History

A 1-year-old 195-kg (429-lb) Quarter Horse colt, procured for resale purposes, was evaluated at the Auburn University Large Animal Teaching Hospital because of continuous bilateral, mucopurulent nasal discharge accompanied by respiratory noise of 6 months' duration. The respiratory noise had worsened during the week prior to the evaluation. The colt had been treated intermittently with systemic broad-spectrum antimicrobials without a positive response.

Clinical and Gross Findings

At the evaluation, the colt was bright, alert, and responsive; heart rate, respiratory rate, and rectal temperature were within reference limits. Bilateral, mucopurulent, malodorous nasal discharge was present with dyspnea and respiratory noise. Mild facial deformation primarily on the left side, at the level of the conchofrontal sinus, was evident. No airflow could be detected from the left nostril. Endoscopy of the upper portion of the respiratory tract was not performed because of worsening of the dyspnea after sedation and fractious behavior of the colt. Radiography of the skull revealed a large area of increased opacity associated with the left nasal passage and left paranasal sinuses region. The nasal septum was deviated axially, partially occluding the right side. On the lateral radiographic view, the margins of the increased opacity were poorly defined and extended over the maxillary and conchal sinuses. No fluid line was noted. Centesis of the left caudal maxillary sinus was performed, and an inspissated, purulent aspirate sample was collected. Cytologic evaluation of the aspirate sample revealed a large number of moderately degenerative neutrophils with the presence of intra- and extracellular bacteria of mixed population. Additional diagnostic imaging techniques were recommended but declined by the owner owing to financial constraints. Exploratory sinusotomy was recommended. The colt was anesthetized and placed in lateral recumbency, and a left frontonasal sinusotomy was performed. A white, firm, nonfriable, mineralized mass that completely filled the ventral conchal, dorsal conchal, rostral maxillary, and frontal sinuses was found. No apparent hemorrhage was present on the mass during the surgical procedure. Evaluation of the caudal maxillary sinus was not possible because of the distortion of the normal architecture of the left paranasal sinuses. Given the location and extent of the lesion, the colt's prognosis was considered poor and the owner elected for intraoperative euthanasia.

Complete necropsy of the colt revealed an 8.0 × 9.5 × 15.0-cm, multilobulated bony mass that extended from, but did not breech, the cribriform plate in a rostral direction to the level of the 210 tooth (Triadan system) and that filled the left rostral and caudal maxillary and conchal sinuses, as well as the left nasal passage (Figure 1). The nasal turbinates and septum could not be identified because the mass occupied the full width of the left respiratory passage. Although not noticeable at surgery, areas of hemorrhage and necrosis were interspersed throughout the mass. There was diffuse submucosal laryngeal and epiglottal edema. No other major macroscopic changes were identified during the necropsy.

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Photograph of a longitudinal section of the left nasal passage of a male Quarter Horse yearling that had continuous bilateral, mucopurulent nasal discharge accompanied by respiratory noise of 6 months' duration and was subsequently euthanized on the basis of diagnostic findings and poor prognosis. Notice that a firm, white, multilobulated mass has completely filled the left nasal passage, destroyed the nasal turbinates, elevated the overlying frontal bone, and compressed the surrounding soft tissues.

Citation: Journal of the American Veterinary Medical Association 248, 7; 10.2460/javma.248.7.773

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Formulate differential diagnoses from the history, clinical findings, and Figure 1—then turn the page →

Histopathologic Findings

The left aspect of the upper respiratory tract (left nasal passage and left frontal and maxillary sinuses) was filled and obliterated by a poorly delineated, densely cellular, infiltrative mass composed of solid sheets and interwoven bundles of moderately pleomorphic spindle cells supported by a delicate fibrous stroma (Figure 2). Cell diameters ranged from 15 to 25 μm, and there was a nuclear-to-cytoplasmic ratio of 4:1. Individual cells had a scant amount of pale eosinophilic, streaming cytoplasm, and cytoplasmic borders blended imperceptibly with the adjacent cells. Extracellular spaces were frequently filled with homogeneous, pale to intensely eosinophilic osteoid. Nuclei were round to oval with finely stippled, marginated chromatin and 1 or 2 distinct basophilic nucleoli. Three mitotic figures were observed in 10 contiguous hpfs (400X). Bony trabeculae that had scalloped, irregular contours and were surrounded by osteoclasts were interspersed throughout the mass. In addition, small areas of hemorrhage with scattered hemosiderin-laden macrophages, edema, and small aggregates of attenuated or degenerating respiratory tract epithelium were also present among the neoplastic cell population.

