Case 180817-1

4-month-old, female Westphalian foal

The foal was presented for removal of a mass associated with the right mandible. Approximately 1.5 months prior, the foal was noted to have sore gums and the mass was observed shortly after. The mass, located caudal to the right incisors, is firm and approximately 5.5 x 3.5 x 3 cm.

A core biopsy of the mass was received in formalin. The specimen was mineralized and required minimal decalcification prior to processing.

Two sections are examined composed of anastomising trabeculae and small spicules of bone that range from lamellar (at the presumed deep margin) to woven (more superficial) separated by wispy fibrous connective tissue with scattered fibroblasts and numerous small to medium caliber blood vessels. Trabeculae at the superficial aspect are lined by plump osteoblasts. The superficial aspect of the biopsy is composed of a thick layer of compacted fibrous connective tissue with numerous fibroblasts. No mitotic figures are noted.

None.

Mass: ossifying fibroma

The mass is composed of well-differentiated, newly formed bone and ample fibrous connective tissue characteristic of an ossifying fibroma. Ossifying fibromas are considered benign though they are expansile and frequently result in destruction of adjacent bone. Other differentials include fibrous dysplasia, which typically lacks the islands of bone lined by osteoblasts noted in this case, or osteoma, which typically lacks the proliferation of fibrous tissue noted here. Fibrous osteodystrophy is considered unlikely considering the observed maturation from woven to lamellar bone and unilaterality of the lesion. A rostral mandibulectomy was performed one week later. No follow up information is available.

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