Author: Sanketh DS, MDS
Unicystic ameloblastoma is considered to be a distinct type of a clinico-radiological and histological presentation of ameloblastoma. This type has been estimated to account for 5 %- 22% of all ameloblastomas reported. Much confusion exists with regards to the terminology used for this lesion. While one group of pathologists prefer to name this entity unicystic ameloblastoma another prefers to refer them as cystic ameloblastoma.
Let’s get some things clear before moving into details of this tumor. Conventional ameloblastoma also refered to as multicystic ameloblastoma is essentially a tumor with multiple islands of tumor cells which may undergo cystic degeneration to form multiple microcysts. Let us try recollecting the histology of multicystic ameloblastoma to understand this. There are either multiple follicular islands undergoing cystic degeneration at the centre of the islands or plexiform networks with the cystic degeneration happening in the connective tissue. This may give rise to the appearance of multiple cysts and hence the name multi-cystic ameloblastoma.
Unicystic ameloblastoma,however, is a cystic tumor, meaning it has the components of a cyst but behaves like a tumor. It is one large cyst with a central lumen, an ameloblastic epithelial lining and a connective tissue capsule.
Three different mechanisms have been proposed for the lesion’s pathogenesis:
1. It has been widely hypothesised to arise from dentigerous cysts and other cysts where the non-neoplastic lining of these cysts undergo an ameloblastic transformation.
2. The reduced enamel epithelium wrapping the developing tooth may undergo an ameloblastic transformation to give rise to a subsequent unicystic ameloblastoma.
3. Multiple microcysts formed in a conventional ameloblastoma may fuse together to form a single cystic cavity to form a unicystic lesion!
Unicystic ameloblastomas are often seen in younger patients with a mean age of 23 years as compared to multicystic ameloblastomas occurring in the third decade. It has an almost equal gender predilection and occurs predominantly in the mandible in the posterior regions (90%). The lesion may cause a local swelling, occasionally pain and signs of lip numbness. Otherwise the lesion is often asymptomatic.
1. This lesion typically occurs as a unilocular lesion, very often surrounding the crown of an unerupted third molar mimicking a dentigerous cyst.
2. It may appear as a unilocular lesion in a “non-dentigerous” relation to the tooth mimicking other cysts depending on the location.
3. Root resorption has been described in numerous cases.
4. Unicystic ameloblastoma ironically, have also been reported to show multi-locular appearences.
Three distinct types of unicystic ameloblastoma have been described.
1. The first type, called luminal unicystic ameloblastoma consists of a fibrous cystic wall with a lumen and an ameloblastic epithelial lining. The lining shows (Vicker’s and Gorlin’s Criteria):
a) a basal layer of cuboidal to columnar cells with a nucleus that is palisaded, hyperchromatic with reverse polarity
b) basilar cytoplasmic vacuolisation
c) sub-epithelial hyalinisation
The superficial epithelial cells resemble the stellate reticulum.
2. The second variant is called intraluminal type with papillary nodules of epithelium projecting into the lumen. These nodules on cross-section, frequently show a plexiform pattern of ameloblastic epithelium. This variant is thus also referred to as plexiform unicystic ameloblastoma.
3. The third variant has follicular or plexiform islands of odontogenic epithelium similar to conventional ameloblastoma in the cystic wall of the lesion. This type is referred to as mural unicystic ameloblastoma.
Even the surgical findings may suggest the lesion removed to be a cyst and requires microscopic analysis for a diagnosis of unicystic ameloblastoma.
These lesions thought to be odontogenic cysts clinico-radiologically are treated by enucleation and are confirmed to be unicystic ameloblastomas only after microscopic analysis. They are less aggressive in behaviour and have much lesser recurrence rates as compared to conventional ameloblastomas and hence are treated with a conservative approach like enucleation and close radiographic observation. Some surgeons believe in a local resection as a prophylactic measure. However, it is agreed that, in the mural variant of the lesion the islands may extend through the fibrous wall and may invade bone. Hence mural unicystic ameloblastomas are treated by local resection of the area of lesion.
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