A pixel detector characterization for the quality assurance

Proton therapy using Pencil Beam Scanning (PBS) offers highly conformal dose distributions for cancer treatment, maximizing tumor control while minimizing damage to surrounding healthy tissues. However, the extreme precision of PBS demands rigorous and efficient Quality Assurance (QA) protocols to guarantee patient safety. QA systems require very high spatial resolution (σx,y < 1−2 mm) to verify complex, heavily modulated dose fields and must be as time-efficient as possible. This thesis focuses on the characterization and dose calibration of a novel high-resolution pixel detector system designed for advanced QA applications. The work was carried out at the Center for Proton Therapy (CPT) at the Paul Scherrer Institute (PSI). Measurements were performed at the Gantry 2 treatment room, comparing the pixel detector’s response with standard dosimetric devices and Monte Carlo simulations implemented in TOPAS. The core objective of the project was to derive and validate a reliable dose conversion factor to translate the raw signal collected by the pixel detector into absolute dose values. This work successfully demonstrated the capabilities of the device, particularly its excellent 2D spatial resolution of delivered spots. However, the full characterization of the detector is not yet complete. During the analysis, deviations from reference values were observed, and a specific structural limitation of the device was identified. Since this was not found to be the sole source of error, these factors must be carefully considered to perform correct quality assurance measurements. Ultimately, this work outlines the current limitations of the detector and highlights the need for further studies and future tests to complete its full validation.