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Beam and Cryostat Scatter Characteristics of the Elekta MR-Linac

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M Duglio

M Duglio*, S Towe , D Roberts , Elekta Limited, Crawley, West Sussex


SU-F-J-149 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

The Elekta MR-Linac combines a digital linear accelerator system with a 1.5T Philips MRI machine. This study aimed to determine key characteristic information regarding the MR-Linac beam and in particular it evaluated the effect of the MR cryostat on the out of field scatter dose.

Tissue phantom ratios, profiles and depth doses were acquired in plastic water with an IC-profiler or with an MR compatible water tank using multiple system configurations (Full (B0= 1.5T), Full (B0=0T) and No cryostat). Additionally an in-house CAD based Monte Carlo code based on Penelope was used to provide comparative data.

With the cryostat in place and B0=0T, the measured TPR for the MR Linac system was 0.702, indicating an energy of around 7MV. Without the cryostat, the measured TPR was 0.669.
For the Full (B0=0T) case, out of field dose at a depth of 10 cm in the isocentric plane, 5 cm from the field edge was 0.8%, 3.1% and 5.4% for 3x3 cm², 10x10 cm² and 20x20 cm² fields respectively.
The out of field dose (averaged between 5 cm and 10 cm beyond the field edges) in the “with cryostat” case is 0.78% (absolute difference) higher than without the cryostat for clinically relevant field sizes (i.e. 10x10 cm²) and comparable to measured conventional 6MV treatment beams at a depth of 10 cm (within 0.1% between 5 cm and 6 cm from field edge).
At dose maximum and at 5 cm from the field edge, the “with cryostat” out of field scatter for a 10x10 cm² field is 1.5% higher than “without cryostat”, with a modest increase (0.9%) compared to Agility 6MV in the same conditions.

The study has presented typical characteristics of the MR-Linac beam and determined that out of field dose is comparable to conventional treatment beams.

Funding Support, Disclosures, and Conflict of Interest: All authors are employed by Elekta Ltd., who are developing an MR-Linac.

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