—CASE REPORT— Histopathological Characteristics of an Ossifying Fibroma Formed in the of a Racehorse

Kazumichi KODAIRA1, Masanori MURANAKA2, Yuji NAITO1, Hirotaka ODE1, Kazuomi OKU1, Toshio NUKADA3, Yoshinari KATAYAMA2* 1Ritto Training Center, Japan Racing Association, Misono 1028, Ritto city, Shiga, 520-3085, 2 Epizootic Research Station, Equine Research Institute, Japan Racing Association, Shiba 1400–4, Shimotsuke city, Tochigi, 329-0412, 3Miho Training Center, Japan Racing Association, Mikoma 2500–2, Miho-mura, Inashiki-gun, Ibaraki, 300-0415, Japan

A 1-year-old male thoroughbred racehorse experienced swelling of the left upper lip. The J. Equine Sci. swelling was attributable to enlargement around the incisive of the interdental space Vol. 21, No. 1 posterior to the third in the left maxilla. Even after two operations to reduce the bulk pp. 7–10, 2010 of the mass, it continued to increase in size. Dyspnea caused by stenosis of the nasal cavity forced us to perform euthanasia, and a pathological examination was conducted. Macroscopic examination of a section of the mass revealed the formation of multiple areas of solid fibrous tissue, and trabeculae within the incisive bone which had displaced the cortical bone. On histology, the mass was composed of trabecular bone-like structures due to the proliferation and aggregation of fibroblasts. Therefore, we diagnosed it as an ossifying fibroma. Equine ossifying fibroma is characterized by development in the , but was formed in the maxilla in this case. Equine ossifying fibroma has not been reported previously in Japan. This is the first case of equine ossifying fibroma identified in Japan. Key words: bone, horse, maxilla, ossifying fibroma

In the World Health Organization International ossifying fibroma in Japan. Here, we summarize a rare Classification of Tumors of Domestic Animals, among case of a horse with an ossifying fibroma formed in the the tumors formed in the of the upper left incisive bone of the maxilla that was examined alimentary system and in bones and joints, osteoma, histopathologically. ossifying fibroma, myxoma, and chondroma are A male thoroughbred horse born in Hokkaido first classified as benign, while osteosarcoma, experienced swelling of the left maxilla at the age of 1 chondrosarcoma, and multilobular bone tumor are year. Over the following 3 months, the swelling became classified as malignant [3, 9]. In addition, fibrous more conspicuous, and caused serious stenosis of the dysplasia and craniomandibular osteopathy are left nasal cavity, which was treated by surgery to partially classified as tumor-like lesions [3]. Osteoma, ossifying reduce the enlarged bone tissue. However, over the fibroma, and fibrous dysplasia, which often develop in next 9 months, the left maxilla again swelled gradually, horses, should be diagnosed carefully because of their leading to dyspnea associated with obstruction of the similar histopathological morphology [9]. Ossifying left nasal cavity. The horse also had difficulty in fibroma has been reported in horses, cattle, cats, dogs, chewing, owing to considerable gingival enlargement antelope, and sheep, and occurs at high incidence in between the incisor and premolar of the left maxilla the of young horses, [1, 2, 4–6, 8, 11]. (Fig. 1). Radiography revealed that the mass was Nevertheless, there have been no reports of equine relatively solid and contained radiopaque trabecular structures (Fig. 2). Another operation was performed This article was accepted February 19, 2010 to remove the tissues that had formed within the *Corresponding author. e-mail: [email protected] enlarged incisive bone, but the mass was too large to 8 K. KODAIRA, M. MURANAKA, Y. NAITO ET AL.

Fig. 1. Clinical appearance of the case. A severely swollen left Fig. 2. Radiograph of the maxilla. Dilatation can be seen in the upper lip can be seen (before the second operation). regional bone of and around the interdental space between the left third incisor and the maxillary bone. Sections 5 mm thick were prepared by slicing in a frontal direction at the arrowed site and were examined histopathologically.

Fig. 4. Histopathological findings regarding the mass. Trabeculae of woven bone are embedded in the fibrous tumor tissue. (Hematoxylin-eosin stain).

