Technical know‐how in stereotactic ablative radiotherapy (<scp>SABR</scp>)

Abstract

Stereotactic ablative radiotherapy (SABR) is an addition to the armamentarium against cancer. The technical requirements for SABR are very stringent, given its very narrow therapeutic window. However, when the principles are strictly followed, it is possible to deliver SABR to extracranial tumours safely and effectively. With the sophistication of present day technology, we are now able to deliver stereotactic ablative radiotherapy (SABR), entailing delivery of ablative individual doses in a very precise and accurate fashion to extracranial tumours, a feat which was unimaginable in the past. SABR is virtually a spin-off of intra-cranial stereotactic radiosurgery (SRS) and has been used to treat primary tumours of the lung, the liver, the kidney, and the prostate, and oligometastatic and oligoprogressive disease.1-3 In order to minimise any collateral damage to surrounding normal parallel tissues like lung and liver or adjacent serial organs such as the oesophagus or spinal cord, highly conformal radiation isodose distribution and very tight margins are used. For tumours that move with respiration, manoeuvres accounting for respiratory motion are necessary to avoid inadequate coverage of the tumour being treated. To facilitate safe and effective delivery of SABR, the technical requirements are very stringent and their importance cannot be overemphasised. The SABR process can be divided into: (1) Proper selection of patients; (2) Immobilisation; (3) Respiratory motion control; (4) CT simulation; (5) Delineation of target volume and organs-at-risk (OARs); (6) Treatment planning; (7) Pre-treatment verification; and (8) Treatment delivery and intra-fractional monitoring.1-3 The subsequent sections will discuss all the above components individually. Eligibility criteria vary among different trials or centres for each disease site or condition.1 However, there are some general stipulations to be met in order for the patient to be eligible for SABR. Firstly, the patient must be able to derive benefit from the procedure either in terms of durable control of the target tumour or symptomatic relief. In other words, there must be a set goal for therapy for the patient. Secondly, the patient must be able to tolerate lying still in the immobilisation device for the SABR treatment. If a robotic radiosurgery system is used, the treatment delivery time can easily exceed 1 hour. However, if a linear accelerator (LINAC)-based system is used and when volumetric modulated arc therapy (VMAT) and beam-flattening filter-free feature are available, the treatment delivery time can be dramatically reduced. Thirdly, the target to be treated must be clearly visualised on imaging as typically, there is very little or no margin expansion around the gross tumour volume (GTV) with SABR. A robust immobilisation is of utmost importance in the SABR process as very tight margins are set around the GTV and in many cases, such as in spinal metastasis, there are critical OARs in the proximity. Immobilisation can be achieved either by the use of rigid external immobilisation devices, by active motion detection and compensation during treatment delivery and by fast treatment delivery. Depending on the treatment machine used and the body site to be treated, different immobilisation devices should be considered.1-3 Among all, spinal SABR requires the most robust immobilisation, especially when a LINAC-based system is used. Li et al. has demonstrated that the dual vacuum system (BodyFIX; Elekta AB) is superior to other immobilisation devices in terms of set-up accuracy and can keep the set-up variation to 2 mm or less with intra-fractional adjustment.4 However, colleagues from VU University Medical Center in the Netherlands showed that based on pre- and post-fraction X-ray imaging during fast lung SABR, simple support devices can result in spine stability that is comparable to that reported with rigid external immobilisation.5 When a robotic radiosurgery system is used,