Table 1.

Technological advances in RT.

Technological development	Changes to RT	Clinical trials enabled By technological development	Limitations of technological development
CT-guided intensity-modulated RT (IMRT) [30]	Significant improvements in radiation dose conformality and reduction in radiation doses to critical normal structures Ability to avoid or reduce doses to local structures	Reduction in doses of radiation to critical organs at risk (e.g. parotid glands in head and neck cancer) [31] Dose escalation strategies	Expensive Insufficient soft-tissue contrast, limiting treatment adaptation Limited ‘beam-on’ tumour monitoring during treatment delivery
Proton therapy	Reduction in radiation dose to normal structures Ability to completely eliminate dose to some normal structures	Trials focused on clinical improvements enabled by reductions in moderate radiation doses to normal tissues [32,33]	Very expensive Insufficient soft-tissue contrast in image guidance Inability to reduce highest radiation doses to normal organs Limited treatment adaptation
MR-guided intensity-modulated RT	Substantial improvement in soft tissue imaging during treatment Online treatment adaptation based on MR-defined ‘anatomy of the day’ Beam-on imaging with MR may enable considerable reductions in high doses to normal organs Detection of radiation response of tumour and normal structures	Anatomically and biologically adaptive RT, based on changes seen on daily MRI during a treatment course (e.g. daily DWI) Intra-treatment, or ‘beam-on’, monitoring of normal organ and tumour movement NTCP reduction with dose reduction, particularly of local structures in close proximity to the tumour	Expensive Risks and complexity associated with the use of MRI New training needed Novel effects of MRI on radiation dose distributions Limited patient eligibility (not an option for those with contraindications to MR-based imaging)

MR, magnetic resonance; DWI, diffusion weighted image; NTCP, normal tissue complication probability.