Malignant transformation of an aneurysmal bone cyst in a dog

OBJECTIVE: To report malignant transformation of an aneurysmal bone cyst in a dog. STUDY DESIGN: Case report. SAMPLE POPULATION: Client-owned dog. RESULTS: A 5-year-old male Labrador retriever was diagnosed with an aneurysmal bone cyst of the distal aspect of the right ulna. Surgical curettage resulted in remission of clinical signs for 33 months. Because of recurrence of clinical signs, a distal ulnar ostectomy was performed. Histopathologic examination of the excised specimen revealed a moderately differentiated chondrosarcoma at the level of the bone cyst. Limb function was normal for 17 months until a subcutaneous mass, determined to be a chondrosarcoma, occurred at the level of the shoulder. A right forequarter amputation was performed. CONCLUSIONS: Aneurysmal bone cysts are benign but may be transformed into a malignancy after surgical manipulation. Complete surgical excision should be attempted where possible. In this case, distal ulnar ostectomy was an effective means of preserving limb function and preventing local recurrence of chondrosarcoma.