some parts of tumour as clusters.

Stroma: 

• Mature fibrous connective tissue (calcification may be seen sometimes).

Treatment:

• It exhibits a milder biologic behaviour than the SMA. So, wide excision is usually not needed.
• Conservative supra-periosteal surgical excision with adequate disease-free margins.
• Recurrence rate lower than SMA.

Desmoplastic Ameloblastoma

• Extensive stromal collagenisation or desmoplasia.
• Hybrid ameloblastoma: Shows features of both Desmoplastic variant & classic follicular ameloblastoma. It is probably a transitional form of desmoplastic ameloblastoma.

Clinical features:

• Benign, locally infiltrative, epithelial neoplasm.
• A variant or sub-type of SMA.
• Painless swelling: chief complaint in most cases.

Radiographic features:

• Well defined borders are usually not seen.
• Mixed radiolucent/ radiopaque in most cases.
• Root resorption is common.
• New bone formation may be seen.
• Shows infiltration in adjacent bone marrow space. It causes ill-defined borders of lesion.

Epidemiology:

• 4-13% of all SMA
• Age – 17-72yrs (43yrs)
• Slight or no male predominance.
• Occurs with almost equal frequency in both jaws. More common in Anterior region.

Pathogenesis:

Microscopy:

• Consists of proliferating, irregular, often bizarrely shaped islands and cords of odontogenic epithelium of varying sizes embedded in a desmoplastic connective tissue stroma.

Epithelial islands: 

• Irregular in shape and have a pointed stellate appearance.
• Pathognomonic animal like outline.
• Peripheral epithelial cells are usually cuboidal, rarely columnar with reversal of polarity.
• Hyperchromatic nuclei may be seen sometimes.

Centre of epithelial islands:

• Hypercellular with spindle shaped or squamatoid or rarely keratinized epithelial cells.
• Microcysts containing eosinophilic amorphous deposits or empty are common within tumour islands.
• Foci of keratinization may be seen sometimes.
• Glandular differentiation with mucous cells formation may be seen in tumour nests.

Connective tissue stroma:

• Extensive stromal dysplasia.
• Moderately cellular, fibrous connective tissue.
• Collagen fibres are thick & numerous. They appear to compress tumour islands.
• Myxoid changes may be seen in stroma around odontogenic epithelium.
• Metaplastic bone formation may be seen.
• Capsule – peripheral fibrous condensation not always seen.

Treatment:

Same as SMA        

                                     

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