Treatment planning strategies for proton therapy in the presence of a cranial fixation clip

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This thesis outlines the project conducted at HollandPTC for the Master Biomedical Engineering, track Medical Physics, at Delft University of Technology.

Metal implants create dosimetric uncertainties in treatment planning and delivery for proton therapy. The purpose of this thesis was to determine the most suitable treatment planning strategy for proton therapy in the presence of cranial fixation clips.
A phantom was designed and imaged using computed tomography. Subsequently, base experiments were performed on a mono-energetic proton beamline to investigate the dose perturbation caused by the clip. It was found that the presence of the clip in the radiation field produces a measurable signal perturbation and that the clip affects the field uniformity.

A total of seven different treatment planning strategies were created. These strategies were based on four main concepts that were determined from a literature review and were identified to have potential effectiveness in addressing cranial fixation clips for brain cancer: implementing a density override on the implant, employing either a single-field optimization (SFO) or multi-field optimization (MFO) technique, implementing beam-specific margins or not, and adding an avoidance margin on the implant.
All strategies, except for the one including an avoidance margin around the implant, were successfully implemented into the treatment planning system. The plans were robustly optimized and met the clinical goals regarding clinical target volume (CTV) coverage and organ at risk (OAR) sparing.

The strategies were evaluated by performing experiments on a clinical treatment gantry. Based on the results of the first set of experiments, a Lynx detector was used, which was currently not able to detect the measured signal in dose in Gray. The aim of the experiments was to gain insights into the agreement between the data from the treatment planning system and the experiments, as well as to investigate what the best strategy is for handling cranial fixation clips in proton therapy.

Based on the results of the conducted experiments, there is an indication that the field homogeneity inside the target volume gets perturbed by the clip. This indicates a high need to investigate the clinical implications of this further. In particular, absolute dose measurements must be added to the presented results.