Synovial sarcomas are relatively uncommon malignant tumors in animals arising from synovioblastic mesenchyme, usually in close proximity to synovial structures such as joints, tendons, or bursae. The neoplasm is characterized histologically by being composed of two intermingled cellular components, a synovioblastic or epithelioid component and a fibroblastic component, where the proportions can vary considerably from tumor to tumor. The disease has been well described for humans, but less so in animals where cases have mainly been seen in dogs.2 The clinical characteristics differ between human and canine cases, and probably also compared to avian cases. Previously only one avian case has been reported, in a Moluccan cockatoo.1
A 4-year-old female Senegal parrot (Poicephalus senegalus) was presented at a clinic with a mass on one wing, on the distal humerus surrounding the elbow joint. The skin was slightly ulcerated over the mass due to self-trauma. The mass had been observed for approximately 1 month and was approximately 2 cm across and 1 cm in cross-section. It was firm in consistency and radiographs revealed an infiltration in underlying bone making it inoperable, as amputation was not an option. The bird was euthanatized and a portion of the mass was sent for histopathology.
A >14-year-old male sulphur-crested cockatoo (Cacatua galerita) was presented at a clinic with a mass on one wing, on the distal humerus, close to the elbow joint. The bird held the affected wing somewhat elevated. The mass, approximately 2 cm across, was soft and fluctuated on palpation. Traumatic hematoma was suggested. Three weeks later the mass was somewhat larger. Cytology showed pure blood, with some signs of infection but no suspicion of neoplasia. Enrofloxacin was given. Three weeks later the bird started biting the mass, puncturing it, causing hemorrhage and infection. Two months after initial presentation the bird was weak and dyspneic. The skin was ulcerated from self-trauma and the mass was now larger, involving most of the humerus and on the dorsal aspect firm in consistency. Pus and faintly yellow fluid oozed from the mass. The owner elected euthanasia and a portion of the mass was sent for histopathology.
A 16-year-old breeding female blue-eyed cockatoo (Cacatua ophthalmicus) living with a male was presented at the same clinic as Case 2, with a drooping left wing and a 2.5-cm swelling just distal to the elbow joint. Radiographs showed only soft tissue swelling without any bone involvement. The bird was sent home to rest, separated from the mate as trauma was suspected. The mass was initially only on the lateral side of the wing, but at the second visit 1 month later the mass had expanded in size and was spread proximal and medial, including the elbow joint. The mass was firm and nonpainful on palpation, well attached to underlying bone. The bird was emaciated, weak, and very lethargic and had the previous day started biting the skin over the mass. The owner elected euthanasia, but did not allow necropsy or further histologic examination of the mass. Of course, only a tentative diagnosis, but the similarities to Case 2 makes it very tempting to put synovial sarcoma as first on the list of differentials.
At histologic examination of the submitted neoplasms (Case 1 and 2) are surprisingly similar in appearance. Both consist of sheets of an abundant spindle cell component that stain faintly positive for collagen with van Gieson staining. A large number of reticulin fibers are seen in these cells with reticulin staining. In this fibroblastic tissue are irregular cavitary areas lined by the second component of the tumor, a mainly single layer of polyhedral epithelioid cells, sporadically growing into villous projections. In the cystic spaces a viscous fluid is found that stains positive with periodic acid-Schiff (PAS) stain, probably a synovial-like mucopolysaccharide secreted by the epithelioid cells as described.2
In humans these neoplasms rarely involve the underlying bone or the nearby joint but do metastasize readily, whereas in dogs the tumors tend to be more aggressive, infiltrative, and in time can destroy bone and destroy the adjacent joints or tendons. The clinical course is highly variable but the metastatic potential seems to be less in canine synovial sarcomas (25%) than that in humans. Metastasis occurs in dogs, but late in the clinical course, which can aid in differentiating this type of tumor from an osteosarcoma. Metastasis in humans occur to the lungs, regional lymph nodes and bones.2
The synovial sarcomas presented here as well as the previously presented avian case1 seem to behave like canine cases, with soft tissue, bone, and joint involvement. Presumably metastasis may occur also in psittacines, but unfortunately in these avian cases not all tissues were examined histologically.
Bottom line: Make your pathologist and yourself happy—always necropsy and histopathology!
1. Kennedy FS, Murphy JD, Colbert TL. In: Proceedings of the Association of Avian Veterinarians. 1994:431–433.
2. Pool RR. Synovial sarcoma. In: Moulton JE, ed. Tumors of Domestic Animals, 3rd ed. Berkeley, CA: University of California Press; 1990:134–143.