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Dosimetric Effects of Irradiation Through a Bilateral Hip Prosthesis in a MRI Linac

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M Wronski

M Wronski1,2*, S Ahmad1 , A Sarfehnia1,2 , A Sahgal1,2 , B Keller1,2 , (1) Sunnybrook Odette Cancer Center, Toronto, Canada, (2) University of Toronto, Department of Radiation Oncology, Toronto, Canada


SU-F-T-374 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

To evaluate the interface effects when irradiating through a hip prosthesis in the presence of an orthogonal 1.5 T magnetic field using Monte Carlo simulations.

A 20x20x38 cm virtual phantom with two 5x5x5 cm sections of bilateral titanium hip prosthesis was created in GPU-based Monte Carlo (MC) algorithm (GPUMCD, Elekta AB, Stockholm Sweden). The lateral prosthesis spacing was based on a representative patient CT scan. A treatment SAD of 143.5 cm was chosen, corresponding to the Elekta AB MRI Linac and the beam energy distribution was sampled from a histogram representing the true MRI Linac spectrum. A magnetic field of 1.5 T was applied perpendicular to the plane of irradiation. Dose was calculated, in voxels of side 1 mm, for 2x2, 5x5, and 10x10 cm treatment field sizes with normal beam incidence (gantry at 90o or 270o) and at 5o and 10o from normal, representing the range of incidence through the bilateral prosthesis.

With magnetic field ON (B-On) and normal beam incidence the backscatter dose at the interfaces of proximal and distal implants is reduced for all the field sizes compared to the magnetic field OFF (B-Off) case. The absolute reduction in doses at the interface was in the range of 12.93% to 13.16% for the proximal implant and 13.57% to 16.12% for the distal implant. Similarly for the oblique incidences of 5o and 10o the dose in the plane adjacent to the prosthetic implants is lower when the magnetic field is ON.

The dosimetric effects of irradiating through a hip prosthesis in the presence of a transverse magnetic field have been determined using MC simulation. The backscatter dose reduction translates into significantly lower hot spots at the prosthetic interfaces, which are otherwise substantially high in the absence of the magnetic field.

Funding Support, Disclosures, and Conflict of Interest: This project was supported through funding provided by ElektaTM.

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