Ameloblastoma is one of the most sinister of odontogenic tumors. It is unencapsu-lated, infiltrates through bone marrow, and requires excision with a wide margin of surrounding bone to prevent recurrence. This tumor is named for its resemblance to ameloblasts, the enamel-forming cells of the tooth germ. This tumor is found mainly in the middle years and occurs chiefly in the body of the mandible; only 15% occur in the maxilla. Small tumors may escape detection unless found on routine dental radiographs. Eventually they expand the jaw and may cause pain or paresthesia. Three distinct types of ameloblastoma exist. The most common type is the solid ameloblastoma, which paradoxically may develop microcystic spaces. It is this type that infiltrates bone and requires aggressive treatment. It typically appears as a multi-locular radiolucent lesion although unilocular tumors do occur. The histologic features of solid ameloblastoma are easily recognized. The tumor cells form islands (the follicular pattern) or anastomosing cords (the plexiform pattern) that are caricatures of the enamel organ. Most peripheral cells are tall, columnar cells resting on a basement membrane that abuts the surrounding fibrous connective tissue stroma. The nuclei of the tall cells migrate to the end most remote from the basement membrane, a property referred to as reverse nuclear polarity. Furthermore, the cytoplasm between the nucleus and the basal end is typically vacuolated. These two features are important identifying characteristics. In the central regions of the epithelial islands or cords, the cells are angular to fusiform to stellate in shape. The resemblance to the stellate reticulum of the epithelial portion of the tooth germ is inescapable. There are histologic variants of ameloblastoma. Acanthomatous, desmoplastic, clear cell, and granular cell variants have been described. A less common type is the unicystic ameloblastoma. It typically appears as a solitary, unilocular radiolucent lesion that may bear no relationship to teeth or alternately may be associated with an unerupted tooth, in which case it radiographically resembles a dentigerous cyst. The epithelium that lines this lesion exhibits the characteristic cytologic features of odontogenic epithelium.
This lesion has a low recurrence rate after simple curettage. In some instances, budding of the tumor cells into the wall of this cystic tumor results in a "mural" tumor. Should the infiltrating tumor cells penetrate the full thickness of the tumor wall and invade surrounding bone, it should be treated as a solid ameloblastoma.
The peripheral ameloblastoma is the third and least common type. It arises from basal cells of the gingival epithelium and clinically appears as a soft tissue mass in the gingival. The peripheral ameloblastoma is the result of neoplastic transformation of basal epithelium in the gingiva and is analogous to cutaneous basal cell carcinoma. The tumor infiltrates underlying connective tissue in a pattern that resembles basal cell carcinoma except that the tumor cells retain odontogenic characteristics. There are rare examples of ameloblastoma in which the tumor cells exhibit cytologic features ordinarily associated with malignancy. Such tumors are referred to as ameloblastic carcinoma and they are capable of metastasis. Whether the rare clear cell odontogenic carcinoma is a separate entity or a clear cell variant of ameloblastoma is unknown. There is little controversy about the treatment of large, solid ameloblastoma. Curettage is so often met with recurrence that most surgeons recommend complete excision with at least a 1.0 cm margin of bone beyond the radiographic edge of the tumor. Because mandibular tumors so often encroach on the inferior border of the mandible, it may not be possible to achieve an adequate margin in which case full-thickness resection is required. For smaller tumors in which a margin is attainable, a lesser procedure that does not interrupt the continuity of the bone may suffice. Some surgeons recommend the tumor bed be treated by peripheral osteotomy, chemical cautery, or cryotherapy as discussed under the section on OKCs.
Large tumors in the posterior maxilla present a special problem because of the presence of the maxillary sinus and proximity of other structures such as the eye. The unicystic variant is ordinarily treated by vigorous curettage unless tumor cells have penetrated the full thickness of the tumor wall, in which case it must be treated more aggressively. Peripheral ameloblastomas are ordinarily recognized when still small and have a good prognosis after simple excision with a margin free of tumor.
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