Stereotactic radiation therapy (SRT)
SRT comprises high-precision irradiation techniques that use multiple, non-coplanar photon radiation beams, and deliver a high dose of radiation to stereotactically localized lesions, applying frame-based and frameless techniques. These lesions were originally mainly located in the brain, but now also include a number of extra-cranial malignancies. With regard to dose fractionation, SRT is divided into stereotactic radiosurgery, in which the total dose is delivered in a single treatment session, and stereotactic radiotherapy, in which the total dose is delivered in multiple fractions, similar to standard radiotherapy.
SRT can be given using a wide variety of treatment devices and may incorporate specialized dose delivery methods such as IMRT. Requirements for SRT comprise secure patient immobilization, accurate tumour relocalization, and a solution for respiratory motion, if relevant. Dedicated SRT equipment is now commercially available and includes the GammaKnife, which incorporates 201 cobalt-60 sources, linacs that combine IGRT with modern immobilization and respiratory motion technology, and the CyberKnife, which incorporates a miniature linac mounted on a robotic arm. All steps involved in SRT must be verified experimentally to ensure reliable and accurate performance of the hardware and software. The most important requirement of a linac used for SRT is the mechanical stability and accuracy of its isocentre. In SRT dose conformity to a relatively small target volume is extremely important, and the target dose homogeneity requirement is often relaxed to allow for optimization of the target dose conformity.
Introduction to References
The various aspects of the early types of stereotactic radiosurgery, including requirements for a QA programme, were summarised in the initial AAPM Report. More recent developments in the field of SRT can be found in J. Van Dyk's Compendium, the IAEA Handbook, and the Handbook of Radiotherapy Physics. A comprehensive discussion of both physical and clinical aspects of SRT is provided in the book edited by Meyer. In 2009, the American College Radiology developed some practice guidelines.