Peter Metcalfe PhD, Presenter: Nothing to Disclose

Brad Oborn PhD, Abstract Co-Author: Nothing to Disclose

Lois Holloway PhD, Abstract Co-Author: Nothing to Disclose

Geoff Delaney FRANZC, Abstract Co-Author: Nothing to Disclose

Michael Barton FRANZC, Abstract Co-Author: Nothing to Disclose

Anatoly Rosenfeld PhD, Abstract Co-Author: Nothing to Disclose

Paul John Keall PhD, Abstract Co-Author: Speaker, Varian Medical Systems, Inc

With the added dose burden and low contrast images provided by current x-ray based cone beam CT there are potential advantages of using MRI. MRI provides excellent soft tissue contrast improving accuracy and precision of target definition and there is the potential to do other analysis such as quantification of motion on the fly. MRI is also showing promise in establishing biological and functional targets, which may be of benefit for radiotherapy. Using an MRI in combination with a radiation source is being examined by at least three other groups. The ViewRayTM system uses MRI guidance in combination with cobalt sources . There are also two MRI linac prototypes i.e. the Utrecht group (1) and the Edmonton group (2). The magnetic field from the MRI perturbs the path of electrons and this has an impact on the dose distribution. This is will be the main focus of this presentation (3, 4).  

The Australian MRI linac Project (AMP) is in the early stage of planning. In preparation for this a number of Geant4 Monte Carlo simulations of beam properties from various prototypes have been undertaken. Skin doses have been simulated in 10 micron voxels at depth of the epidermal layer with a series of beam entrance and exit angles for magnetic fields representative of the proposed AMP design.

These simulations indicate skin dose enhancement due to re-entrant electrons. The skin dose enhancement is angular orientation dependent. For example at 1 T the central axis exit surface dose enhancement (10 micron surface voxel) at +45 degrees is negligible whereas at -45 degrees it is around 100% (relative increase). These values vary strongly between the different magnetic field strengths and directions representing the different designs. For example at 0.4 T the increase is around 70% for both +/- 45 deg. Conclusions:  

This is a challenge that needs to be addressed in the MRI-linac design and also in radiation treatment planning dose calculation and beam angle optimization. Interface dose effects are an issue for both in line and transverse magnetic field linac designs. References 1. J. Langendijk et al Radiotherapy and Oncology 86 (2008) 25-29 2. B. Fallone et al Med Phys 34 (2007) 2547 3. B. Oborn et al Med Phys 36 (2009) 3549-3559 4. B. Oborn et al Med Phys 37 (2010) 5208-5217  

MRI and its potential use in radiotherapy image guidance.

Metcalfe, P, Oborn, B, Holloway, L, Delaney, G, Barton, M, Rosenfeld, A, Keall, P, Is There a Future Role for MRI in Radiation Therapy Image Guidance?.  Radiological Society of North America 2011 Scientific Assembly and Annual Meeting, November 26 - December 2, 2011 ,Chicago IL. http://archive.rsna.org/2011/11012896.html