Malignant transformation of a solitary osteochondroma to chondrosarcoma in a cat

An 8-year-old male neutered Domestic Shorthair cat presented due to left forelimb lameness. Clinical examination revealed a firm, non-mobile protuberance on the craniomedial aspect of the left distal radius. Histological and radiographic findings were consistent with a solitary osteochondroma. Reoccurrence of clinical signs and acute increase in the mass size occurred 1.5 years after diagnosis. A left forequarter amputation was performed due to concerns over malignant transformation. Histology was consistent with chondrosarcoma. Feline osteochondromas are rare, benign osseous neoplasms: they are classified as solitary, multiple or synovial. To the best of the authors' knowledge, this is the first reported case of malignant transformation of a solitary osteochondroma in a cat. In contrast to the belief that solitary asymptomatic osteochondromas have minimal clinical significance in cats, this case report suggests greater vigilance and re-examination for repeat staging after diagnosis of this neoplasm is warranted.