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teeth, but are usually cone-shaped. Histologically, they show the normal arrangement of centrally placed fibro-vascular pulp tissue surrounded by dentin with an outer surface covered by enamel in the crown area and cemen-tum in the root part.

Compound odontoma is one of the more frequently encountered odontogenic lesions [34, 127]. Data on age of occurrence show the same wide range as with complex odontoma: 0.5-73 years of age, which is due to the fact that compound odontomas may also remain unnoticed for a long time.

In contrast with almost all other odontogenic lesions that have the posterior mandible as the preferred site, compound odontomas have a definite predilection for the anterior maxilla [118, 153]. They may cause swelling or disturbed tooth eruption. Radiographical-ly, a radiolucency containing multiple tooth-like radi-opaque structures is seen. Treatment consists of enu-cleation and there is no recurrence.

Compound odontomas may contain areas identical to those of the calcifying odontogenic cyst including ghost cells (see Sect. 4.4.3.6) [22, 61, 80].

4.4.3.5 Odonto-Ameloblastoma

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Odonto-ameloblastoma is a very rare neoplasm that combines the features of ameloblastoma and odonto-ma, including the presence of enamel and dentin [73, 181]. The ameloblastoma component determines clinical presentation and behaviour. Radiographically, the soap-bubble appearance of ameloblastoma is combined with radiopaque masses due to the odontoma component [97].

4.4.3.6 Calcifying

Odontogenic Cyst

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Calcifying odontogenic cysts occur both in the maxilla and in the mandible in equal proportions. They most commonly lie intraosseously, but cases in the gingiva are also seen. The lesion is most commonly found in the 2nd and 3rd decades [19, 20, 173].

In its most simple form, calcifying odontogenic cyst is a cavity with a fibrous wall and an epithelial lining. This epithelial lining closely mimics that seen in unicys-tic ameloblastoma, but, in addition, there are intraepi-thelial eosinophilic ghost cells lacking nuclei that may undergo calcification. Ghost cell masses may also herniate through the basal lamina to reach the underlying stroma where they can act as foreign material and evoke a giant cell reaction (Fig. 4.24). In the fibrous stroma adjacent to the basal epithelial cells, homogenous eosino-

Fig. 4.24. Calcifying odontogenic cyst closely resembles amelo-blastoma, but the presence of large intraepithelial aggregates of ghost cells rules out the latter diagnosis

philic material resembling dentin may be found in varying amounts. Dentin-like material and ghost cells together may form mixed aggregates.

To this simple unicystic structure other features may be added, thus creating different subtypes with different names [173].

The proliferative calcifying odontogenic cyst shows multiple intramural daughter cysts with an epithelial lining similar to the main cyst cavity. The solid (neo-plastic) calcifying odontogenic cyst has been described by a variety of other terms: dentinogenic ghost cell tumour, epithelial odontogenic ghost cell tumour, calcifying ghost cell odontogenic tumour, and cystic calcifying odontogenic tumour [81]. This lesion combines the morphology of an ameloblastoma with intra-epitheli-al and stromal ghost cells with a dentin-like material. The most recent WHO classification proposes the diagnostic designations calcifying cystic odontogenic tumour (ICD-O:9301/0) and dentinogenic ghost cell tumour (ICD-O:9302/0) to discern between the cystic and the solid lesion [181].

Also, calcifying odontogenic cysts may occur in association with other odontogenic tumours, in most instances ameloblastoma and odontoma [124]. All variants of calcifying odontogenic cyst may show melanin pigment [60, 162].

Ghost cells, either intraepithelially or in the stroma, separate calcifying odontogenic cyst from ameloblasto-ma. The solid variant of calcifying odontogenic cyst is similar to craniopharyngioma [7, 111].

Intraosseous calcifying odontogenic cysts cause bony expansion. The peripheral type forms a gingival soft tissue swelling. Radiographically, the lesion shows a lucent appearance when located intraosseously, mostly with variable amounts of radiopacities. Treatment of the calcifying odontogenic cyst consists of enucleation in cases of intraosseous location or excision for peripheral ones.

Recurrences have been documented for both the cystic and the solid variant [124, 143]. If the lesion is combined with an ameloblastoma, this latter component dictates the most appropriate therapy, which consists of surgical excision including a margin of healthy tissue as already discussed [124].

4.4.4 Odontogenic Tumours -Malignant

Both odontogenic epithelium as well as odontogenic mesenchyme may show neoplastic degeneration, causing either odontogenic carcinomas or odontogenic sarcomas [73, 159, 181]. All entities to be mentioned show the clinical presentation and course as well as the radiographic appearance of an intraosseous malignant tumour.

Fig. 4.25. Ameloblastic carcinoma combines the presence of epithelial nests with peripheral palisading and cytonuclear atypia

4.4.4.2 Ameloblastic Carcinoma

4.4.4.1 Malignant Ameloblastoma

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Malignant (metastasising) ameloblastoma is an am-eloblastoma that metastasises in spite of an innocuous histologic appearance. The primary tumour shows no specific features that are different from ameloblasto-mas that do not metastasise. Therefore, this diagnosis can only be made in retrospect, after the occurrence of metastatic deposits. It is thus clinical behaviour and not histology that justifies a diagnosis of malignant amelo-blastoma [181]. This definition profoundly differs from that given in the previous WHO classification [73]. At that time a malignant ameloblastoma was described as a neoplasm in which the pattern of an ameloblastoma is combined with cytological features of malignancy, a definition not based on behaviour but on histology. It is obvious that disparate views on what represents a malignant ameloblastoma can give rise to confusion [41, 159]. It has to be emphasised that to avoid mixing up different entities, the term malignant ameloblas-toma should be reserved for metastasising ameloblas-tomas whereas the ameloblastomas with atypia should be called ameloblastic carcinomas, a type of lesion to be discussed in Sect. 4.4.4.2 [104]. Confusion may also arise through the use of the term atypical ameloblastomas to denote lesions with a fatal outcome for various reasons, either metastasis, histologic atypia or relentless local spread [3].

Metastatic deposits of malignant ameloblastomas are mostly seen in the lung [76, 77, 159]. Apart from metastasis, malignant ameloblastoma shows no features that are different from conventional ameloblastoma (see Sect. 4.4.1.1).

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Ameloblastic carcinoma, an entity that has only recently been recognised, is a lesion with the histologic features of both ameloblastoma and squamous cell carcinoma [42, 181]. This tumour may arise de novo or from a pre-existing benign odontogenic tumour or cyst [159].

Most cases of ameloblastic carcinoma occur in the mandible [30]. They show a wide age range with a mean of 30.1 years [30].

Ameloblastic carcinoma is characterised by cells that, although mimicking the architectural pattern of ame-loblastoma, exhibit pronounced cytological atypia and mitotic activity, thus allowing the distinction between ameloblastic carcinoma and ameloblastoma (Fig. 4.25). Metastatic lesions are described in the lungs and in the lymph nodes [36, 151].

4.4.4.3 Primary

Intraosseous Carcinoma

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Primary intraosseous carcinoma is a squamous cell carcinoma arising within the jaw, having no initial connection with the oral mucosa, and presumably developing from residues of the odontogenic epithelium [73, 181]. It ranges from well to poorly differentiated [42], mainly occurs in the posterior mandible and is more often seen in males [190].

The tumour may arise from a still recognisable precursor lesion such as the epithelial lining of an odontogenic cyst [67, 90]. Also, enamel epithelium has been documented as a tissue of origin [154].

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