Calibrazione di un sistema per il monitoraggio dell'energia di fasci di protoni terapeutici

Hadrontherapy is a form of radiation therapy which employs hadron beams, typically protons and carbon ions, to deliver the prescribed amount of dose to destroy cancer cells, at the same time attempting to preserve the surrounding healthy tissues thanks to a selective and localized energy release. The most advanced and employed beam delivery system in proton therapy is the active scanning technique, which involves the use of monoenergetic pencil beams (FWHM in the order of 10 mm) controlled with fast steering magnets to cover the volume target. Therefore, for the safety of the treatment, a precise and fast monitoring of the beam parameters is mandatory, notably the number of ions irradiated to each spot and their energy, which defines the penetration range in body of the patient. Nowadays, the measurements of the beam fluence, position and shape are performed by large area planar gas ionization chambers while the online monitoring of the energy is still an open issue and therefore the range accuracy must be daily guaranteed by safe checks of the accelerator settings and quality assurance (QA) measurements of the beam. This study is part the MoVe-IT experiment, an INFN project, where the medical physics group of the University of Turin is involved in the development of new beam monitors for radiobiological applications based on solid state detectors, which could overcome the limitations of ionization chambers. In particular, this thesis work investigates the use of innovative thin silicon detectors (UFSDs) based on the Low Gain Avalanche Diode (LGAD) technology for the direct measurement of the energy of therapeutic proton beams exploiting the time-of-flight (ToF) technique. The main advantage of these sensors comes from their excellent time resolution (tens of ps in 50 µm of active thickness), which makes them suitable for single particle detection and timing applications, indeed a precision in the order of the ps on the ToF is needed to keep the uncertainty on the equivalent range in water below the clinical tolerance of 1 mm. A prototype of telescope of two UFSD strips sensors aligned along the beam direction has been built and tested at CNAO (Pavia) and at the Trento Proton Therapy Centre, covering the therapeutic proton energies from 60 up to 230 MeV (range in water from 3 up to 32 cm) for clinical values of intensity (10^8 - 10^10 p s^-1 cm^-2). Starting from the measured time differences of coincident protons crossing the two detectors for different beam energies and positions of the sensors, a self-calibration method, independent from any a priori knowledge of the beam parameters, will be described with the aim of removing the systematic errors due to the experimental setup. The system here proposed is transparent (may be placed on the treatment line) and could be adopted for routine quality control checks substituting the present measurements of depth-dose profiles performed with ionization chambers immersed in water phantoms, which require a longer execution time compared to the few seconds of irradiation needed to the telescope. Moreover, the use of silicon detectors provides a direct measurement of the beam energy which is independent both from the ionization density and environmental parameters. The future perspectives of the system will be the online monitoring of the energy during the treatment and the application in dose delivery methods which require a fast modulation of the beam energy during the irradiation.