Successful remission of a recurrent metacarpal osteosarcoma of a Warmblood gelding after radiation therapy

A 15-year-old Warmblood gelding was presented with recurrence of a histologically confirmed osteosarcoma of the left second and third metacarpal bones 3 years after initial diagnosis and treatment. Initially, surgical excision was performed, followed by local injection of carboplatin 11 weeks later. The horse was sound after surgery and was used for low-level dressage and jumping. Three years later, the horse developed an acute onset of lameness in the left forelimb and a painful soft tissue swelling at the proximomedial metacarpal region. Radiographs showed a spiculated periosteal reaction on the medial aspect of the third metacarpal bone, indicating re-activation of the lesion. Stereotactic radiation therapy was performed with a protocol consisting of three fractions under general anaesthesia (Mo-We-Fr schedule), and a total dose of 30 Gy. The technique used was volumetric-modulated arc therapy guided by cone beam computed tomography for daily positioning. Following the radiation therapy, the horse became sound at walk, and 1 month later, it became sound at trot. Follow-up by veterinary examination 18 months after final treatment revealed no lameness at walk and trot, and a stable radiographic appearance.