Three-dimensional dose verification of adaptive radiotherapy at an MR-linac

Abstract

The vision of accurate real-time image-guided radiotherapy (RT) has led to the development of MR-Linacs consisting of MRI scanners integrated within RT treatment units. The safe clinical operation of these devices requires to calculate and verify complex clinical dose distributions in patients exposed to the static magnetic field of the MR-scanner. The effects of the magnetic field on patient dose are particularly pronounced at tissue-air interfaces where an increase in dose occurs due to the electron return effect (ERE). State of the art Monte Carlo (MC) dose calculation algorithms account for this effect, and form the basis for the implementation of unprecedented on-line adaptive treatment strategies. Of particular concern are the treatment sites involving the lung and the pelvic region where the ERE can impact significantly on the dose. In this thesis, I have developed 3D-dosimetry techniques based on PRESAGE(R) dosimeters for the validation of these novel techniques. In contrast to most conventional radiation detectors, this dosimeter is not affected by the magnetic field and provides uncompromised 3D dose information, suitable for end-to-end testing of adaptive workflows in MRI-guided RT. The hitherto inability of retrieving dose using the 3D dosimeters periphery was solved, enabling for the first time, measuring the ERE in 3D and benchmarking the results with MC calculations. By exploiting the full volume of PRESAGE(R) dosimeters and using a commercial phantom, I developed a reproducible and versatile methodology that was easily adapted to perform a variety of tests at the Elekta MR-linac. Different treatment sites and clinically available workflows were assessed, and excellent 3D agreement between calculations and measurements confirmed their correct implementation. With the same methodology, I performed the first dose verification of MLC-tracked RT at the Elekta MR-linac.