History

A 10-month-old Quarter Horse filly was evaluated because of a large mandibular mass. The mass was readily visible, extending beyond the margins of the lips. The mass was first noticed by the owner when the horse was 4 months old. Treatment for exuberant granulation tissue was initiated by a veterinarian at that time; however, the mass did not respond to the treatment and continued to enlarge to the point of disfigurement. The surface of the mass was ulcerated. Except for the mass, no abnormalities were detected on physical examination. Radiographs of the rostral aspect of the mandible were obtained (Figure 1).

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Lateral (A) and dorsoventral (B) radiographic views of the rostral aspect of the maxilla and mandible of a 10-month-old Quarter Horse filly evaluated because of a mandibular mass. On the dorsoventral radiographic view, the radiopaque structures on either side of the head are rings on the horse's halter.

Citation: Journal of the American Veterinary Medical Association 229, 11; 10.2460/javma.229.11.1727

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Determine whether additional imaging studies are required, or make your diagnosis from Figure 1—then turn the page

Radiographic Findings and Interpretation

An irregular, well-defined osseous mass (13 × 10 × 8 cm), displacing the soft tissues of the rostral aspect of the mandible caudally and ventrally, can be seen (Figure 2). The soft tissues are mildly thick. The caudal margin of the mass extends to the caudal fourth of the mandibular symphysis. However, the mandibular incisors appeared unaffected, and the periodontal ligaments appeared radiographically normal. Radiographic findings were compatible with a nonaggressive, mineralized mass of the rostral aspect of the mandible. Chronic juvenile ossifying fibroma, osteoma, and fibrous dysplasia are fibro-osseous lesions that may affect the mandible in horses.

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The same lateral radiographic image as in Figure 1. The osseous mass is well-defined with mildly irregular, welldefined margins and a well-defined, narrow transition zone compatible with a nonaggressive lesion (arrowheads).

Citation: Journal of the American Veterinary Medical Association 229, 11; 10.2460/javma.229.11.1727

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Comments

On the basis of gross inspection, chronic juvenile ossifying fibroma, osteoma, fibrous dysplasia, squamous cell carcinoma, oral papilloma, myxoma, myxofibrosarcoma, and exuberant granulation tissue should be considered as differential diagnoses.¹ Squamous cell carcinoma, myxoma, and myxofibrosarcoma tend to be aggressive, destructive tumors, whereas oral papilloma tends to have a coalescing, wartlike appearance.¹

In general, radiography is useful in differentiating oral cavity masses. In the horse reported here, all except the fibro-osseous lesions could be immediately ruled out by use of radiography. Moreover, juvenile ossifying fibroma, osteoma, and fibroma (fibrous dysplasia) tend to have distinct radiographic appearances. Juvenile ossifying fibromas usually have soft tissue opacity, with mild to moderate mineralization that replaces normal bone tissue.²

Histologic characterization of a mass is often useful in differentiating fibro-osseous lesions. In cases of juvenile ossifying fibroma, formation of bone by osseous metaplasia develops within a fibromatous border; irregular spicules of osteoid and bone are rimmed with osteoblasts in a fibro-osseous stroma. Osteomas tend to blend into contiguous bony structures, protrude from their source, and maintain a distinct border.² The histologic characterization of osteomas varies. However, they are generally bordered by a layer of connective tissue that surrounds fibrous and bony trabechular layers.² Mandibular and maxillary fibrosarcomas are believed to be exceedingly rare in horses. The mandibular and premaxillary fibro-osseous lesions in young horses previously diagnosed as fibrosarcomas were probably examples of juvenile ossifying fibroma.^3,4

Equine juvenile ossifying fibroma is a proliferative, fibro-osseous mass typically located near the rostral aspect of the mandible or, less commonly, the maxilla of young horses.³ Smaller masses are usually covered with smooth mucosa, whereas larger masses may ulcerate. Disfigurement of the lip and nearby teeth is common as the masses expand. In the horse of this report, rostral mandibulectomy⁴ was performed as described. Incomplete resection of the tumor often results in local recurrence; postoperative radiographs were helpful in confirming the margin of resection and proximity to the mandibular symphysis. The margins of the resected mass were decalcified and examined histologically. The histopathologic diagnosis of the resected mass was consistent with ossifying fibroma. Complete resection of the mass was accomplished with rostral mandibulectomy, and cosmesis was considered good.