The Future of Image-guided Radiotherapy

Dose-escalated radiotherapy using increasingly con-formal techniques from three-dimensional conformal radiotherapy to step-and-shoot intensity-modulated radiotherapy (IMRT) through to volumetric-modulated arc therapy (VMAT) has been shown to improve outcomes in a number of different solid tumours [1e5]. This has been achieved while minimising the potential toxicity to sur-rounding normal tissue. However, the accuracy of radio-therapy delivery is limited by volume delineation, set-up error and intra-/inter-fraction organ deformation, motion and rotation. These uncertainties are reduced using optimal planning and image-guided radiotherapy (IGRT). The use of cone-beam computed tomography (CBCT) has improved the precision of radiotherapy, enabled the safe delivery of stereotactic ablative body radiotherapy and can produce results comparable with surgery [6]. Despite the successive incremental improvements in radiotherapy, the limitation of CBCT is that there is loss of image quality compared with the planning scan and it is suboptimal for many tissues [7]. Magnetic resonance imaging (MRI) provides improved soft-tissue contrast in many parts of the body and offers far superior in-room imaging for radiotherapy. Although other MRI guidance technology exists [8], at present only the MR-linear accelerator (MR-linac) is available in the UK. The MR-linac combines a 1.5T magnet with a linac. This allows diagnostic quality images to be taken during radiotherapy treatment with the potential for real-time adaptive plan-ning and delivery. University Medical Centre Utrecht developed the prototype [9,10] with further advances made in collaboration with commercial partners, Elekta AB and Philips. The international MR-linac Consortium was formed in 2012 to introduce this new technology on a background of collaboration, research and robust evidence of clinical benefit. The consortium includes Elekta AB