Ameloblastomas are locally aggressive benign tumors that arise from the mandible, or, less commonly, from the maxilla. Usually present as a slowly but continuously growing hard painless lesion near the angle of the mandible in the 3 to 5decades of life, which can be severely disfiguring if left untreated.
The most common form of ameloblastoma - the multicystic form - was formerly known as adamantinoma of the jaw. However, ameloblastoma is unrelated histologically to adamantinoma of the bone, and this terminology should be abandoned to avoid confusion.
Ameloblastomas are the second most common odontogenic tumor (odontoma is the most common overall, but ameloblastoma is the most common lucent lesion) and account for up to one-third of such cases.
They are slow-growing and tend to present in the 3 to 5decades of life, with no gender predilection .
Ameloblastomas typically occur as hard, painless lesions near the angle of the mandible in the region of the 3 molar tooth (48 and 38) although they can occur anywhere along the alveolus of the mandible (80%) and maxilla (20%). When the maxilla is involved, the tumor is located in the premolar region and can extend up into the maxillary sinus.
Although benign, it is a locally aggressive neoplasm with a high rate of recurrence. Approximately 20% of cases are associated with dentigerous cysts and unerupted teeth.
Unsurprisingly, ameloblastomas arise from ameloblasts, which are part of the odontogenic epithelium, responsible for enamel production and eventual crown formation. Four forms have been described in the literature: unicystic, solid (multicystic), desmoplastic, and peripheral (extraosseous).
There are no specific histological features to differentiate between ameloblastomas and craniopharyngiomas, with only location differentiating between two tumors . Nonetheless, there are histological features of craniopharyngiomas not generally encountered with ameloblastomas, such as irregular calcified masses and foci of metaplastic bone or cartilage .
Radiograph and CT
Multicystic ameloblastomas account for 80-90% of cases which are classically expansile "soap-bubble" lesions, with well-demarcated borders and no matrix calcification. Resorption of adjacent teeth and “root blunting” is often a feature. When larger it may also erode through the cortex into adjacent soft tissues.
Unicystic ameloblastomas are well-demarcated unilocular lesions that are often pericoronal in position. These are commonly found in the posterior mandible, particularly at the molars. They are indistinguishable from other unilocular pericoronal lesions, such as dentigerous cysts, ameloblastic fibromas and odontogenic keratocysts on CT.
In general, ameloblastomas demonstrate a mixed solid and cystic pattern, with a thick irregular wall, often with solid papillary structures projecting into the lesion. These components tend to enhance vividly which is very helpful to distinguish them from other lucent lesions of the mandible.
Treatment and prognosis
Ameloblastomas tend to be treated by surgical en-bloc resection. Local curettage is associated with a high rate of local recurrence (45-90%).
Simple unilocular lesions are less common but have a better prognosis and can be treated by enucleation:
- simple (no nodule) variant will not be diagnosable on radiography, as it will be indistinguishable from other more common cysts
- luminal variant has a single nodule projecting into the cyst
- mural variant has multiple nodules (often only microscopic) in the wall of the cyst. The latter has an elevated risk of recurrence
Malignant behavior is seen in two forms :
- frankly malignant histology
- metastases despite well-differentiated 'benign' histology
General imaging differential considerations include:
- dentigerous cyst: the relationship between ameloblastomas and dentigerous cysts is a controversial one; 20% of ameloblastomas are thought to arise from pre-existing dentigerous cysts
- odontogenic keratocyst (OKC): usually unilocular with thin poorly enhancing walls
- odontogenic myxoma: can be almost indistinguishable
- aneurysmal bone cyst (ABC)
- fibrous dysplasia
- giant cell reparative cysts/granulomas