Development of systems based on silicon detectors for beam monitoring, treatment verification and microdosimetry in Particle Therapy

Charged Particle Therapy (CPT) has gained a signi cant interest for the treatment of solid tumors thanks to the favorable depth-dose deposition which allows for the delivery of a well-conformed dose to the tumour while sparing healthy tissues e ectively. In addition, charged particles, such as carbon ions, exhibit a larger biological effectiveness in comparison to conventional photons, thereby providing the potential to treat radioresistant tumors with a higher success probability. This thesis presents the development and testing of new system based on silicon detectors to further improve the delivery of the dose to the tumor. In particular, this work explores the use of silicon detectors for beam monitoring, particle range verification and treatment quality assurance through microdosimetric measurements.

The state-of-the-art of beam monitors in CPT are the gas- lled Ionization Chambers (IC), which measure beam position, shape and particle flux. Even though ICs are widely used in clinics showing good radiation hardness, the slow charge collection time of 100 s and the low sensitivity of 1000 particles prevent ICs from being used for the development of faster and more precise irradiation modalities. The medical physics group of the University of Torino and the Nuclear Institute for Nuclear Physics (INFN) is working on the development of new detectors based on silicon sensors for applications in beam monitoring and range veri cation during the treatment. Thin planar silicon sensors appear to be a promising alternative to ICs, allowing for the direct discrimination of particles, thanks to the short charge collection time of 1 ns and to the sensitivity to the single particle.

In this context, an innovative proton and carbon ion counter is presented in this work. The detector exploits strip-segmented planar silicon sensors with an active thickness of a few tens of m read out by a front-end electronics based on a 24-channel ASIC for the discrimination of the particles' signals. The detectors were tested with clinical beams at the Centro Nazionale di Adroterapia Oncologica (CNAO) in Pavia, Italy. The proton counting efficiency shows a dependence on the beam energy because of transversal dimension and pile-up effects whereas an efficiency between 94 and 98 % with lower energy dependence was measured for carbon ion beams. In addition, the particle counter was integrated with the CERN PicoTDC, a Time-to-Digital Converter with a minimum bin size of 3 ps. A measurement of the distribution of the time interval between consecutive crossing particles was performed and was found to be compatible with the accelerator radio-frequency period.

The second application investigated in this thesis is a novel range verification system based on the Prompt Gamma Timing (PGT) technique. The PGT method provides the assessment of the particle range by measuring the time of flight between the primary particle transit time and the detection of the Prompt Gamma (PG) photons emitted by the fast de-excitation of nuclei left in an excited state by nuclear interactions. The setup relies on a strip-segmented silicon sensor and a LaBr₃(Ce) scintillating crystal coupled with Silicon PhotoMultiplier to detect the primary particles and the PG photons, respectively. The signal readout is based on the PicoTDC. The preliminary measurements were conducted with 398 MeV/u carbon ions at sub-clinical rate in CNAO, showing promising results.

Finally, the thesis reports the microdosimetric measurements and simulations performed with a 3D Silicon-On-Insulator (SOI) microdosimeter developed by the Centre for Medical Radiation Physics (CMRP) in Wollongong, Australia. The aim is to compare the Relative Biological Effectiveness (RBE) calculated using the Microdosimetric Kinetic Model (MKM), based on microdosimetric measurements, with the RBE computed by a Treatment Planning System (TPS) using the Local Effect Model (LEM). The 3D SOI microdosimeter was placed in an RW3 phantom and was irradiated with different carbon ion plans at CNAO, acquiring microdosimetric spectra along the beam direction. A good agreement between experiment and Monte Carlo simulation was found, providing RBE₁₀ values ranging between 1.2 and 2.8. The prescribed LEM-based biological dose of 3 GyE in a cubic Spread-Out-Bragg- Peak was found to be 33 % larger than the MKM-based biological dose, consistently with other results found in the literature. This result confirms the reliable use of the 3D SOI microdosimeters as a quality assurance tool for RBE prediction in particle therapy.