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Exploiting the Rotational Symmetry of Tomotherapy to Reduce Dose Perturbations From MRI-Guided Radiotherapy: A Monte Carlo Investigation

Y Yang

Y M Yang*1,M Geurts1,J B Smilowitz1,E Sterpin2,B Bednarz1 (1)University of Wisconsin Madison, Madison, WI,(2)Universite catholique de Louvain, Center of Molecular Imaging, Radiotherapy and Oncology, Brussels, Belgium

SU-E-T-503 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

There is growing excitement over new treatment modalities that integrate magnetic resonance (MR) image guidance with radiotherapy delivery systems. The therapeutic gain from reducing tumor position uncertainty using intra-fraction MR imaging during radiotherapy will only be achieved by adequately accounting for impact of the magnetic field on dose distributions during treatment delivery. We propose that integrating MR-image guidance with a more rotationally symmetric modality such as helical Tomotherapy delivery might reduce or even eliminate dose perturbations due to magnetic fields. The purpose of this study was to investigate the magnetic field effect on clinical Tomotherapy dose distributions in two disparate patient geometries using Monte Carlo methods.

The Monte Carlo code Geant4 was used for all simulations performed in this study. To simulate magnetic resonance imaging guided radiation therapy (MRIgRT), we investigated dose distributions from Tomotherapy patient plans subjected to homogeneous transverse magnetic fields of varying field strengths. For this study, perturbations in the dose distributions from a prostate and lung case are presented.

Results show that perturbations in the overall dose profile due the magnetic field can increase the dose heterogeneity to the target volume when a transverse magnetic field was applied to the patient geometry. The perturbations were dependent on the heterogeneity of the patient geometry, symmetry of the primary beam fluence, and magnetic field strength. The heterogeneous lung geometry demonstrated a significant impact to the tumor volume due to the magnetic field, whereas relatively homogeneous prostate geometry showed minimal dose perturbations.

This study demonstrated that the effect of external magnetic fields can be mitigated by exploiting a more rotationally symmetric treatment modality.

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