**1. Introduction**

Stereotactic body radiation therapy (SBRT) is derived from the concept of radiosurgery. The American Society of Radiation Oncology (ASTRO) describes it as high-dose, image-guided radiotherapy treatment with tumor ablative intent in a limited number of fractions. Other names used are extracranial stereotactic radiosurgery or stereotactic ablative radiotherapy (SABR) [1].

The success of radiosurgery in intracranial tumors raised interest in its application in the management of extracranial tumors. However, the development of extracranial SBRT has been much later than that of radiosurgery due to the constant internal movement of the organs by respiration and bowel movements. At the cellular level, SBRT produces cellular chromosomal damage, endothelial cell apoptosis, microvascular dysfunction, and increased lymphocyte recruitment.

ASTRO has published recommendations for SBRT treatment, and the American Association of Medical Physics Task Group 101 report has expanded on them [2, 3]. It is necessary to use systems that improve volume delineation and image fusion, including magnetic resonance and/or positron emission tomography, advanced planning algorithms, image-guided radiotherapy systems, intrafraction motion control methods, and patient immobilization systems to achieve stable and reproducible patient positioning, for which various devices that suppress or limit motion have been developed [4].

The process involves a sequence of phases, and the same applies to SBRT treatments. These phases include patient immobilization, motion assessment and management, image acquisition, image set analysis and processing, planning image fusion,

**Figure 1.** *Hepatocellular carcinoma planning in Cyberknife®.*

volume delineation, radiation planning, quality assurance testing, patient setup in the treatment unit, acquisition of guidance images to allow target relocalization, treatment initiation, real-time monitoring of treatment integrity, and patient stability and tolerance [4].

The main obstacle that must be overcome to perform SBRT treatments involves respiratory-related motion control. Positioning errors during treatment or between treatments must also be taken into account. Image-guided radiation therapy or imaged-guided radiation therapy (IGRT) ensures target relocalization and beam alignment, which is indispensable in SBRT [4].

In SBRT, high-energy photons are used as the source of therapeutic radiation, although charged particles can also be used. There is no standard or absolute consensus solution for achieving a tightly focused high-dose distribution within the planning target volume and rapid dose fall-off outside it, the combination of beam angles or arcs best suited, and each case may present a new planning challenge. These treatments are tailored and personalized to each patient and each tumor [4]. Planning example is shown in **Figure 1**.

SBRT treatment sessions are longer than conventional treatments, so patient comfort is another important aspect, controlling patient changes in position between the time of treatment verification by imaging and treatment or even during treatment [4].
