Clinical communication — Kliniese mededeling Three cases of osteoma and an osseous fibroma of the paranasal sinuses of horses in South Africa

INTRODUCTION Neoplasia of the paranasal sinuses in horses is uncommon. Tumours reported to develop within the sinuses include squamous cell carcinoma, adenocarcinoma, fibrosarcoma, haemangiosarcoma, osteoma, ossifying fibroma and ameloblastic odontoma. The most common neoplasm involving the paranasal sinuses of the horse is squamous cell carcinoma. A group of fibro-osseous lesions, often with overlapping histologic classification, have been reported in the paranasal sinuses of horses. These include osteomas and osseous fibromas. Clinical signs of most paranasal sinus neoplasms are generally related to their size, location and pattern of growth. Typical clinical signs include facial distortion, nasal discharge, and obstruction of normal airflow on the affected side. Diagnosis and extent of masses are usually established on interpretation of radiographs, excisional biopsy, histopathology and more recently computed tomography. Metastases have rarely been recorded. Osteomas are benign, slow-growing tumours involving the maxillary and conchofrontal sinuses. They are uncommon in all domestic species but are more often recognised in horses and cattle, and they have been reported in foals as young as 4 months of age. The clinical signs may, however, only become evident after several years have elapsed owing to their expansile nature. Male horses are predominantly affected. Osteomas are reported to be generally amenable to surgical excision, as they usually have a sessile or pedunculated attachment over a small base and are not infiltrative. Histologically, osteomas consist of initial cancellous bone that may become increasingly compact with time. Soft tissue spaces between the bony trabeculae contain one or more centrally located small-calibre blood vessels, a sparse population of spindle cells and a moderately fibrillar connective tissue matrix, within which adipose tissue and haemopoietic elements may be present. Osseous or ossifying fibromas are large, solitary, expansile lesions that are rare in all species but reported most frequently in horses. They have an apparent predilection for the rostral mandible of the horse. These lesions may replace alveolar and cortical bone, cause loosening and loss of teeth, interfere with mastication of food, and predispose the jaw to pathological fracture. Most cases are reported to occur in animals less than 1 year of age. Osseous fibromas and some cases of osteoma are morphologically similar. There is a possibility that an osseous fibroma may mature into an osteoma. Histologically, osseous fibromas exhibit irregularly-shaped spicules of osteoid and woven bone rimmed with osteoblasts, which are randomly formed in a moderately vascularised, densely-cellular fibroosseous stroma. The proliferative cells resemble fibroblasts that transform into osteoblasts along the margins of developing bone spicules. There is neither cartilage formation nor a recognisable periosteal membrane border. In a few scattered areas, large, well-developed bony trabeculae may occur, as well as osteoclastic resorption and patchy deposition of lamellar bone. In these areas soft tissue between bone spicules may lose cell and fibre density, thus resembling osteoma histologically. This study reports 3 cases of osteoma and 1 osseous fibroma at the Onderstepoort Veterinary Academic Hospital, University of Pretoria, South Africa (OVAH). Three of the 4 cases were presented within the last 2 years.