Tumor Trailing for Liver SBRT on the MR-Linac

Purpose: Tumor trailing is a treatment delivery technique that continuously adjusts the beam aperture according to the last available time-averaged position of the target. This study investigates whether tumor trailing on a magnetic resonance (MR) linear accelerator (linac) can improve target coverage in liver stereotactic body radiation therapy (SBRT) in the case of baseline motion. Methods and Materials: For 17 patients with oligometastatic liver disease, midposition SBRT treatment plans (3 × 20 Gy, 11-beam intensity modulated radiotherapy) were created for the Elekta Unity MR-Linac. Treatment was simulated using an in-house-developed delivery emulator. Respiratory motion was modelled as the superposition of periodic motion (patient-specific amplitude, 4-second period) and the following baseline motion scenarios: a continuous linear drift (0.5 mm/min), (2) a single shift halfway through treatment (10 mm), (3) a periodic drift (amplitude: 5 mm, period: 5 minutes), or (4) MR imaging–measured baseline drifts. Delivered dose was calculated under full consideration of the patient and machine motion interplay. In addition, trailing was experimentally validated on the MR-Linac using a programmable motion phantom. Results: The average simulated delivery and beam-on times were 15.9 and 8.7 minutes, respectively. An imaging frequency of ≥1 Hz was deemed necessary for trailing. Trailing increased the median gross tumor volume D98% dose by 1.9 Gy (linear drift), 1.2 Gy (single shift), 0.7 Gy (periodic drift), and 0.5 to 1.5 Gy (measured drifts) per fraction, compared with a conventional delivery. In the phantom experiments, the 3%/2 mm local gamma pass rate nearly doubled to 98% when using trailing. Conclusion: Tumor trailing on the MR-Linac restores target dose in liver SBRT in the case of baseline motion for the presented patient cohort.