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The Dosimetric Impact of Both a Rigid and a Flexible Radiofrequency Coil in a Hybrid 1.5T MRI and 6 MV Linac System for Online MR-Guided Radiotherapy

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S Hoogcarspel

S Hoogcarspel*, S Crijns, M Vulpen, J Lagendijk, B Raaymakers, University Medical Center Utrecht, Utrecht, Utrecht

SU-E-J-32 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To investigate the dosimetric impact of both a rigid and a flexible radiofrequency coil in a hybrid 1.5T MRI and 6 MV linac system for online MR-guided radiotherapy.

Methods: We evaluated the dosimetric impact of a flexible Synergy MR coil (Philips, Best) on both a simple and a complex irradiation strategy for treating spinal bone metastases by Monte-Carlo simulations. This tumor site, in combination with this MR coil may be chosen for the first in-man treatments with the UMC Utrecht MRL-system. Second, we investigated the dosimetric impact of a rigid pelvis coil as used in the interventional Philips MRI HIFU system. This coil consists of a hard plastic shell with the actual coil elements and electronics inside it. We took a separate look at the influence of the shell and the coil components on the dose distribution.

Results: For the flexible coil, the dose to the target volume was underestimated up to 2.2% as a result of beam attenuation in the MR coil in case a single posterior-anterior field was applied (Figure 1). This underestimation however, decreased to 1% when a stereotactic treatment strategy was employed (Figure 2). For the rigid HIFU coil, the shell without electronic components introduced a homogenous 4% dose decrease when an anterior-posterior rectangular field was utilized. The dose to the skin however, was increased dramatically as a result of additional electrons generated inside the shell (figure 3). The shell with the electronic components yielded severe streaks in the dose distribution (figure 4).

Conclusion: It is feasible to use the Flexible Synergy MR body coil for MR guided radiotherapy without any modification to the coil. The rigid HIFU pelvis coil however, is with its current design, unsuitable for MRgRT.

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