Published online by Cambridge University Press: 05 November 2015
Recent advances in technology have enabled more complex radiotherapy to be delivered routinely and the use of stereotactic or modulated techniques have become standard for many indications. These developments allow the radiation dose to conform more closely to the planning target volume (PTV) while more effectively avoiding normal tissue. This can allow the dose to be escalated without increasing the risks of normal tissue toxicity.
Some of these techniques require specialised machines, described later in the chapter, and recently there has also been interest in modifying conventional linear accelerators to achieve the same aim. For instance, the increased use of cranial stereotactic techniques for both malignant and non-malignant indications has led both to an increase in specific machines and to the development of linear accelerators which deliver advanced radiotherapy to both cranial and extracranial sites. Rotational therapy, a technique that was used by some centres in the 1960s, has been ‘rediscovered’ and, when combined with the modulated techniques of conventional intensity-modulated radiotherapy (IMRT), has led volumetric-modulated adaptive therapy (VMAT), delivering radiation in a way similar to tomotherapy.
Advances in imaging have also contributed to radiotherapy development. For example, use of pretreatment MRI or PET scans can help to define planning target volumes, and cone beam CT scans during treatment can check on internal organ movement and set-up accuracy. The ability to ‘fuse’ these images with the CT planning scan has greatly assisted both radiotherapy planning and treatment delivery.
Intensity-modulated radiation therapy
IMRT is a technique which uses beams that, unlike the flat or wedged beams of conventional RT, have changing dose intensity across them, as shown in Figure 6.1 (Webb, 2003; Bortfeld, 2006). Modulation was originally produced by using low melting point compensators, but is now usually generated by moving the multi-leaf collimators (MLCs) during ‘beam on’ time. The aim of this is to build up the desired dose distribution by producing appropriately modulated beams.
The basic principle of IMRT can be seen in Figure 6.2, which shows three modulated beams to cover a PTV with an isodose distribution that wraps around the organ at risk (OAR) and conforms well to the target volume. In practice more beams, usually between 5 and 9, are needed to give an acceptable distribution.