Breathtaking precision: Towards MR-guided MLC tracking for lung SBRT

This thesis aimed to investigate the technical feasibility and dosimetric benefits of different online adaptive radiotherapy techniques on the Unity MR-linac, especially MR-guided multileaf colliamtor (MLC) tracking. Additionally, we focussed on the validation of online adaptive radiotherapy workflows by developing and testing a plastic scintillation dosimeter (PSD)-based QA device.

To compensate for respiratory motion during lung stereotactic body radiation therapy (SBRT), the treatment margins are typically extended to encompass the entire range of tumour motion. Active motion management, such as MLC tracking and trailing, offers the potential to reduce these treatment margins to maximize healthy tissue sparing. Chapter 2 provided the first experimental validation of MR-guided MLC tracking on the Unity MR-linac for a full step-and-shoot lung intensity modulated radiation therapy (IMRT) delivery. Based on phantom experiments we demonstrated that a prediction filter effectively mitigates the system latency induced by the tracking workflow. Radiochromatic film-based experiments demonstrated that MLC tracking effectively restored the static dose for both central and peripheral targets. Furthermore, it was demonstrated that MR-guided trailing combined with a midP approach highly effectively mitigates baseline motion, resulting in improved target coverage.

While the Unity MR-linac only supports step-and-shoot IMRT in clinical mode, volumetric modulated arc therapy (VMAT) can often spare even more healthy tissue with equal or better target coverage. In chapter 3 MLC tracking was integrated with a research VMAT interface. The challenge with combining VMAT with MLC tracking is that both dynamic modes require MLC motion. Nevertheless, chapter 3 demonstrated the technical feasibility of VMAT combined with MR-guided MLC tracking on the Unity MR-linac. Similarly to chapter 2, chapter 3 showed through film-based experiments that applying MLC tracking restored the static dose map. In addition, we developed a novel motion-encoded and time-resolved virtual Hexamotion platform to demonstrate the excellent time-resolved performance of MR-guided MLC tracking in pseudo-3D.

The increased treatment complexity and motion-delivery interplay during MR-guided MLC tracking require dedicated motion phantoms with integrated dosimeters to validate the delivery in an end-to-end fashion. Currently, available MR-compatible motion phantoms often use radiochromic film as a dosimeter. Despite the advantages of film dosimetry, it lacks temporal resolution. In Chapter 4, we validated the performance of a time-resolved PSD for use in a 1.5 T MR-linac. We demonstrated that the PSD performed highly accurate dosimetry readings that were consistent with a previously validated microDiamond. The PSD's performance was not affected by MR scanning or by motion. Lastly, the PSD experienced only minimal angular dependence, which was the first report of such performance.

In Chapter 5, we developed a new, commercially available QA device that integrated radiochromatic film and four PSDs in a cassette that seamlessly integrates with an existing motion phantom. This combination simultaneously harnesses the spatial resolution of film with the time-resolved dosimetry of PSDs. Chapter 5 demonstrated that this new commercial MRI4D scintillator cassette is suitable for use in the 1.5 T MR-linac. When combined with a motion phantom, the MRI4D scintillator cassette provides accurate, meaningful, patient-specific, and motion-integrated quality assurance for various adaptive radiotherapy scenarios.