MRTgRT MRI/linac integration Jan J.W. Lagendijk a, * , Bas W. Raaymakers a , Alexander J.E. Raaijmakers a , Johan Overweg b , Kevin J. Brown c , Ellen M. Kerkhof a , Richard W. van der Put a , Bjo¨rnHa˚rdemark d , Marco van Vulpen a , Uulke A. van der Heide a a Department of Radiotherapy, University Medical Center Utrecht, Heidelberglaan, The Netherlands, b Philips Research, Hamburg, Germany, c Elekta Oncology Systems, Crawley, UK, d RaySearch Laboratories, Stockholm, Sweden Abstract Purpose/objectives: In radiotherapy the healthy tissue involvement still poses serious dose limitations. This results in sub-optimal tumour dose and complications. Daily image guided radiotherapy (IGRT) is the key development in radiation oncology to solve this problem. MRI yields superb soft-tissue visualization and provides several imaging modalities for identification of movements, function and physiology. Integrating MRI functionality with an accelerator can make these capacities available for high precision, real time IGRT. Design and results: The system being built at the University Medical Center Utrecht is a 1.5 T MRI scanner, with diagnostic imaging functionality and quality, integrated with a 6 MV radiotherapy accelerator. The realization of a prototype of this hybrid system is a joint effort between the Radiotherapy Department of the University of Utrecht, the Netherlands, Elekta, Crawley, U.K., and Philips Research, Hamburg, Germany. Basically, the design is a 1.5 T Philips Achieva MRI scanner with a Magnex closed bore magnet surrounded by a single energy (6 MV) Elekta accelerator. Monte Carlo simulations are used to investigate the radiation beam properties of the hybrid system, dosimetry equipment and for the construction of patient specific dose deposition kernels in the presence of a magnetic field. The latter are used to evaluate the IMRT capability of the integrated MRI linac. Conclusions: A prototype hybrid MRI/linac for on-line MRI guidance of radiotherapy (MRIgRT) is under construction. The aim of the system is to deliver the radiation dose with mm precision based on diagnostic quality MR images.  c 2007 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 86 (2008) 25–29. Keywords: MRI; IGRT; Accelerator; Position verification The ideal radiotherapy dose distribution is tailored to the tumour clonogenic cell density and radiosensitivity [2]. From TCP model analysis it is shown that, in case of a homo- geneous tumour, a large macroscopic tumour (the GTV) needs the highest dose, a smaller GTV a lower dose and the tumour infiltration in normal tissue (CTV minus GTV) re- quires the lowest dose (around 70% of a GTV dose [9]). The definition of such a dose distribution is complicated by daily positioning uncertainties which lead to PTV margins (ICRU 50 [10,27,34]). Because of normal tissue involvement in the PTV, conflicting dose constraints occur, e.g. the rectum in the PTV for prostate treatments or the parotid glands in the PTV of head and neck cancer [31]. This often means that the tumour dose is limited by normal tissue tolerance. Stud- ies on the pattern of failure after radiotherapy show that the location of recurrences is mostly local and inside the original GTV [4,6,8,37] indicating the need for a GTV boost [7]. Image guided radiotherapy (IGRT) aims to decrease posi- tioning uncertainty in order to reduce the PTV margins and thus minimize the dose limiting normal tissue involvement in the PTV. A large variety of IGRT technology is being tested clinically. Techniques based on cone beam CT, megavolt CT, ultrasound and implanted fiducial markers each try to find their own place with their specific capacities and limitations [11,16,18,20,26]. Problems in IGRT are still the combination of limited visibility of the tumour itself and the absence of real intrafraction imaging. In potential, diagnostic quality MRI (1.5–3 T) yields superb soft-tissue visualization and provides several imaging modalities for di- rect on-line imaging during movements. MRI for treatment guidance would thus offer visualization of both tumour and surrounding organs at risk which can lead to a further reduction of the margins. Therefore, at the University Med- ical Center Utrecht, a diagnostic quality 1.5 T MRI scanner integrated with a 6 MV radiotherapy accelerator for on-line, intrafraction IGRT is under construction. This is a joint ef- fort between the Radiotherapy Department of the Univer- sity Medical Center of Utrecht, the Netherlands, Elekta, Crawley, U.K., and Philips Research, Hamburg, Germany. Radiotherapy and Oncology 86 (2008) 25–29 www.thegreenjournal.com 0167-8140/$ - see front matter  c 2007 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.radonc.2007.10.034