Fig. 3. Macroscopic findings on a frontal-direction slice of the mass (arrows in Fig. 2). The tissue appears to be a mixture of soft areas showing gelatinous degeneration (black arrow) and areas with thickened or increased cancellous bone (red arrows).

remove completely. The left maxilla rapidly enlarged again over the following 3 months. Dyspnea caused by Fig. 5. High magnification image of the mass. Fibroblast-like obstruction of the nasal cavity forced us to perform cells are aggregated to form a trabecular structure. euthanasia with an intravenous injection of an excess of Numerous osteoblast-like cells adjoining or shifting from fibroblasts can be seen around the trabeculae. sodium thiopental, and post mortem examination was (Hematoxylin-eosin stain). OSSIFYING FIBROMA IN A RACEHORSE 9 performed. proliferating fibrous tissue as the fibrous area. It is Macroscopic examination revealed a palm-sized mass possible that the gelatinous area was the area of the in the incisive bone between the incisor and premolar surgical reduction, because hemosiderin deposition of the left maxilla. The mass was white and indurated. and edema are histopathological findings often In the frontal direction, in an excised portion of the observed after surgery. interdental space of the enlarged incisive bone We diagnosed this case on the basis of the posterior to the left third incisor, an area of gelatinous histopathological findings as an ossifying fibroma. An soft tissue and another area of thickened or increased osteoma is a tumor that generates collagen fibers and cancellous bone were observed (Fig. 3). Soft X-ray vessels in well-differentiated cancellous or compact imaging of the excised portion of the interdental space bone and its stroma. On the other hand, fibrous of the enlarged incisive bone showed no bony dysplasia is generated from a substrate that consists structures in the area of the gelatinous tissue, while in mainly of collagen fibers and vessels, and shows tissue the surrounding solid area that contained cancellous morphology with scattered, undifferentiated trabecular bone, thickening and multiplication of the trabeculae structures. Ossifying fibroma has morphological of the cancellous bone could be seen. However, the characteristics intermediate between osteoma and compact bone of the incisive bone was thinned by the fibrous dysplasia in terms of histology [10]. In this case, proliferating mass. the bone tissue was woven bone. Well-differentiated On histopathology, the proliferating mass within the lamellar bone was observed in the osteoma. The enlarged incisive bone was seen to consist of both trabeculae arise via metaplasia of fibrous connective fibrous and osseous elements (Fig. 4). In the fibrous tissue and are not typically rimmed by osteoblasts in area, proliferating fibroblast-like spindle cells which fibrous dysplasia. Therefore, we diagnosed ossifying had oval middle-sized nuclei and fusiform slightly fibroma. basophilic cytoplasm were seen in combination with Ossifying fibroma is often reported in horses. In collagen fibers and blood vessels. Few mitotic figures most cases, the tumor develops in the mandible. In were observed. many countries, because ossifying fibromas occur in In the osseous elements, there were irregularly horses as young as 2 to 14 months, this tumor is known shaped osteoid spicules and woven bone rimmed with as equine juvenile mandibular ossifying fibroma [5]. osteoblasts. The fibroblast-like spindle cells underwent Ossifying fibroma is classified as a benign tumor transformation to osteoblasts along the margins of the histologically because it shows low anaplasia, no developing bone spicules (Fig. 5). Cartilage and metastasis, and is characterized by local invasiveness, necrosis were not observed in the mass. Osteoclasts that is, enlarging fibrous bone growth that leads to were clearly observed at the trabecular margin. The destruction of preexisting trabeculae [3]. In this case, fibroblast-like cells were partially aggregated, and some tumor tissue extended into the incisive bone, mild calcification was observed. In addition, the indicating that this ossifying fibroma was relatively marginal trabeculae had numerous osteoclasts. invasive. The gelatinous areas were composed of fibroblast- A preliminary diagnosis of ossifying fibroma can be like cells and collagen fibers with edema. The made from its clinical course, its site of development, fibroblast-like cells showed oval middle-sized nuclei and the age at onset, and the findings on macroscopic were fusiform. Some mitotic figures were seen. These examination, palpation, and radiography, with cells showed morphological features that were the same subsequent definitive diagnosis after biopsy. Total as those of the proliferating fibroblast-like cells in the removal via mandibulectomy is considered ideal for fibrous area. No osseous elements could be seen. tumors of the mandible, and is reported to reduce Toluidine blue staining revealed no mucinous recurrence and improve the outcome [7]. However, as substance that would indicate metachromasia in the in this case, when the tumor develops in the maxilla, edematous connective tissue. Some hemosiderin total resection may be impossible depending on the deposits were observed in the connective tissue. No range and invasiveness of tumor tissue into stellate cells, which resemble myxoma tumor cells, were surrounding tissue, increasing the likelihood of observed. From these histological findings, the recurrence. Therefore, radiotherapy should be gelatinous area was considered to consist of the same combined with tumor tissue reduction surgery. 10 K. KODAIRA, M. MURANAKA, Y. NAITO ET AL.

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