Image-guided stereotactic body radiation therapy (SBRT): an emerging treatment paradigm with a new promise in radiation oncology

Stereotactic radiosurgery (SRS), coupling radiation with a stereotactic guiding device, was first performed by Leksell in 1951 [1]. SRS is now an established treatment option for many benign and malignant tumours. With the advances in technology, including patient/target immobilisation, tumour/target tracking, image-guidance, and radiation planning and delivery, SRS to extracranial sites has become a reality. Extracranial stereotactic radiosurgery/radioablation or stereotactic body radiation therapy (SBRT) is defined by the American Society for Therapeutic Radiology and Oncology (ASTRO) and the American College of Radiology (ACR) as a " treatment method to deliver a high dose of radiation to the target, utilising either a single dose or a small number of fractions with a high degree of precision within the body [2]. " Like any novel therapeutic approach in medicine, SBRT needs to be performed with caution and ideally in the clinical trial setting, especially in view of the biologically potent dose prescription as high as 20 to 30 Gy per fraction. Practice guidelines should be followed to avoid the risk of severe complications [2]. Teamwork is essential for the success of this new treatment, and the team should include not only medical physicists, but also dosimetrists, radiation therapists, nurses, radiologists, and radiation oncologists. Some essential components needed for the clinical implementation of SBRT include: patient immobilisation and accurate reposition from simulation session to each treatment session, accounting for motion or tracking " moving target " e.g., lung tumours, fusion of various imaging studies, construction of tight dose distributions covering tumour with rapid fall-off the adjacent normal tissues, as well as the availability of image-guidance. The article in this issue of Biomedical Imaging and Intervention Journal [3] illustrates the proper conduct of SBRT, which includes the use of immobilisation device, accurate repositioning of the patient with KV-X Ray as image-guidance, 4D-CT to account for tumour motion, proper fusion of PET/CT and MRI with simulation CT, use of visicoil/bony landmarks for image-guidance, and construction of tight isodose around the tumour. Radiobiologically, the dose fraction regimens used in SBRT ranging from 6 to 30Gy, are aimed to yield substantially more potent biological and clinical effects. Applying linear-quadratic formula, Fowler and colleagues have compared the relative biological effectiveness of various SBRT fractionation schemes with the conventional fractionation scheme for non-small cell lung cancer [4]. The conventional 60Gy in 30 fractions (2 Gy per fraction) and 60Gy in 3 fractions (20Gy per fraction) have biological equivalent doses …