Second presentation
Three years after the initial treatment described above, the horse was presented with an acute onset of forelimb lameness (3/5 AAEP) and a painful swelling of the left proximomedial metacarpal region. Anaesthesia of the distal limb (perineural anaesthesia of the palmar digital nerves, and subsequently an abaxial sesamoid nerve block) was negative. Radiographs showed a spiculated periosteal reaction on the medial aspect of the third metacarpal bone (Fig 5 ), indicative of a reactivation of the lesion. On ultrasonography, a vascularized soft tissue swelling with bone proliferations was observed in the proximal metacarpal region between the second and third metacarpal bone. Because of the risk of complications due to the interrupted cortex, surgical excision of the remaining tumour was not considered a viable treatment option and the horse was referred to Equinox Healthcare GmbH for radiation therapy. Stereotactic radiation therapy was performed under general anaesthesia in three fractions (Mo-We-Fr) consisting of 10 Gy each, with a total dose of 30 Gy. For anaesthesia, the horse was sedated with 0.01 mg/kg butorphanol and 1.1 mg/kg xylazine intravenously. For induction a 14G catheter was placed and 2.2 mg/kg ketamine and 0.1 mg/kg diazepam was injected intravenously. The anaesthesia was maintained with boluses of 0.55 mg/kg xylazine and 1.1 mg/kg ketamine given every 10 minutes. The mean anaesthesia duration was 35 minutes. For radiation therapy planning, an initial computed tomography (CT) scan was performed in left lateral recumbency with the left forelimb extended and placed in a vacuum cushion (BlueBAG³). The CT scan was imported to the treatment planning system (Monaco 64) and the organs at risk and the target volumes (gross tumour volume, clinical target volume and planning target volume) were defined (Fig 6a and b ). The technique used was image guided volumetric modulated arc therapy (IG-VMAT), allowing high precision and accuracy of the dose application. Because of geometrical limitations, the arc was limited to a 200 degree pitch circle in total with a starting angle of 325 degrees. Each radiation fraction, the leg was positioned in the vacuum cushion and the position on the treatment table was adjusted with laser (Fig 6c) . In the second step, a cone-beam CT scan was performed before each treatment for fine adjustments of the position of the target volume. The horse tolerated the general anaesthesia well and all recovery phases were uneventful without assistance. Generally, there is no requirement for bandages following radiation therapy, as the treatment does not induce skin wounds, and the skin does not become more sensitive to ultraviolet light afterwards. A non-steroidal anti-inflammatory treatment (phenylbutazone 2.2 mg/kg bwt, per os) was installed for three weeks until full analgetic effect of the radiation therapy was obtained. Radiation therapy was well tolerated without noticeable side effects. Four weeks after the radiation therapy, the patient became sound at walk and another month later sound at trot. Follow-up radiographs were obtained every six months after treatment and revealed stable appearance of the bone surface (Fig 7 ). At the veterinary follow-up, eighteen months after the radiation therapy treatment, the horse was still sound at walk and trot.