15-year-old, castrated male, mixed breed, (Canis familiaris) dogA 15-year-old castrated male mongrel dog developed a mass in the left hemimandible around the first molar tooth. The owner reported that the dog pawed at its mouth. Two 1 cm x 2 cm incisional biopsy specimens from the buccal and lingual aspects of the mass were processed en toto for histologic examination by a reference laboratory; the diagnosis was osteosarcoma. One month later, the dog was admitted to the Purdue University Veterinary Teaching Hospital for total left hemimandibulectomy.

Gross Description:  

A left hemimandibular surgical specimen containing the entire mass had its margins painted prior to immersion in 10% neutral buffered formalin and submission to the Purdue University Animal Disease Diagnostic Laboratory (ADDL), where the specimen was transferred to a formic acid decalcifying solution. A firm to hard fibrous and bony mandibular mass surrounded the neck and roots of the first molar tooth and measured about 2.5 cm from rostral to caudal margins and 2 cm from medial to lateral aspects (Figs. 2-1, 2-2). The mass on cross-section consisted mostly of hard, white tissue that infiltrated alveolar and cortical bone and adjacent soft tissue (Fig. 2-3).

Histopathologic Description:

The tumor consisted of a spindle-cell proliferation resembling periodontal stroma that appeared to be centered midway between the neck of the tooth and its apex. At its apparent site of origin, the tumor had provoked osteoclastic destruction of alveolar bone, adjacent cortical compacta, periodontal ligament and bone of the alveolar crest. Symmetric growth of the mass expanded the lingual and buccal borders of the hemimandible, again by stimulating osteoclastic removal of the cortical compacta at a rate that allowed development of a thin, incomplete shell of periosteal new bone that partially contained the tumor. At the gingival sulcus, the incomplete and partially resorbed periosteal shell of reactive bone nearly abutted the junctional gingival epithelium. Upward expansion of the tumor into gingival lamina propria led to ulceration and granulation tissue formation. On the buccal surface, the tumor was also partially bound by a thin periosteal shell of reactive bone that ended at the former level of the alveolar crest, which had been replaced by neoplastic tissue. Here, the tumor extended above the level of the periosteal reaction into the gingiva. Neoplastic tissue was composed of fusiform cells in scanty fibrous stroma with light but diffuse infiltration by neutrophils. The fusiform cells had an elongated oval nucleus, small nucleolus, no mitotic figures in 15 high-power fields, and scanty pale eosinophilic cytoplasm with indistinct cell borders (Fig. 2-5). The stroma was moderately vascular with numerous irregular trabeculae of osteoid and partially mineralized woven bone. Bony trabeculae were bordered by one layer of osteoblasts. A few osteoclasts were adjacent to bony spicules. There was little fibrous collagen in tumoral stroma; most of the Massons trichrome-stained collagen was in the bony trabeculae. A preliminary diagnosis of ossifying fibroma was reported with the final diagnosis to follow examination of remaining (central) tissue.

Sections of the fully decalcified central portion of the tumor were histologically similar to the initial peripheral sections, except that 1 to 3 mitotic figures were found per ten high-power fields. Neoplastic tissue was not found in soft tissue ventral to the hemimandible or in mandibular or soft tissue caudal to the mass. Histologic impression was complete excision of an ossifying fibroma.

Morphologic Diagnosis:  

Mandibular ossifying fibroma

Lab Results:  

No abnormalities were detected in the available lateral radiographic view of the skull because of superimposition of the hemimandibles. However, in the computed tomographic (CT) scan, an expansile and lytic lesion, about 1.8 cm in width and 2 cm from rostral to caudal borders, was evident in the dorsal aspect of the left hemimandible, surrounding the neck and roots of the first molar tooth (Fig. 2-4). The mass destroyed alveolar and cortical bone, but had well-defined borders with a short transition zone. There was slight swelling, but no postcontrast enhancement of adjacent soft tissues. Thoracic and abdominal radiographs were within normal limits and free of evidence of metastatic neoplasia.


