Program Information
Tumour Trailing for Liver SBRT On the MR Linac
M F Fast*, A JAJ van de Schoot , T van de Lindt , C Carbaat , UA van der Heide , JJ Sonke, Netherlands Cancer Institute, Amsterdam, The Netherlands
Presentations
MO-L-GePD-JT-3 (Monday, July 31, 2017) 1:15 PM - 1:45 PM Room: Joint Imaging-Therapy ePoster Theater
Purpose: Tumour trailing is a delivery technique which continuously adjusts the shape of the beam aperture according to the time-averaged (mid) position of the target. Unlike MLC tracking, trailing only compensates the low-frequency components of motion, thus easing the requirements on image frequency and system latencies. This study investigates whether tumour trailing on the MR-Linac can improve target coverage in liver SBRT treatments in the case of substantial baseline drifts.
Methods: Liver SBRT treatment plans (3x20 Gy to PTV D95%, 11-beam IMRT, mean homogeneity index 1.4) for three patients were created in research Monaco v5.19.02 for the 1ATL Elekta MR-Linac. Following departmental practice, plans were generated on the mid-position CT with an anisotropic patient-specific GTV-to-PTV expansion. We then simulated treatment delivery using in-house developed software. Target motion was modelled as cosā“ trajectory (amplitudes from 4dCT, 4 sec period) centred around the mid-position, with 1 cm/20 min and 5 mm/20 min baseline drifts added in cranial and posterior direction respectively. The trailing delivery was based on the mid-position of the target, calculated from three previous respiratory cycles. Based on the simulated delivery, a modified treatment plan was created to mimic the actual patient and aperture motion. Delivered dose can then be calculated in Monaco.
Results: For the three liver patients, average peak-to-peak respiratory motion was 8.6 (3) mm in superior-inferior (anterior-posterior) direction. Average PTV (GTV) volumes were 122 (44) cc. The mean delivery time was 19.8 min. The average increase in GTV D100% of the trailing delivery compared to a conventional delivery without active motion mitigation was 3.0 Gy per fraction.
Conclusion: Tumour trailing on the MR-Linac improves target dose in liver SBRT treatments in cases of large baseline drifts for the presented patient cohort. Following this proof-of-principle, future investigations should focus on the impact on organ-at-risk dose and margin reductions.
Funding Support, Disclosures, and Conflict of Interest: NKI-AvL is part of the Elekta Atlantic MR-linac Research Consortium and we acknowledge financial and technical support from Elekta AB under a research agreement.
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