Introduction
Osteosarcoma is a highly malignant primary bone tumour which affects the
primitive bone-forming mesenchymal cells. These tumour cells produce
osteoid or immature bone within the medullary cavity and cause
significant local tissue destruction due to bone lysis, bone production
or both. In dogs and cats, osteosarcoma is the most common primary bone
tumour and the incidence and clinical behaviour are well documented.
They occur mostly in the appendicular skeleton and can metastasise to
other sites in the body, particularly to the lungs and other bones
(Thompson and Pool, 2002; Henry and Higginbotham, 2010; Withrow et al.,
2013). In horses, osteosarcoma is rare and less documented. When it
occurs, it usually affects the mandible and maxilla in young horses. In
this area, it causes significant damage to the face and dental arcade,
interfering with normal mastication (Bush et al., 2007; Henson, 2007).
Fewer case reports document osteosarcoma within the appendicular
skeleton and vertebral column (Barber et al., 1983). Limb involvement in
horses most often results in uncontrollable lameness and usually has a
bad prognosis (Zaruby et al., 1993; Nelson, 1998; Gutierrez-Nibeyro et
al., 2010; Jenner et al., 2010). The aetiology and pathogenesis are
unknown although microtrauma, viral infection, exposure to radiation and
genetics have been hypothesized as predisposing factors (Oblak et al.,
2012; Sadykova et al., 2020). In human medicine, several reports presume
that rapid bone growth and the presence of bone stress are risk factors
for tumour formation (Gelberg et al., 1997; Cotterill et al., 2004). It
has been suggested that prolonged mastication can induce similar
excessive stresses in the mandible, which can lead to the appearance of
tumours at this site in horses (Bush et al., 2007).
Diagnosis can be made based on clinical, radiological, ultrasonographic,
cytological, histological and post-mortem findings. Surgical excision
remains the treatment of choice but both human and canine survival rates
are considerably higher when surgical resection is combined with
chemotherapy (Bregazzi et al., 2001; Szewczyk et al., 2015). In human
and small animal medicine, radiation therapy is generally additionally
utilized to relieve pain and retard metastasis and further growth of
osteosarcoma (Schwarz et al., 2009; Oblak et al., 2012; Szewczyk et al.,
2015). In conventional radiation therapy, both neoplastic and normal
cells are damaged and prevented from mitosis. This therapy can achieve
local tumour control as normal cells have a better regeneration, but it
is associated with a high rate of complications (Liptak et al., 2004;
Timmerman et al., 2014; Martin et al., 2021). Stereotactic body
radiation therapy (SBRT) is a novel treatment approach which delivers a
high dose of radiation extremely precise and accurate to the tumour. By
this technique, the exposure of normal tissues to radiation is
minimized, which lessens the damage of normal cells and the occurrence
of adverse effects related to conventional radiation therapy (Farese et
al., 2004; Martin et al., 2021). As canine osteosarcoma has been proven
to be more resistant to radiation compared to other tumours, it is
preferable to administer a higher dose per fraction to obtain a strong
response during treatment (Walter et al., 2005; Fitzpatrick et al.,
2008). Few reports are available that document radiation therapy in
horses, leaving the effectiveness and tolerance of radiation therapy as
a treatment for osteosarcoma in horses unclear. This case report
describes the clinical, radiographic and histological findings of an
osteosarcoma in the second and third metacarpal bones of a horse and the
treatment consisting of a combination of surgical resection,
chemotherapy and SBRT.