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Fig. 4.26. Clear cell odontogenic carcinoma is characterised by Fig. 4.27. At higher magnification, clear cell odontogenic carci-clear cells forming epithelial nests noma is seen to contain clear cells as well as eosinophilic cells

Swelling of the jaw and pain are most often the presenting signs. Surgery and postoperative radiotherapy seem to provide the best results. The tumour may metastasise to regional lymph nodes as well as lungs. Prognosis is poor with almost 50% of the patients failing loco-regionally within the first 2 years of follow-up [190].

ICD-O:9341/1

Clear cell odontogenic carcinoma was initially reported as a clear cell odontogenic tumour [56]. As these lesions not only behave aggressively locally, but may also me-tastasise, the currently used diagnostic term clear cell odontogenic carcinoma appears to be more appropriate [43, 75]. The tumour is mostly seen in elderly patients [5].

The tumour is composed of cells with clear cytoplasm. These cells form nests and strands, intermingled with smaller islands of cells with eosinophilic cytoplasm (Figs. 4.26, 4.27). Also, squamous differentiation has been reported [17]. Cells at the periphery of the nests may show palisading. The clear cells stain positive for glycogen as well as for epithelial markers keratin AE1/AE3, cytokeratin 8/18, cytokeratin 19 and epithelial membrane antigen (EMA) [89].

The anterior mandible appears to be the site of predilection. Metastases are found in lymph nodes as well as in the lungs and the skeleton. Recurrent disease is seen in more than 50% of cases with documented follow-up. Death due to the tumour has also been reported [5, 89].

Differential diagnosis includes metastatic renal cell carcinoma, the clear cell variant of mucoepidermoid carcinoma, and ameloblastoma with clear cells. Met-astatic renal cell carcinoma may be ruled out on clinical grounds. The clear cell variant of muco-epidermoid carcinoma can be identified with stains for mucin production. Differentiation from ameloblastoma with clear cells may be problematic and it has been proposed that these lesions represent the same entity [16]. Clear cell carcinoma of minor salivary gland origin is another differential diagnosis (see Chap. 5).

4.4.4.5 Malignant Epithelial Odontogenic Ghost Cell Tumour

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Malignant epithelial odontogenic ghost cell tumour, also called odontogenic ghost cell carcinoma is a tumour that combines the elements of a benign calcifying odon-togenic cyst with a malignant epithelial component. Only a few cases of this tumour have been reported, thus precluding any conclusions regarding clinicopathologic features. Malignancy has been demonstrated by local aggressive growth and distant metastasis [84]. The tumour apparently arises most often from malignant transformation of a pre-existing benign calcifying odontogenic cyst [81].

4.4.4.6 Odontogenic Sarcoma

ICD-O:9270/3

The WHO discerns between ameloblastic fibrosarcoma, ameloblastic fibrodentino- and fibro-odontosarcoma and odontogenic carcinosarcoma [73, 181]. The amelo-blastic fibrosarcoma consists of malignant connective tissue admixed with epithelium similar to that seen in an ameloblastoma or ameloblastic fibroma [152]. If there is also dentin, this is known as an ameloblastic fibroden-tinosarcoma, and if there is also enamel, it is called ameloblastic fibro-odontosarcoma. This subclassification has no prognostic significance [2]. These tumours may arise

4.4.4.4 Clear Cell

Odontogenic Carcinoma

Table 4.3. Fibro-osseous lesions [13, 181]

de novo or from a pre-existing ameloblastic fibroma or ameloblastic fibro-odontoma [98].

Those extremely rare lesions that combine carcino-matous and sarcomatous elements, but are recognisable as odontogenic from the epithelial component, resemble ameloblastic carcinomas, and have been called odonto-genic carcinosarcoma or odontogenic carcinoma with sarcomatous proliferation [152].

4.5 Fibro-Osseous Lesions

The current classification of maxillofacial fibro-osseous lesions includes fibrous dysplasia, ossifying fibroma and osseous dysplasia [13, 158]. Table 4.3 gives an overview of the various entities in this group.

4.5.1 Fibrous Dysplasia

Fibrous dysplasia is composed of cellular fibrous tissue containing trabeculae of woven bone. It occurs in three clinical subtypes: monostotic, which affects one bone, polyostotic, which affects multiple bones, and Albright's syndrome, in which multiple bone lesions are accompanied by skin hyperpigmentation and endocrine disturbances [13]. Activating missense mutations of the gene encoding the a subunit of the stimulatory G protein are a consistent finding in the various forms of fibrous dysplasia [123].

Craniofacial fibrous dysplasia is usually of the monostotic type [44]. The disease mostly occurs during the first three decades, although cases are occasionally seen at an older age. The maxilla is more often involved than the mandible. In the maxilla, fibrous dysplasia may extend by continuity across suture lines to involve adjacent bones [13].

Fibrous dysplasia shows replacement of the normal bone by moderately cellular fibrous tissue containing irregularly shaped trabeculae consisting of woven bone without rimming osteoblasts that fuse with adjacent bone. Jaw lesions may also show lamellar bone (Fig. 4.28). Sometimes, tiny calcified spherules may be present [158].

Fig. 4.28. In the jaws, fibrous dysplasia may consist of both woven and lamellar bone, as shown in this photomicrograph taken with the use of partly polarised light to enhance the collagenous scaffold of the bone

Fibrous dysplasia has to be distinguished from other lesions characterised by the combination of fibrous tissue and bone: ossifying fibroma, osseous dysplasia, low-grade osteosarcoma and sclerosing osteomyelitis. However, none of these is composed of woven bone trabeculae fusing with adjacent uninvolved bone. Ossifying fibroma and osseous dysplasia both show much variety in appearance of mineralised material and stromal cellu-larity, low-grade osteosarcoma invades through the cortical bone into soft tissues and sclerosing osteomyelitis shows coarse trabeculae of lamellar bone, whereas the intervening stroma is not cellular but oedematous with sprinkled lymphocytes [158].

Fibrous dysplasia clinically presents as a painless swelling of the bone involved. Radiographically, the classical appearance is described as orange-skin or ground-glass radiopacity without defined borders [13]. Usually, fibrous dysplasia is a self-limiting disease. Therefore, treatment is only required if there are problems due to local increase in size of the affected bone. Sometimes, an osteosarcoma may arise in fibrous dysplasia [39].

4.5.2 Ossifying Fibroma

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Ossifying fibroma, formerly also called cemento-ossi-fying fibroma is a well-demarcated lesion composed of fibrocellular tissue and mineralised material of varying appearance. It occurs most often in the 2nd through the 4th decades. The lesion shows a predilection for females, is mostly seen in the posterior mandible [13] and may occur multifocally [70].

Chromosomal abnormalities have been observed in ossifying fibromas [31, 49, 138]. Data are still too scarce to determine their pathogenetic significance.

Fibrous dysplasia

Ossifying fibroma Conventional

Juvenile trabecular Juvenile psammomatoid Osseous dysplasia Periapical osseous dysplasia

Focal osseous dysplasia Florid osseous dysplasia Familial gigantiform cementoma

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