MR-linac QA

Radiotherapy plays a crucial role in cancer treatment. An important innovation in the delivery of treatment is the MR-linac, a device that combines magnetic resonance imaging with a linear accelerator. This system enables precise visualization of tumors and surrounding tissues before the start of the treatment session, allowing daily variations to be incorporated directly into the adapted treatment plan while the patient is on the treatment table. This approach is known as online inter-fraction adaptive radiotherapy. In addition to imaging before treatment, the MR-linac can also acquire images during radiation delivery to track tumor motion, enabling techniques such as beam gating and intra-fraction drift correction. This is known as online intra-fraction adaptive radiotherapy. To ensure the accuracy of these treatments, quality assurance (QA) procedures are performed. Some of these involve measurements with dosimeters, which quantify the delivered dose. To validate the complete workflow, end-to-end (E2E) tests are conducted that encompass all clinical steps. This thesis investigates the accuracy of measurement devices used on the MR-linac and the accuracy of MR-linac workflows. Several key conclusions were drawn from the conducted studies. First, two devices used for dose measurements, a plastic scintillation detector and a Delta4 phantom, were evaluated and shown to be capable of time-resolved dosimetry with a high temporal resolution. Next, a motion platform was characterized and assessed for use in the MR-linac, enabling the integration of motion into dosimetric measurements. Combining these devices provided a tool capable of performing dosimetry during controlled motion of the detector. After the assessment of these devices, two E2E tests were performed to evaluate the online inter- and intra-fraction adaptive workflows. For the online inter-fraction adaptive workflow, 3D gel dosimeters were employed, demonstrating excellent geometric and dosimetric accuracy and providing volumetric information not available from 1D and 2D dosimeters. To evaluate online intra-fraction adaptive workflows, a novel E2E testing approach was developed, demonstrating that motion-inclusive reference dose distributions are crucial for reliable accuracy assessment. This study demonstrates that intra-fraction adaptive treatments can be delivered with high accuracy. Finally, the feasibility of log file–based dose reconstructions during intra-fraction motion was explored. These calculations showed good agreement with measured dose distributions and hold promise as a tool for patient-specific QA of intra-fraction adaptive workflows. Overall, this thesis presents a comprehensive evaluation of measurement devices and workflows for the MR-linac. By characterizing detectors, developing motion-integrated measurement tools, and performing E2E tests of both online inter- and intra-fraction adaptive workflows, it establishes practical methods for QA and verification of treatment accuracy.