Ameloblastoma classification

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] ; Associate Editor(s)-in-Chief: Simrat Sarai, M.D. [2], Vamsikrishna Gunnam M.B.B.S [3]

Overview

Ameloblastoma may be classified based on histopathology into six subtypes including follicular, plexiform, acanthomatous, basal cell, granular cell, and desmoplastic. Based on the location, ameloblastoma may be divided into either intra-osseous or extra-osseous. Based on the radiologic features, ameloblastoma may be classified into four groups including solid or multicystic, unicystic, peripheral, and malignant.

Classification

Ameloblastoma may be classified into following subtypes based on the location

• Intraosseous
  • Intraosseous ameloblastoma is locally aggressive
  • Intraosseous ameloblastoma may include the histological subtypes such as follicular, plexiform, acanthomatous, unicystic, granular cell, basal cell, or desmoplastic
• Extra-osseous
  • Extraosseous ameloblastoma may include the histological subtypes such as follicular, plexiform, or basal cell
  • Extraosseous ameloblastoma is benign
  • Extraosseous ameloblastoma is also known as peripheral ameloblastoma

Based on radiology, intraosseous ameloblastoma may be subclassified into two groups which includes the following:

• Solid/multicystic
  • More commonly reoccur
• Unicystic
  • Unlikely to reoccur
  • Classically found in younger individuals

Ameloblastoma may be classified into following subtypes based on the clinicoradiologic into four groups:

• Solid or multicystic
  • Solid ameloblastoma is the most common form of the lesion.
  • Approximately 86% of the ameloblastoma are solid. It has a tendency to be more aggressive than the other types and has a higher incidence of recurrence.
  • Multicystic ameloblastomacan infiltrate into the adjacent tissue and may metastasize and has the ability to recur.
  • It is prevalent in a slightly older age group than the unicystic ameloblastoma.
  • Radiographically, the appearance is generally multilocular or unilocular.
• Unicystic
  • Unicystic ameloblastoma has a large cystic cavity with intraluminal, luminal, or mural proliferation of ameloblastic cells.
  • Unicystic ameloblastoma is a less aggressive variant and it has a low rate of recurrence although lesions showing mural invasion are an exception and should be treated more aggressively.
  • The unicystic ameloblastoma usually appears as a “cystic” lesion with either an intramural or an intraluminal proliferation of the cystic lining.
  • Radiographically, it can resemble a well-circumscribed slow-growing radiolucency.
• Peripheral
  • Histologically, the peripheral ameloblastoma appears similar to the solid ameloblastoma.
  • Peripheral ameloblastoma is uncommon, usually presenting as a painless, non-ulcerated sessile or pedunculated gingival lesion on the alveolar ridge.
  • Peripheral ameloblastoma mostly appears in the alveolar mucosa. It is a soft-tissue version of an ameloblastoma but may also involve the underlying bone.
• Malignant
  • The malignant ameloblastoma is a rare entity. It is defined as an ameloblastoma that has already metastasized but still maintains its classical microscopic features.
  • The WHO classification of odontogenic tumors (2005) defines malignant ameloblastoma as, “an ameloblastoma that metastasizes in spite of a benign histological appearance".
  • Even if metastasis is absent, ameloblastoma with cytological atypia is defined as ameloblastic carcinoma.
  • Thus, malignant ameloblastoma is defined as a retrospective diagnosis that can only be made when metastasis occurs.
  • In majority of cases, it not only maintains the histological characteristics of the parent tumor but also continues to display similarly indolent clinical behavior.

Differentiating features of three different subtypes of ameloblastoma is shown below in a tabular form:

Subtypes of Ameloblastoma	Percentage of Ameloblastoma	Age	Sites affected	Additional features
Multicystic/Conventional Ameloblastoma (also known as Solid Ameloblastoma)	Majority ofameloblastomas are multicystic/conventional ameloblastomas (over 80%)	The tumor is usually seen in middle-aged adults, around the age of 40 years		It is more prevalent among African-American population
Unicystic Ameloblastoma (also known as Cystic Ameloblastoma)	About 20% of all ameloblastomas are unicystic ameloblastomas	It affects men more than women, with 65% of the affected individuals being males It is usually seen in younger individuals with an average age of 25 years	A majority of these tumors occur in the lower jaw (over 90%)	The prognosis for unicystic ameloblastoma is better than conventional ameloblastoma, because they are less aggressive in growth However, they can recur after surgery, like conventional ameloblastomas
Peripheral Ameloblastoma	Generally, around 2% of ameloblastomas are of this type	Peripheral ameloblastoma typically affect individuals aged between 40-60 years Both men and women are equally affected	The common sites of tumor occurrence include: Soft tissues of jaw Gingiva Buccal mucosa	They are generally less aggressive than intraosseous tumors that arise in the bones (intraosseous ameloblastoma tumors)