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Photomicrographs of the paranasal mass from the horse in Figure 1. A—The densely cellular and infiltrative mass is composed of solid sheets and intersecting bundles of moderately pleomorphic spindle cells. Few turbinate bony septae (asterisks) remain, and the neoplastic cell population is separated from the overlying respiratory tract epithelium by a band of dense fibrocollagen. H&E stain; bar = 200 μm. B—Moderately pleomorphic spindle cells have produced extracellular matrix consistent with osteoid (arrowhead). H&E stain; bar =100 μm. C—There is extensive osteoclastic (arrow) resorption of the few remaining nasal turbinate bony trabeculae (which have scalloped, irregular contours) embedded throughout the neoplastic cell population. H&E stain; bar = 100 μm.

Citation: Journal of the American Veterinary Medical Association 248, 7; 10.2460/javma.248.7.773

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Morphologic Diagnosis and Case Summary

Morphologic diagnosis and case summary: poorly productive, osteoblastic osteosarcoma of the left paranasal sinuses in a 1-year-old colt.

Comments

Primary malignant bone tumors of the equine nasal, maxillary, and frontal bones are rare.¹ Although osteosarcomas are the most common bone tumor in humans, dogs, and cats (comprising 80% and 70% of malignant bone tumors in dogs and cats, respectively),² osteosarcoma is a rare tumor in horses. In horses, osteosarcomas have been mainly identified in the head and, more specifically, in the mandible of young horses.³ A few case reports have described osteosarcomas within the caudal maxillary sinus,³ nasal cavity,^3,4 appendicular skeleton,^3,5,6 and vertebral column.⁷ The etiopathogenesis of osteosarcomas is unknown, although trauma, viral infection, exposure to radiation, and genetic factors have been hypothesized as predisposing factors. Trauma as a predisposing factor is supported by the fact that most osteosarcomas in humans and small animals develop in weight-bearing bones such as the proximal portion of the humerus, distal portion of the radius, distal portion of the femur, and proximal portion of the tibia.^6,7 It is postulated that osteosarcomas develop in the mandible of large animal species because of the trauma associated with persistent chewing.⁷

Osteosarcomas are malignant tumors that produce osteoid (ie, unmineralized bone matrix), which may mineralize to produce new bone.⁸ These tumors grow by infiltration with extensive destruction of the surrounding tissue.¹ Histologically, osteosarcomas vary in differentiation and compactness but generally consist of proliferating lamellar bone and intratrabecular fatty and hematopoietic marrow.⁹ Although the tumor in the case described in this report was histologically classified as poorly productive osteoblastic, osteosarcomas in horses are often fibroblastic.³ Because of the small number of reported cases of equine osteosarcoma, lack of attempted treatments, and high frequency of euthanasia among affected animals, it is difficult to assess any correlation between histologic subtype and prognosis. Differential diagnoses for sinus neoplasia should include benign cyst, osteoma, chondroma, chondrosarcoma, fibroma, fibrosarcoma, osteomyelitis, osteosarcoma, osteochondroma, spindle cell sarcoma, mastocytoma, hemangiosarcoma, angiosarcoma, and lymphosarcoma.^1,10–19

In horses, paranasal sinus tumors are commonly treated by surgical excision, with the addition of ancillary treatments such as radiotherapy and chemotherapy as required.^20,21 Resection of benign lesions (eg, osteomas) via a frontonasal flap procedure can be associated with a good long-term prognosis^1,15,22; however, the aggressive nature and the complex anatomic location of most sinonasal tumors usually prevent complete resection, and consequently, affected horses have a poor prognosis following surgical treatment of these lesions.¹ With regard to malignant tumors, excision alone has also been associated with a high rate of recurrence and a poor prognosis because of the tumors' invasive nature and advanced stage at the time of diagnosis and the limited possibilities for achievement of sufficient excisional margins.¹ However, even when combining excision and radiation therapy to treat osteosarcoma in horses, expectations should be tempered. This combination may only relieve signs of pain, retard the growth rate of this type of neoplastic process,⁸ and be associated with considerable expense. In the colt of the present report, the neoplastic mass had achieved a considerable size, filling the left paranasal sinuses and almost completely obliterating the nasal passage. In view of the tumor's advanced stage at evaluation and its anatomic location resulting in difficult resection, the associated poor prognosis, and the client's financial constraints, no attempt was made to remove the tumor. Further investigations of horses with paranasal osteosarcoma and the efficacy of various alternative treatments are warranted to provide future treatment options. Although rare in horses, osteosarcoma of the paranasal sinuses should be considered as a differential diagnosis in equids with clinical signs of sinus disease, especially when nasal discharge, facial deformation, and respiratory obstruction are present.