For the first time since the 1950's the linac c-arm gantry for radiotherapy has been overtaken with the introduction of the revolutionary TomoTherapy Hi-Art® ring gantry.  Hi-Art® is a CT ring gantry with an on-board linac which helically rotates and shares the same x-ray target for both CT image guidance and megavoltage treatment.

Image guided radiotherapy - IGRT - is now being seen on all types of treatment machine. Hi-Art's on-board CT imaging has many benefits over conventional portal imaging and cone beam CT.  This system was designed from the outset to facilitate ADAPTIVE radiotherapy.

IGRT’s purpose is adaptive radiotherapy - using daily imaging during treatment to adapt the treatment if needed, to ensure the prescribed treatment plan is actually delivered to the patient.  There are different levels of adaptation, and it is very easy to assume that if a system has some kind of on-board imager, then treatments can be accurately adapted.

The two different levels of adaptation are firstly, to adapt the position of the patient to re-align anatomy so that at every treatment session, the radiation beam(s) are correctly hitting the target and avoiding normal tissues.  The first pre-requisite for this, is to have a good quality CT beam that allows the clear visualisation of soft tissue anatomy.  TomoTherapy's megavoltage beam clearly meets this goal.

The second level is to adapt the treatment so that the dose is recalculated based on changes to the patient that cannot be overcome by positional adaptive. For example, a patient losing weight can result in changes in anatomy size, position and shape, so that without adapting, an inaccurate treatment possibly delivering too little dose to the tumour, and too much radiation to normal tissues would result.

TomoTherapy technology accurately performs both positional and dose adaptive radiotherapy.  Its excellent quality CT images allow the visualisation of fine soft tissue detail, but also the ability to accurately calibrate the hounsfield numbers of these CT images to dose.  This provides the essential requirements for true CT adaptive radiotherapy.