During the treatment of tumors by radiation therapy, the energy beam is focused on its target using previously acquired imagery. 
Because physicians don’t actually see the tumor, which can also be moving as in the case of lungs, the precision is compromised causing a lot of surrounding tissue to be damaged by radiation. Now scientists at University Medical Centre Utrecht designed a system that combines a 1.5 T MRI scanner with a 6 MV accelerator into one package. The group is planning to start clinical trials of the MRI-guided radiation therapy (MRIgRT) system, which, if successful, will undoubtedly become a popular choice in radiation oncology.
From the abstract:
The prototype is a modified 6 MV Elekta (Crawley, UK) accelerator next to a modified 1.5 T Philips Achieva (Best, The Netherlands) MRI system. From the initial design onwards, modifications to both systems were aimed to yield simultaneous and unhampered operation of the MRI and the accelerator. Indeed, the simultaneous operation is shown by performing diagnostic quality 1.5 T MRI with the radiation beam on. No degradation of the performance of either system was found. The integrated 1.5 T MRI system and radiotherapy accelerator allow simultaneous irradiation and MR imaging. The full diagnostic imaging capacities of the MRI can be used; dedicated sequences for MRI-guided radiotherapy treatments will be developed.
Abstract with link to full article: Integrating a 1.5 T MRI scanner with a 6 MV accelerator: proof of concept
Press release by Institute of Physics: Breakthrough in radiotherapy promises targeted cancer treatment…
Images: Top: Sketch of the MRI accelerator concept. The 1.5 T MRI is shown in blue (1), the 6 MV accelerator (2) is located in a ring around the MRI. The split gradient coil (3) is shown in yellow and in orange the superconducting coils (4) are shown. The light blue ring around the MRI indicates the low magnetic field toroid (5) in the fringe field. Side: MRI of a pork chop with and without the radiation beam on.