Based on histology ameloblastoma is classified into six subtypes:

The following six histopathologic subtypes have been identified for ameloblastoma:

• Follicular
• Plexiform
• Acanthomatous
• Granular cell
• Basal cell
• Desmoplastic

Subtypes of Ameloblastoma	Features
Follicular	Histologically, follicular ameloblastoma are the most common type of ameloblastoma. The follicular pattern consist of islands of epithelial cells with a central mass of polyhedral cells or loosely arranged angular cells resembling stellate reticulum, which are surrounded by well organized single layer of cuboidal or tall columnar cells with nuclei placed at the opposite pole of basement membrane resembling pre-ameloblasts. This peripheral cell layer tends to show cytoplasmic vacuolization. Cystic formation is often seen in the center of the epithelial islands due to degeneration of stellate reticulum like cells.
Plexiform	The tumor epithelium is arranged in form of trabeculae which is bound by a layer of cuboidal or columnar cells and stellate reticulum like areas are usually minimal. Cyst formation occurs but is usually due to stromal degeneration rather than cystic change in the epithelium. The stroma consists of loose, vascular sparsely cellular connective tissue.
Acanthomatous	It resembles a typical follicular ameloblastoma except it shows extensive squamous metaplasia sometimes with keratin formation within the epithelial islands.
Granular cell	The tumors are referred to as granular cell ameloblastoma, when the central stellate reticulum cells show extensive granular cell transformation i.e. in the form of sheets of large eosinophilic granular cells. Ultrastructurally, it is seen that the granules consist of pleomorphic, osmiophilic, lysosome like organelles.
Basal cell	This variant shows predominant basaloid pattern consisting of darkly stained cells with minimal cytoplasm and little evidence of palisading at the periphery resembling those seen in basal cell carcinoma.
Desmoplastic	Desmoplastic ameloblastoma is characterized by extensive stromal collagenization or desmoplasia surrounding compressed small/irregular islands of odontogenic epithelium making it a distinct entity. Cyst formation is common and ameloblast like areas are present only in small foci. Calcification in the fibrous stroma and occasional bone formation can also be seen. Histochemical evaluation of the collagen suggests that the dense stroma is not scar tissue but represents active de novo synthesis of extracellular matrix proteins. It typically presents radiographically as a mixed radiolucency and radiopacity mimicking a fibro osseous lesion. In contrast to typical ameloblastoma, this variant frequents the maxilla and the anterior region of the jaws.
Unicystic	Unicystic ameloblastoma is considered as an in-situ or superficially invasive form of ameloblastoma and consists of a single cyst lined by ameloblastic epithelium. It presents clinically similar to a dentigerous cyst and is usually associated with an impacted tooth (usually 3rd molars). They are usually seen in younger patients with an average age of diagnosis being 22 years and generally involve the mandible. It histologically presents as a single cyst lined by ameloblastomatous epithelium and is divided into several subgroups based on pattern and extent of ameloblastomatous proliferation in relation to cyst wall. Unicystic ameloblastoma may be classified into four subtypes based on a classification by Reichart et al. which was modified by Ackermann et al. 1. Luminal unicystic ameloblastoma: It is a cystic lesion lined by epithelium which exhibits columnar differentiation and reverse polarization of the basal cell layer. The connective tissue adjacent to the epithelium often exhibits a uniform, thin band like area of hyalinization. 1.2. Luminal and intraluminal unicystic ameloblastoma: It is the simple type but exhibits one or more nodules projecting into the lumen. No extension is seen into the surrounding connective tissue wall. Occasionally, this form of unicystic ameloblastoma can produce an intraluminal plexiform pattern of odontogenic epithelium that lacks typical ameloblastomatous differentiation and is called as unicystic plexiform ameloblastoma. 1.2.3. Luminal, intraluminal and intramural unicystic ameloblastoma: Here there is occurrence of intramural proliferation of ameloblastoma along with subgroup 1.2 features. 1.3. Luminal and intramural unicystic ameloblastoma: It exhibits a cyst with a luminal lining in combination with intramural nodules of SMA tissues. The intramural ameloblastoma tissue may be seen as an infiltration from the cyst lining or as free islands of follicular ameloblastoma.
Peripheral	Peripheral ameloblastoma present histologically with follicular or plexiform pattern as well as acanthomatous pattern. In most cases, the tumor is well separated from the overlying epithelium but many are confluent with the overlying mucosa.
Other rare varients	Clear cell variant which may contain clear PAS positive cells are localized in the stellate reticulum like areas. Papilliferous keratoameloblastoma show occurrence of areas of ameloblastoma with keratinisation, tumor islands with papilliferous appearance along with cystic areas resembling odontogenic keratocysts. Mucous cells can also be demonstrated rarely in ameloblastoma.