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Shielding Design for An MR Guided Radiotherapy (MRgRT) Suite

M Carlone

H Alasti, M Islam, A Rink, M Carlone*, S Breen, D Jaffray, Princess Margaret Hospital, Toronto, ON

SU-E-J-165 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: An MRI on rails is being installed at our institution to provide MR-guided brachytherapy and teletherapy (MRgRT). The facility is comprised of collinear brachytherapy, MR-Sim, and linac suites. The MR-on-rails translates into the adjacent suites for imaging. An existing bunker and adjacent space were renovated to permit the MR to be moved away from the linac during irradiation. This work describes the design and radiation shielding characteristics of the linac suite within the MRgRT facility.

Methods: The original linac vault was designed with a two-corner maze design to operate at 18 MV. The maze was removed and replaced with three direct-shielded doors: a sliding console door allow for personnel and gurney access to the bunker; and two double sliding doors through which the MRI translates. The double doors employ an S-shaped coupling surface to minimize radiation leakage when they are closed. These doors are also RF-shielded as part of the MR RF cage. The shielding was designed for a workload of 5 x 10⁶ cGy per year; 6 MV beam; maximum dose rate: 1400 MU/min; shielding level 0.05 mSv/year for members of the public and 1 mSv/year for nuclear energy workers.

Results: The total thickness of the linac-MRI sliding doors was 7 inches of lead and 2 inches of stainless steel, producing an equivalent thickness of 8 inches (20.3 cm) of lead. The dose rate at the distal surface of the linac-MR door is 0.08 mSv/year or 10.9 μSv/hr. The final design includes a copper RF shield and bladder to provide RF shielding.

Conclusion: A facility comprising three collinear brachytherapy, MR-Sim, and linac facility has been designed to meet radiation and radiofrequency shielding limits.

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