A 2-year-old umbrella cockatoo (Cacatua alba) was presented with a 1-month history of swelling in the ventral proximal radius/ulna region of the right wing. The owner attributed the initial cause of the swelling to a fall the bird sustained in its cage. The owner noticed no signs of pain, but the affected area continued to increase in size over the month-long period. Upon presentation there was a 4 cm × 4 cm mass on the ventral surface of the right wing at the humero-radial/ulnar joint. All other parameters measured on the external physical examination were within normal limits.
A series of diagnostic tests were performed including a complete blood count (CBC), plasma biochemical analysis, full body radiographs and a cytologic evaluation of a fine needle mass aspirate. Results of the CBC (reference ranges in parenthesis) were leukocytosis, 20.4×103 cells/µl (5–12×103 cells/µl) and monocytosis 15% (0–1%).1 All other parameters on the CBC and plasma chemistry panel were within normal limits. Full body radiographs were then taken; at which time a fine needle aspirate (FNA) was obtained from the center of the wing mass. Cytologic examination of the FNA revealed low numbers of erythrocytes and an occasional cluster of smudged nuclei, but no evidence of microorganisms, inflammatory or neoplastic cells.
The initial radiographic interpretation stated that the affected wing demonstrated a highly aggressive, lytic, and mildly proliferative bone lesion in the proximal aspect of the radius. Some calcified material was also present within the soft tissues. Final radiographic interpretation of the wing mass, apparent aggressive bone neoplasia of the left wing. The mass was surgically removed and submitted for histopathologic evaluation. When removing the mass it was noted that there was a thin, 2 mm osseous shell to the mass, with a center filled with the blood tinged transudate. Postoperative recovery was uneventful.
The results of the histopathologic examination showed bone sections predominated by a sheet of loosely associated plump fibroblast and collagen fibers. There were areas of both endochondral and intramembranous ossification associated with the connective tissue stroma. Microscopic examination revealed an extensive connective tissue stroma with foci of new bone production. The cause of the lesion is suggestive of callus formation or other causes of bone production such as a bone cyst.
A second radiographic interpretation of this case, 2 weeks after the initial surgery, again supported the appearance of a primary bone tumor such as an osteosarcoma, chondrosarcoma or fibrosarcoma. Subsequent biopsy samples were taken after a regrowth of the cyst and the proximal radius was removed. The results of the pathologic interpretation were consistent with that of a post-traumatic callus formation. On four different occasions over a 60-day period, multiple samples of the cysts and radius were submitted for histopathologic evaluation and neoplastic tissue was never identified.
Two other cockatiel (Nymphicus hollandicus) cases were presented with the same clinical signs, a fast-growing subcutaneous mass. The mass locations on these cockatiel cases were: 1) bilateral distal ventral wings and 2) cranial area. Although there was no confirmation of trauma as an initiating cause of these lesions, these locations are prone to potential initiating trauma. The masses were successfully removed without later regrowth.
These avian periosteal cysts may appear to be primary bone tumors on radiographic interpretation. Rare and unusual bone cyst formations in other animal species may also be misdiagnosed as primary osteosarcoma on clinical examination including radiology.2 These cases provide supportive evidence that psittacine species may be susceptible to post-traumatic bone cyst formation. Differentiation between a primary bone tumor and periosteal bone cyst must be made through biopsy evaluation not radiographic interpretation.
1. Altman RB, Clubb SL, Dorrestein GM, Quesenberry K. Appendix 1, Table 1: Hematologic reference ranges of common psittacine species. In: Altman RB, Clubb SL, Dorrestein GM, Quesenberry K, eds. Avian Medicine and Surgery. Philadelphia, PA: W.B. Saunders Co.; 1997:1005–1012.
2. Bindseil E, Schultz M, Kallestrup Sorensen AK. Report of an epidermoid bone cyst in a phalanx of a dog. Vet Rec. 1984;114: 640–641.