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.