PhD defence by Mads Fjelbro Klavsen

On Wednesday 22 May 2024, Mads Fjelbro Klavsen defends his PhD thesis “Dosimetry for online monitored MR-Linac radiotherapy".

Time: 13:00

Place: Bldg. 341, aud. 23 & Zoom: https://dtudk.zoom.us/meeting/register/u5YvceqgqzMqGdWO6zs_v3f8dtIZMInI89mq
Please be aware that the PhD defense may be recorded - This will also be informed at the beginning of the PhD defense.

Principal supervisor: Senior Scientist Claus E. Andersen
Co-supervisor: Medical Physicist Claus F. Behrens
Co-supervisor: Professor Ivan Vogelius
Co-supervisor: Senior Researcher Christina Ankjærgaard

Examiners:
Senior Scientist Lars R. Lindvold, DTU Elektro
Professor Faisal Mahmood, Odense University Hospital
Principal Research Scientist Hugo Palmans, National Physical Laboratory

Chairperson at defence:
Senior Researcher Kristina Jørkov Thomsen, Department of Physics 

In conventional radiotherapy, the patient is commonly MR-scanned to create an image of the cancer position and size prior to radiation treatment in a linear accelerator, Linac. A recent innovation combines these two facilities into one, the MR-Linac, which enables real-time high soft-tissue contrast imaging during irradiation treatments such that the patient is not treated “in blind” as in conventional radiotherapy, where the imaging is performed prior to treatment. Additionally, some MR-Linac machines have the ability to process images so fast, that the irradiation beam can be turned on and off based on organ positions. This allows a whole range of treatments not prior possible. It also allows daily adaptations of treatment plans to the patient’s anatomy based on fresh MR-images. 

To fully exploit this new technology, the dosimetry must be thoroughly tested and validated. For this, a detector system that can provide time-resolved dosimetry without image distortion is essential. Many conventional dosimeters contain materials such as metals or graphite which disturbs MR-images or in other ways are affected by the magnetic field. This PhD project have investigated the compatibility and use of a plastic scintillator-based detector system in the MR-Linac. The scintillator converts the deposited energy into light which is transported by optical PMMA fibers to a detection system. It was found that the system could provide time-resolved dosimetry in a range of machines without disturbing the MR-images. One machine was found to have a dose-rate transient which caused a lower dose output right as the beam started. The effect of this dose-rate transient was investigated along with measurements at three other MR-Linacs of the same type. The dose-rate transient appears to be machine specific and that not all machines have one. For certain treatment plans this dose-rate transient could influence the delivered dose such that the cancer receives less radiation than intended.

In order to give the optimal treatment to patients the medical physicists need the best data to make decisions. This project has shown that the plastic scintillator system is an excellent candidate for time-resolved dosimetry for advanced treatment plans in an MR-Linac, and therefore to provide such needed data.