Ossifying fibroma

Contributor Comment:  

The case was reported as a brief communication in Vet Pathol.1 Benign fibro-osseous proliferations of bone in veterinary species include ossifying fibroma, osteoma, and fibrous dysplasia.2,3 Osteomas are typically solitary osteosclerotic lesions that arise from the surface of bones of the jaw or skull; trabeculae of woven bone constitute the bulk of the tumor, are rimmed by one layer of well-differentiated osteoblasts and, in many cases, are oriented perpendicular to the surface of the tumor.3 Fibrous dysplasia 3 is a tumorlike lesion that can involve one or multiple bones, often in young animals. It arises within the bone, rather than from the periosteal surface, and its ample fibrous stroma contains only thin, curved trabeculae of woven bone. The bony trabeculae are generally not rimmed by osteoblasts, which distinguishes it from ossifying fibroma or osteoma, and are regularly spaced but without orientation relative to the periosteal surface.

Ossifying fibroma has histologic features that are intermediate between those of osteoma and fibrous dysplasia, although there can be overlap among the three entities.2 Ossifying fibroma is an expansile, lytic, and invasive mass that develops within the bone, particularly the mandible. Its bony trabeculae are rimmed by osteoblasts as in osteoma, but are arranged haphazardly and contribute relatively less to the fibro-osseous stroma.

Importantly, from a prognostic perspective, ossifying fibroma must be differentiated from malignant tumors, such as osteosarcoma. That distinction can be based on the lower cellularity, bland cytologic features, and low mitotic index of ossifying fibroma. Furthermore, bony trabeculae of ossifying fibroma tend to be better developed than in osteosarcoma and are bordered by a single layer of osteoblasts that are distinct from the tumor cells. However, histologic examination of excisional biopsy specimens and knowledge of the anatomic location of the tumor may be necessary for accurate diagnosis.

JPC Diagnosis:  

Gingiva, tooth, and alveolar and cortical bone: Ossifying fibroma

Conference Comment:  

Ossifying fibromas are most commonly reported in young horses, generally less than one year of age, and usually present as a protruding mass from the rostral mandible.4 There have also been reported cases in cats, dogs, and sheep.4 In horses, the differential diagnosis for ossifying fibroma includes fibrous osteodystrophy, fibrous dysplasia, osteoma, and osteosarcoma.4 Fibrous osteodystrophy presents as a symmetrical, bilateral lesion with numerous osteoclasts.4 In fibrous dysplasia, bone spicules are not rimmed by osteoblasts and are more uniform.4 Osteomas are composed of more normal appearing bone, while osteosarcomas have invasive, pleomorphic cells with a higher mitotic index.4 Osteomas are most common in horses and cattle and have been reported as large as 14 cm in diameter.3

Many tumors diagnosed as osteomas in dogs are actually multilobular tumors of bone.3 Histologically, the multilobular tumor of bone consists of multiple vari-shaped nodules of bone and/or cartilage at various stages of differentiation separated by a fibrovascular stroma.2,4 These tumors are slow growing but can be invasive and may metastasize.4 They have been reported in dogs, cats, and horses.4 Osteomas are dense masses of welldifferentiated bone that protrude from bone surfaces.


1. Miller MA, Towle HAM, Heng HG, Greenberg CB, Pool RR: Mandibular ossifying fibroma in a dog. Vet Pathol 45:203-206, 2008
2. Slayter MV, Boosinger TR, Pool RR, D+�-�mmrich K, Misdorp W, Larsen S: Benign tumors. In: Histological Classification of Bone and Joint Tumors of Domestic Animals, 2nd series, vol. 1, p. 5-7. Armed Forces Institute of Pathology, Washington, DC, 1994
3. Thomson KG, Pool RR: Benign tumors of bone. In: Tumors in Domestic Animals, ed. Meuten DJ, 4th ed., pp. 248-255. Iowa State Press, Ames, IA, 2002
4. Thompson K: Bones and joints. In: Jubb, Kennedy and Palmers Pathology of Domestic Animals, vol 1 ed. Maxie MG, 5th ed., pp.110-124. Elsevier, Philadelphia, PA, 2007

Click the slide to view.

2-1. Mandible, dog.

2-2. Mandible, dog

2-3. Mandible, dog

2-4. Mandible, dog

2-5. Mandible, dog.

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