Technical note: MLC racking performance on the Elekta unity MRI-linac

Recently, multileaf collimator (MLC)-tracking has been technically and clinically demonstrated showing promising improvements of radiotherapy of mobile sites. Furthermore, magnetic resonance imaging (MRI)-guided treatments have shown to provide superior targetting performance due to on-line soft-tissue imaging. Hitherto, the combination of MLC-tracking and MRI has not been investigated using clinically released hardware. In this note we aim to describe the technical feasibilty of such a combination on a clinically operating MRI-linac. The MLC-tracking system is characterized by quantifying the latencies and geometric errors produced by the system. In order to reach optimization recommendations, the tracking system was first characterized using a quasi-ideal position sensor, isolating the performance of the MLC only. Subsequently, the analysis was repeated using real-time MRI as the positioning source for the MLC. For the isolated MLC, we found latencies of 20.67 ms and minimal overshooting behaviour. The latencies for MRI-guidance were 347.45 ms at 4 Hz imaging and 204 ms at 8 Hz. We showed that MLC-tracking on the Elekta Unity using integrated MRI is technically supported and feasible. The isolated analysis of the MLC demonstrated the negligible contribution of the MLC in MRI-guided tracking. The latency and geometric errors caused by the sampling properties of MRI exceed the MLC-related errors by several factors. Most gain for real-time MRI-based adaptive radiotherapy can therefore be realized by optimizing and accelerating the MRI acquisition process.