MO-E-J-6B-03: In Room Magnetic Resonance Imaging Guided Radiotherapy (MRIgRT)

Precise, soft-tissue based, on-line position verification and treatment monitoring is a prerequisite for image guided radiotherapy (IGRT). The system being developed is a 1.5 T MRI scanner integrated with a 6 MV radiotherapy accelerator. Basically, the design is a modified 1.5 T Philips Achieva MRI scanner with a small, single energy (6 MV) accelerator rotating around it. The technical feasibility of simultaneous irradiation and MR imaging will be discussed. The magnetic interference between the MRI system and the linear accelerator has been solved by modifying the active shielding of the magnet, minimising the magnetic field at the components of the accelerator and completely nullifying the magnetic field at the location of the accelerator gun section. The MRI system is being homogenized and the radiation thickness has been minimised to allow beam transmission through the midplane of the closed bore MRI system. A special gradient coil set has been designed with a radiation window allowing a 24 cm field size in the caudal cranial direction for every gantry angle. The dose deposition kernel in the presence of a transverse magnetic field has been investigated. The kernel has been quantified and special effects like the electron return effect (ERE) around air cavities, have been investigated. It is shown that integrating MRI functionality with a radiotherapy accelerator is technically feasible. Ultimately the system may be used for daily treatment optimisation by providing the imaging information for on-line treatment planning. On-line MRI may also provide treatment monitoring and treatment response assessment required for further biological optimisation.