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Fig. 4.36. Central giant cell granuloma shows osteoclast-like giant cells in a loose fibrocellular stroma. Cherubism has an identical appearance

Osseous dysplasia lacks encapsulation or demarcation, but tends to merge with the adjacent cortical or medullary bone [13].

The several subtypes of osseous dysplasia are distinguished by clinical and radiological features. Periapical osseous dysplasia occurs in the anterior mandible and involves only a few adjacent teeth. A similar limited lesion occurring in a posterior jaw quadrant is known as focal osseous dysplasia [167]. Florid osseous dysplasia is non-expansile, involves two or more jaw quadrants and occurs in middle-aged black females [13]. Familial gigantiform cementoma is expansile, involves multiple quadrants and occurs at a young age. This type of osseous dysplasia shows an autosomal dominant inheritance with variable expression, but sporadic cases without a history of familial involvement have also been reported [1, 189]. Simple bone cysts may be seen with florid and focal osseous dysplasia [59, 62].

Osseous dysplasia has to be distinguished from ossifying fibroma. Osseous dysplasia is a mixed radiolu-cent-radiodense lesion with ill-defined borders in the tooth-bearing part of the jaws, either localised or occupying large jaw areas depending on the type. In contrast, ossifying fibroma is usually a localised lesion that expands the jaw, and is predominantly radiolucent with radiodense areas [164].

Osseous dysplasia also has to be differentiated from sclerosing osteomyelitis [140]. Sclerotic lamellar bone trabeculae and well-vascularised fibrous tissue with lymphocytes and plasma cells define sclerosing osteomyelitis, whereas cementum-like areas and fibrocellular soft tissue are lacking [53].

The various forms of osseous dysplasia do not require treatment unless necessitated by complications such as infection of sclerotic bone masses, as may occur in florid osseous dysplasia, or facial deformity, as may be seen in familial gigantiform cementoma.

4.6 Giant Cell Lesions

Central giant cell granuloma, cherubism and aneurys-mal bone cyst all show osteoclast-like giant cells lying in a fibroblastic background tissue. The fibroblastic tissue may vary in cellularity from very dense to cell-poor. Mitotic figures may be encountered but are usually not numerous and not atypical. The giant cells mostly cluster in areas of haemorrhage, but they may also lie more dispersed among the lesion (Fig. 4.36). Bone formation, if present, is usually confined to the periphery of the lesion. Radiologically, all three types of giant cell lesions have a lucent, quite often multilocular appearance. Multiple giant cell lesions may occur in association with Noonan's syndrome as well as with neurofibromatosis [38, 134]. Further discussion will only include giant cell granuloma and cherubism as they are confined to the jaws.

4.6.1 Central Giant Cell Granuloma

Central giant cell granuloma is mostly seen before the age of 30. The lesion is restricted to the jaws, the mandible being more often involved than the maxilla. Its aetiology is unknown. Lesions with a histologic appearance identical to that of the central giant cell granuloma may occur in the gingiva and are called giant cell epulis (see Chap. 3). Sometimes, lesions combine the appearance of a giant cell granuloma with that of an odontogenic fibroma [108].

Clinically, central giant cell granuloma manifests itself as a localised jaw swelling. Radiographically, it is a radiolucent lesion that may be either uni- or multilocu-lar. As the lesion is not encapsulated, removal is sometimes followed by recurrence. If there is recurrence, hy-perparathyroidism should be ruled out as the brown tumours associated with this latter disease are identical to giant cell granulomas. Also, distinction has to be made between giant cell granulomas and true giant cell tumours. There has been much discussion whether giant cell granulomas with more aggressive behaviour than usually observed could represent a gnathic manifestation of this latter lesion [4, 163].

4.6.2 Cherubism

In cases of cherubism, two or more jaw quadrants contain lesions histologically similar to giant cell granuloma. The disease occurs in young children, often with a history of other afflicted family members. The genetic defect responsible for cherubism has been localised to chromosome 4p16.3 [112].

