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Performance Characterization of a Clinical 1.5 T MR-Linac

B Keller1*, A Kim1, V Iakovenko1, M Ruschin1, C McCann1, D Beachey1, B Chugh1, A Elzibak1, A Sahgal2, A Sarfehnia1, (1) Sunnybrook Health Sciences Centre, Odette Cancer Centre, Department of Medical Physics, Toronto, ON, CA, (2) Sunnybrook Health Sciences Centre, Odette Cancer Centre, Department of Radiation Oncology, ON, CA

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: To report on our institutional experience with a clinical MR-Linac in its first year of use, in terms of quality control performance, unscheduled down time statistics and most frequent faults.

Methods: Daily, weekly, and monthly quality control (QC) tests were established for both the linac and MRI of our 1.5 T MR-Linac treatment machine (Elekta AB,Stockholm,Sweden). Tests were categorized as safety, dosimetric, or imaging. Results were captured/plotted using our machine quality control database (QATrack+, v0.3.0.16, Ottawa, ON), an open source software application used for our fleet of linacs. The following MR safe/conditional equipment was used: MR IC Profiler (Sun Nuclear,Melbourne,Fl), Exradin A1SL ion chamber (Standard Imaging,Middleton,WI), reference dosimetry water tank (in house), energy check device (in house), MR to MV coincidence phantom (Elekta,Stockholm,Sweden), and Quasar MRID distortion phantom (ModusQA,London,ON). Unscheduled downtime was software activated by the treatment unit therapists. Faults were recorded manually in an electronic spreadsheet with date and time stamp.

Results: Over the initial 6 months, output constancy using the IC Profiler was 1.000 ± 0.004 at gantry=0. Monthly energy measurements of TPR20/10 were 0.699 +/- 0.001. MR to MV transform stability was within 0.1 mm measured daily. Gradient non-linearity distortion was less than 1 mm for all points within 15 cm of the isocenter, and this has maintained itself over time. The variations in flatness and symmetry were respectively as follows: 25.1%±0.2% (x baseline=25.1%), 19.5%±0.2% (y baseline=19.5%), -0.77%±0.14% (x baseline=-1.1%), -0.28%±0.11% (y baseline=-0.1%), with a tolerance band of 1.5%. In terms of downtime, patients were transferred to a standard linac for 7.1 % of fractions due to either a magnetron change, modulator tank replacement, or a cooling issue.

Conclusion: The radiation beam characteristics and MRI quality control tests have been robust to date. Having rapid access to machine parts would decrease downtime.

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Keywords

Not Applicable / None Entered.

Taxonomy

TH- External beam- photons: Quality Assurance - Linear accelerator

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