Unicystic Ameloblastoma

Unicystic ameloblastoma is a variant of ameloblastoma that presents as a single cystic cavity lined by ameloblastic epithelium. It makes up about 10 to 15% of ameloblastomas.

It affects younger patients than conventional ameloblastoma, with a mean age in the third decade, and is found mostly in the posterior mandible, often around an impacted third molar, where it mimics a dentigerous cyst.

Pathophysiology. Three histological forms are described: luminal, with epithelium confined to the lining; intraluminal, with a plexiform proliferation into the lumen; and mural, with infiltration of the connective tissue wall by ameloblastoma.

BRAF V600E mutations are found in a large share of mandibular ameloblastomas, and that gives a target for treatment. Mural invasion decides the risk of recurrence.

A painless, slow growing swelling of the jaw with expansion of the cortex, tooth displacement, delayed eruption or facial asymmetry. Many are found on radiographs of an unerupted tooth. Pain, paraesthesia or discharge is uncommon.

Rapid growth, lip numbness, mobile teeth or cortical destruction suggests more aggressive disease and needs urgent biopsy.

• Panoramic radiograph: a unilocular, well defined radiolucency, often around the crown of an impacted tooth, with root resorption or scalloped margins.
• Cone beam CT or MRI for extent, cortical perforation and soft tissue spread.
• Incisional biopsy, taken from the cyst wall, because the lining alone may not show the tumour, and a negative sample does not exclude mural disease. Final classification needs examination of the whole specimen.
• Molecular testing for BRAF V600E on the specimen, when targeted treatment is considered.
• Staging imaging only if malignant transformation is suspected.

Differential diagnosis.

• Dentigerous cyst
• Odontogenic keratocyst
• Radicular and residual cyst
• Conventional multicystic ameloblastoma
• Glandular odontogenic cyst
• Central giant cell granuloma
• Adenomatoid odontogenic tumour

• Luminal and intraluminal types: enucleation with removal of the associated tooth and curettage, with or without peripheral ostectomy and a modified Carnoy solution (without chloroform). Preliminary decompression for 6 to 12 months may shrink large cysts.
• Mural involvement (found on final histology), or larger lesions: marginal or segmental resection with a 1 cm margin of normal bone and reconstruction.
• Reported recurrence after enucleation is about 10 to 20% for luminal and intraluminal types, and 30 to 60% for mural disease. Figures vary between series and are of lower certainty.
• Unresectable, recurrent or metastatic BRAF V600E-mutant disease: targeted therapy by a specialist team, with dabrafenib 150 mg orally twice daily and trametinib 2 mg orally once daily until progression or unacceptable toxicity. Check cardiac function, skin, eye and liver tests, and avoid in pregnancy. Evidence is from case reports and small series.
• Antibiotics only for infection with spread: amoxicillin 500 mg orally three times daily for 5 days (reduce to 500 mg every 12 hours at eGFR 10 to 30), or clindamycin 300 mg four times daily in penicillin allergy.

Monitoring. Clinical and radiographic review every 6 months for 2 years, then annually for at least 10 years, as late recurrence is common.

Referral and escalation. Refer to oral and maxillofacial surgery for planning and to a head and neck oncology team for recurrent or unresectable disease. Urgent referral for rapid growth, numbness, cortical destruction or soft tissue extension.