The expansion of the affected jaw areas causes the angelic face leading to the lesion's designation: cherubism.

With the onset of puberty, the lesions lose their activity and may mature to fibrous tissue and bone.

There may also be a component consisting of immature odontogenic tissue due to developing tooth germs lying within the lesional tissue. This is a fortuitous finding without any clinical relevance.

4.7 Neoplastic Lesions of the Maxillofacial Bones, Non-Odontogenic

To be included in the following text, lesions should be mainly confined to the maxillofacial bones.

Fig. 4.37. At high magnification, the vacuolated nature of the chordoma cells are clearly visible, as is their epithelial cohesion

4.7.1 Osteoma

ICD-O:9180/0

Osteomas are lesions composed of compact lamellar bone with sparse marrow cavities filled with fatty or fibrous tissue. In the maxillofacial skeleton, they most commonly occur in the frontal and ethmoid sinus; less often, the maxillary antrum and the sphenoid sinus are involved [137]. They may also occur in the jaw bones as a manifestation of Gardner's syndrome [183].

Paranasal osteomas as a group are common lesions [92]. Clinically, they cause sinusitis and headache or other signs of sinonasal disease. Similar bony outgrowths at the palate or mandible are called tori.

4.7.2 Chordoma

ICD-O:9370/3

Chordomas are malignant tumours derived from embryonic remnants of the notochord. The mostly occur at either the cranial or caudal end of the vertebral column [57]. Chordomas show a slight male predominance; they may occur at any age [57].

Three different types of chordoma are discerned: conventional, chondroid, and dedifferentiated. Conventional chordoma consists of lobules separated from each other by fibrous bands. These lobules contain ovoid cells with small, dark nuclei and homogeneous eosinophilic cytoplasm. Other cells show large vesicular nuclei and abundant cytoplasm with vacuoles. Sometimes, these cells contain only one single vacuole causing a signet-ring appearance or vacuoles surrounding the nucleus: these latter cells represent the so-called physaliphorous cells thought to be pathog-nomonic for chordoma. In general, the cell density is maximal at the periphery of the lobules; more centrally, the cells lose their epithelial cohesion and may lie isolated in an abundant mucoid matrix (Fig. 4.37). Although there may be atypia, mitotic figures are infre quent. The lesion invades adjacent structures. Immu-nohistochemically, chordoma is characterised by posi-tivity for S-100 as well as vimentin and a broad variety of epithelial markers [131].

Chondroid chordoma (ICD-O:9371/3) denotes avariant of chordoma that contains cartilaginous areas indistinguishable from chondrosarcoma [57]. However, chondrocytic differentiation in chordomas probably represents a focal maturation process [51]. Neither has the distinction between conventional and chondroid chordoma any clinical significance [29]. Therefore, it is advocated that this designation be dropped. Dedifferen-tiated chordomas are those lesions that contain areas of chordoma as well as an additional malignant mesenchy-mal component that may be a fibrosarcoma, an osteosarcoma or, most likely, a poorly differentiated sarcoma [64].

Chordoma has to be distinguished from chondro-sarcoma. Positivity for epithelial markers is a consistent feature in chordomas and is absent in chondrosarco-ma [132]. Other look-alikes, such as extraskeletal myx-oid chondrosarcoma, myxoid liposarcoma and myxo-papillary ependymoma, also lack positivity for epithelial markers [28]. Chordoid meningiomas may also mimic chordomas, but there are no physaliphorous cells, nor is there positivity for cytokeratins in this meningioma subtype [125].

The differential diagnosis of chordoma should also include pleomorphic adenoma. Both lesions may show epithelial clusters as well as single cells with vacuolated cytoplasm lying in a mucoid matrix. Moreover, positivity for S-100, vimentin, and epithelial markers is displayed by both. Positivity for myoepithelial markers, however, is restricted to pleomorphic adenoma.

Chordomas manifest by destroying adjacent structures resulting in cranial nerve dysfunction. Rarely, they cause a swelling in the neck due to lateral growth. Their site precludes radical surgical treatment. Mostly, thera-

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