**3. Classification**

WHO has recently provided the 5th edition of Classification of Head and Neck Tumours. Ameloblastoma classification is almost identical to that of 2017, with one new entity that will be mentioned in further text [4].

Ameloblastoma is primarily divided into five types:


Conventional ameloblastoma, earlier known as multicystic or solid ameloblastoma, is the most common type and comprises about 90% of cases. Clinically, it is a slow growing, benign neoplasm with locally aggressive behavior [3]. It is of vital importance to distinguish radiographic features of ameloblastoma to the earlier mentioned term of multicystic ameloblastoma. Multilocular radiographic presentation of ameloblastoma in no way should be considered as the reason why conventional type was named multicystic in the past classifications. On the contrary, it was reported that ameloblastomas appear equally as multilocular or unilocular radiolucencies [15, 16]. However, opinions about radiographic features contradict and radiographic evaluation alone is in no case sufficient for adequate diagnostics (**Figure 1**). Histologically, a decent number of ameloblastoma variants have been found, such as follicular, plexiform, acanthomatous, desmoplastic, basaloid and granular cell. Plexiform and follicular are the two most prevalent histological patterns. It is worth mentioning that ameloblastoma can simultaneously display both histological patterns [3]. Additionally,

**Figure 1.** *Conventional ameloblastoma of distal part of maxillae.*

desmoplastic ameloblastoma is from 2017 no longer recognized as separate type, but is classified as histological variant because of its distinctive histological appearance. It possesses a pathognomonic histological feature of extensive stromal dysplasia, epithelial islands within a highly collagenous connective tissue, and metaplastic bone formations in some cases [2, 3].

Unicystic type is the second most common ameloblastoma making from 5% to 15% of all cases. This type is most frequently found in younger patients, with different clinical, radiological and histopathological features from conventional type [16]. Unicystic ameloblastomas can be predominantly found in the posterior mandible and are often associatied with an unerupted tooth, resembling dentigerous cyst (**Figure 2**). It is thought to be less aggressive and has a lower recurrence rate, which mainly depends on the histological variant. Luminal and intraluminal variants have a good response to conservative treatment with approximately 10% of recurrence, but conservatively treated mural variant has a high recurrence comparable to that of conventional type [2].

Peripheral or extraosseous ameloblastoma is rare variant that has about 1% ratio among all ameloblastomas [17]. This variant has gone through a terminological

### **Figure 2.**

*Radiological features of unicystic ameloblastoma in the mandible: (a) orthopantomographic image; and (b) CBCT image.*

### *Perspective Chapter: Ameloblastoma – Present and Future Concepts of Managing DOI: http://dx.doi.org/10.5772/intechopen.107403*

evolution from its first appearance in late nineteenth century until 1959, when the term "peripheral ameloblastoma" was used for the first time [18]. Stanley and Krogh [19] introduced this term in their study and from that point on, "epithelial epulis" and "alveolar border ameloblastoma" fell out of favor. This type mostly affects middleaged patients with higher prevalence in the mandible. It is considered to be amenable to conservative surgical therapy, recurring in a small number of cases [2]. From histological point of view, it has a similar pattern to conventional ameloblastoma consisting of ameloblastic epithelium islands [3].

Metastasizing ameloblastoma was defined as a histologically benign type of ameloblastoma which metastasizes to distant sites by WHO classification from 2017 [14]. It is particularly rare type of ameloblastoma and despite its affiliation with benign tumors, it metastasizes to distant sites and makes treatment unpredictable with a high recurrence rate [20]. It is most commonly found in lungs, but other sites, such as brain and kidneys have also been reported [21].

According to the 5th edition of Classification of Head and Neck Tumors by WHO, adenoid ameloblastoma is introduced as a new entity. It is described as epithelial odontogenic tumor with cribriform architecture, ameloblastoma-like component and presence of duct-like structures. It is also characterized by possible presence of dentinoid, ghost and clear cells [22]. The hybrid histological pattern including both ameloblastoma and adenomatoid odontogenic tumor characteristics was reported in approximately 40 cases in the literature [23]. Moreover, adenoid ameloblastoma is considered as more biologically aggressive type with higher recurrence rate than conventional ameloblastoma. In contrast to other ameloblastoma types, BRAF V600E mutations are not present in the adenoid type [23].
