Program Information
Beam and Cryostat Scatter Characteristics of the Elekta MR-Linac
M Duglio*, S Towe , D Roberts , Elekta Limited, Crawley, West Sussex
Presentations
SU-F-J-149 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose:
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.
Methods:
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.
Results:
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.
Conclusion:
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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