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.