Gel dosimetry for a MR-linac: magnetic field and time dependency

Gel dosimetry for a MR-linac: magnetic field and time dependency

van Aert, Emy (TU Delft Mechanical, Maritime and Materials Engineering)

Denkova, A.G. (mentor)
Wolthaus, Jochem (mentor)
Woodings, Simon (mentor)
Schaart, D.R. (graduation committee)
Djanashvili, K. (graduation committee)

Delft University of Technology

2020-06-02

The goal of this research was to perform a 3D end-to-end test on a MR-linac to check the whole workflow using a clinical treatment plan. Dosimetric gel was used to obtain 3D spatial information, with the phantom in the same position for irradiation and scanning. In order to achieve this, fundamental elements of gel dosimetry needed to be investigated. In the MR-linac, irradiation is delivered in the presence of a permanent magnetic field. Therefore, the dosimetric response within a 1.5 T magnetic field should be validated. It is also important to investigate the time-dependence of the gel. It is preferable to read-out the gels within approximately one hour, so that the phantom does not have to be moved. Ideally, scanning and irradiation would be done at the same time, to see the dynamical dose delivery. The VIPAR gel was used for this research. The experiments demonstrated that R2 values for doses irradiated with magnetic field were the same as R2 values for the same dose irradiated without magnetic field. R2 values are still proportional to the dose. It was also shown that it is possible to scan the phantom within 20 minutes after irradiation. Sensitivity is at its highest after approximately 8 hours and stays stable afterwards, so scanning after 8 hours will improve the read-out accuracy. It was also possible to make a fit for the R2 versus time plots, which makes it possible to correct for change over time. The fit can be divided in two linear parts if time is plotted on a logarithmic scale, one fit for the time points before 7 hours, one for the time points after 7 hours. The partial doses acquired by the gel during radiation delivery were estimated. The equivalent R2 values then agreed with the extrapolated fit to within 4%. This is a good indication that dynamic gel (4D) dosimetry may be achievable. A protocol for a relative end-to-end test was also developed. From the preliminary results, it appeared that a relative end-to-end test can be performed with the read-out of gel within 1 hour. A new MR sequence needs to be developed. For this end-to-end test, the sequence needs to scan a larger volume with a higher resolution, therefore, the scan time will increase and real-time dosimetry will not be possible. Changing the MR sequence might also change the optimal irradiation-scanning interval and the R2 versus time curve. To perform absolute dosimetry, an extra calibration would be required.

gel dosimetry
MR-linac
magnetic field

http://resolver.tudelft.nl/uuid:4408f7c6-d16f-4cab-90c0-60dbf03d8a27

2020-10-31

Student theses

master thesis

© 2020 Emy van Aert