Chondroid and osseous metaplasia occurs throughout the upper aerodigestive tract and must be distinguished from neoplasia. These metaplastic lesions typically develop in areas of chronic inflammation.¹ Metaplasia is especially prone to occur secondary to ill-fitting dentures or in areas of redundant or “flabby” alveolar ridge tissue. Most lesions have bone with overlying hyaline and fibrocartilage that blends into the surrounding fibrous tissue (Fig. 12.1, eFig. 12.1). The chondroid tissue is typically bland and resembles normal, mature cartilage. In rare cases cellular atypia may raise concerns of chondrosarcoma. Chondroid metaplasia of the true or false vocal cords also can occur, and, as was discussed in Chapter 1, cartilaginous nodules are normally seen in the anterior portion of the thyroarytenoid ligament. Finally, cartilaginous or osseous metaplasia often develops in sarcomatoid carcinomas, and one should exclude a concomitant malignancy when either is present.

Chondromas are much less common than well-differentiated chondrosarcomas and one should be wary about making this diagnosis.² In the upper aerodigestive tract, “chondroma” may best be used for small, circumscribed lesions that are fully excised. Indeed, it may be more prudent to diagnose lesions nonspecifically as well-differentiated chondroid neoplasms when only a small portion of mature-appearing cartilaginous tissue is seen in a biopsy specimen. Chondromas are composed of central, well-circumscribed hyaline cartilage devoid of cytologic atypia with rare or absent mitotic figures (eFig. 12.2). They are surrounded by a thin layer of fibrous tissue. These lesions are benign and should generally not recur after resection.

Osteomas can develop throughout the upper aerodigestive tract, often associated with adjacent bone. A unique osteoma referred to by some as a “choristoma” occurs in the dorsum of the tongue, most commonly in young to middle-aged women.³ These vary in size, but do not generally grow to more than 2 cm. They are usually well-circumscribed and are composed of dense, mature, laminated bone (Fig. 12.2, eFig. 12.3). They do not recur after resection.

Giant cell reparative granuloma may be central and entirely located within mandible or maxilla.⁴,⁵ It not infrequently, however, involves the nasal cavity or paranasal sinuses and can be sampled as a nasal or sinonasal mass. These lesions are thought to be reactive and secondary
to trauma, hemorrhage, or inflammation, although a neoplastic origin cannot be excluded. Patients are typically younger than 30-years. Histologically, aggregates of giant cells are seen within a fibrovascular stroma, often associated with hemorrhage (Fig. 12.3, eFig. 12.4). The giant cells tend to be smaller, with fewer nuclei than those seen with
true giant cell tumors. It has been noted that they histologically resemble solid aneurismal bone cysts. These lesions are not associated with USP6 translocations.⁶

Unlike giant cell reparative granuloma, true giant cell tumors are rare within the head and neck. When they arise here, they have most frequently been reported in the sphenoid and temporal bones, or within the larynx.⁷,⁸ Within the larynx, these tumors present in adults over a wide age range and are much more common in men. Most have arisen from the thyroid or cricoid cartilages, and patients typically present with hoarseness or obstruction. Tumors have averaged approximately 4 cm in greatest dimension and are grossly infiltrative, sometimes with associated hemorrhage or cystic degeneration. Microscopically, the lesions are identical to those more often seen in the long bones and are composed of innumerable giant cells admixed with macrophages and fibroblasts (Fig. 12.4). The giant cells have numerous (often >20) bland nuclei, similar to those of the surrounding epithelioid to spindled mononuclear cells. Mitotic figure are often noted. Patients with laryngeal giant cell tumors do well, and the tumors do not recur after resection or other treatment. Malignant and aggressive giant cell tumors of the sphenoid have been reported; however, some of these may actually represent giant cell-rich osteosarcoma, as they have been noted in patients with Paget disease and in children.⁸

Fibro-osseous lesions frequently develop within the bones of the jaw. They sometimes involve the sinonasal structures or extend from the alveolar ridge (these are mentioned briefly in the discussion of fibroma within the Chapter 11). As such, they sometimes must be diagnosed with biopsy samples from the upper aerodigestive tract.

Fibrous dysplasia typically presents centrally within the jaw bones but may involve the sinonasal tract.⁹ Depending on its primary site of growth, it can present with facial swelling, visual impairment, or sinonasal obstruction. When the lesions involve multiple noncontiguous bones, the disease is considered polyostotic and McCune-Albright syndrome should be excluded. The lesions typically present in children and young adults and may appear lytic, sclerotic, or mixed, depending on the age of the lesions. These are typically benign and stop growing with skeletal maturation. Classically, fibrous dysplasia is composed of intermixed, irregular fragments of woven bone with a cellular stroma. The woven bone is typically not lined by osteoblasts, and the cellular stroma is composed of bland spindled cells (Fig. 12.5, eFig. 12.5). Some osteoblastic rimming and lamellar bone formation can be seen, and one should not use that feature to exclude a diagnosis of fibrous dysplasia.

Ossifying fibromas, like most fibrous dysplasias, are also usually centrally located in the jaw bones. They too may extend into the sinonasal tract, especially certain variants, such as juvenile psammomatoid ossifying fibroma.⁹,¹⁰,¹¹ Whereas juvenile psammomatoid ossifying fibromas often
occur (as the name implies) in adolescents and young adults (although they also occur in older adults), conventional ossifying fibromas occur in patients who are, on average, in their fourth decade of life. The lesions have variable radiolucency and density, depending on the proportion of the lesion that is bony. These lesions are also benign, although juvenile psammomatoid ossifying fibroma has been noted to be locally aggressive.

Conventional ossifying fibromas are histologically similar to fibrous dysplasia with intermixed stroma and bone or cementum. The stroma varies in cellularity and is composed of bland spindled cells. Mitotic figures may be frequent. The bone and cementum form irregular ossicles, similar to those seen with fibrous dysplasia, however, more abundant osteoblastic rimming is typically seen. The radiographic features can be helpful in distinguishing these two entities, but in some instances both radiographic and microscopic features are overlapping and a diagnosis of “benign fibro-osseous lesion” is appropriate. Juvenile psammomatoid ossifying fibromas are similar and composed of bony spicules and spherules admixed with fibrous stroma. Mineralization or calcification is present as basophilic spheres (psammomalike) within the pink ossicles of bone (Fig. 12.6, eFig. 12.6).

Chondromyxoid fibroma is a benign tumor that has been reported to involve the craniofacial bones and may present as a sinonasal or skull-based mass.¹²,¹³ The tumors typically present in adults. Histologically,
tumors are composed of nodules of bland spindled or stellate cells within a loosely chondroid and myxoid matrix (Fig. 12.7, eFig. 12.7). Well-formed cartilage is almost always absent. Typically, the periphery of the nodules is more cellular than the center (eFig. 12.8). Occasional giant cells can be present. These tumors should be distinguished from chondrosarcomas and chordomas. Neither chondrocytes nor physaliferous cells should be present with chondromyxoid fibroma. Tumors recur after excision in one-third of cases.¹⁴