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Magnetic Field Dose Effects On Robustness of Dose Delivery for Lung Stereotactic Body Radiation Therapy

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A Kim

A Kim*, S Al-Ward , C McCann , P Cheung , A Sahgal , B Keller , Sunnybrook Health Sciences Centre, Toronto, ON


SU-I-GPD-T-567 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: MRI-linac systems aim to use the soft tissue contrast of MRI to target disease in real-time. Due to the ever-present magnetic field during delivery there are magnetic field effects on the dose distribution. This is especially evident at density interfaces such as in the lung. The objective here is to determine the effects of the magnetic field on the robustness of dose delivery of lung SBRT to small inter-/intra-fractional translational shifts. The Elekta MRI-linac was considered here.Materials and

Methods: 5 NSCLC patients were selected. Patients were simulated with 4DCT with targets contoured by a staff oncologist. The max inhale phase was used for planning with the aim to demonstrate how translational shifts would affect the dose distribution in the GTV. The Monaco treatment planning system was used for planning. Each case was optimized with IMRT, full arc VMAT, and half arc VMAT (ipsilaterally placed). For each beam geometry, one plan was optimized without the magnetic field and one plan with a B0=1.5 T magnetic field transversely placed to the beam. Prescription dose was 52 Gy in 4 fractions. To quantify robustness, a 3 mm translational shift of the beam isocenter was performed in the 3 ordinate directions for each plan (i.e. 3 beam geometries, with B0 on and off). The GTV coverage and hot spots were evaluated to quantify robustness.

Results: Preliminary results show that there are spurious dosimetric effects due to translational shifts of the beam isocenter. These shifts cause the plan DVHs to stray from optimality. The effect is most severe for the VMAT half arc and with the magnetic field on.

Conclusion: The comprehensive set of results can inform what shifts are tolerable for a given beam geometry and magnetic field state, and can potentially be used to justify more sophisticated motion management techniques.

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