## 1 Introduction  
Approximately 40-70% of patients with cancer will develop bone metastases [1] during the timespan of their disease [2], resulting in a significant reduction in their quality of life (QoL) as well as an increase in healthcare costs and service utilization compared to patients without bone metastases [3]. The incidence increases because of improving cancer survival rates [4]. Spinal metastasis may mainly affect the thoracic spine (70%), followed by the lumbar spine (20%) [5]. Out of these patients, 10% will develop spinal cord compression (SCC), a serious disabling condition that requires urgent treatment [6]. Spinal metastasis can also lead to significant complications--such as pain, due to destruction of bone tissue and/or neurological complaints [7]--in up to 90% of patients [8]. The spinal metastasis incidence is more common during the evolution of these primary tumors: lung (24%), breast (24%), liver (12%), prostate (11%), and kidney (11%) [1]. Most symptomatic lesions develop due to breast, prostate, and lung cancer (56-74%), and are most common in the thoracic spine (51-67%). Pain occurs due to the destruction of bone tissue and/or nerve root and/or SCC. Both RT and surgery are effective in treating these symptoms [9]. Symptons relief can be achieved with conventional fractionated conformal external beam radiotherapy (cEBRT) because it is convenient and cost-effective. Nevertheless, given the longer survival of patients with metastatic cancer, we should try more radical treatments due to the improvements that are evident in systemic therapies, such as targeted therapies and immunotherapies. In addition, the low radiation tolerance of the spinal cord and cauda equine requires optimal radiation dose delivery for durable tumor control [2]. However, since the treatment dose is limited by organs at risk (OARs), it is not possible to reach ablative doses within the tumor volume with cEBRT. Complete response rates of 17-34% and partial responses of 49% have been described with cEBRT, with an overall response rate of 60-66%. In addition, it has been reported that a retreatment incidence between 7.4 and 21.5% can be seen, depending on the fractionation schedule [10; 11]. On the other hand, SBRT allows for greater dose fall-off, minimizing doses to the spinal cord and to other surrounding tissues. This allows dose escalation to the tumor volume, which improves the chances of overcoming it. With SBRT, tumor control rate is greater than 80% [12; 13; 14; 15], reaching 91.6% in patients with more than 5 years of survival [16]; shows a low incidence of serious complications such as myelopathy (0.4%) [2; 17]; and enables patients to preserve function [2]. So, spine SBRT obtains high local control rates (84-90%) and it is a safe treatment with low toxicity [18]. Traditionally, metastatic spinal lesions have been treated with doses and fractions of RT considered safe for the spinal cord and used for the purpose of pain relief and local control (LC) of these lesions. The doses commonly used in this regard are 10 fractions of 3 Gy, 5 fractions of 4 Gy, or 1 fraction of 8 Gy. We are now able to deliver higher, biologically effective and radical doses because we can better tailor the dose and limit the toxic dose at the level of the spinal cord. SBRT is usually administered in 1 to 5 fractions with more than 6 Gy per fraction. Disease progression at the spinal level is associated with neurological morbidity, pain, and limited adjuvant treatment options, making SBRT ideal for this location as it improves local disease control and improves symptoms [19]. The Canadian Association of Radiation Oncology defines SBRT as the precise delivery of highly conformal, image-guided, hypo-fractionated external beam radiation therapy that is delivered in one or a few fractions to an extracranial target with biologically equivalent doses to a conventional fractionated radical treatment [20]. SBRT regimens use doses of 16-18 Gy in one fraction, 24-26 Gy in 2 fractions, 24-27 Gy in 3 fractions, and 30-40 Gy in 5 fractions [19; 21].