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BEST IN PHYSICS (MULTI-DISCIPLINARY): First Experimental Investigation of Simultaneously Tracking Two Independently Moving Targets On An MRI-Linac Using Real-Time MRI and MLC Tracking

P Liu1*, Y Ge2, B Dong3, G Liney3, D Nguyen1,4, E Hewson1, D Waddington1, R O'Brien1, P Keall1, (1) ACRF Image X Institute, The University of Sydney, Eveleigh, NSW, AU, (2) Prince of Wales Hospital, Randwick, NSW, AU, (3) Ingham Institute for Applied Medical Research, Liverpool, NSW, AU, (4) School of Biomedical Engineering, University of Technology Sydney, Ultimo, NSW, AU


(Tuesday, 7/14/2020) 3:30 PM - 5:30 PM [Eastern Time (GMT-4)]

Room: Track 2

The imaging capabilities of MRI-Linacs enable real-time motion-management for radiotherapy treatments with multiple targets, for example, locally advanced prostate cancer, lung cancer or oligometastases. The purpose of this work was to develop and characterize a novel multi-target MLC tracking algorithm that utilizes MRI imaging to simultaneously track the motion of two treatment volumes.

The multi-target MLC tracking algorithm separated each beam aperture into two segments corresponding to two targets. The MLC leaf positions of each segment were calculated independently in response to that target’s motion. The two segments were then recombined into one deliverable MLC field that encompassed motion of both targets.

Two spherical targets were irradiated with a 6MV conformal field on a 1T inline MRI-Linac. The targets were translated in opposite directions with sinusoidal and patient respiratory motion. Cine-MRI images were acquired at 5 Hz and the position of each target was calculated using template matching. The geometric uncertainty and tracking latency was characterized using EPID images.

The average difference between each target center and the corresponding aperture center with tracking was 0.6 ± 5.8 mm for patient respiratory motion, compared to 4.0 ± 8.8 mm without tracking. The primary source of geometric uncertainty was latency, which was measured from sinusoidal motion to be 300 ms. Accounting for latency reduced the average difference to 0.6 ± 2.1 mm, which is the uncertainty arising from the limits of MRI resolution, template matching and leaf motion. No difference in performance was observed between the two targets.

This work is the first demonstration of simultaneous MLC tracking of two moving targets. By ensuring dose coverage even in the case of independent target motion, this technology will enable MRI-linacs to deliver multi-target treatments with reduced dose to healthy tissue, opening up new treatment indications for this emerging device.

Funding Support, Disclosures, and Conflict of Interest: This work is supported by NHMRC program grant APP1036075. Liu and Waddington are funded by CINSW Early Career Fellowships. Nguyen is funded by NHMRC and CINSW Early Career Fellowships. Keall is an inventor on a licensed patent on MLC tracking and additional unlicensed patents on MRI-Linac technology.


MRI, Image Guidance, MLC


IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined- IGRT and tracking

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