• hadrontherapy
• proton therapy
• particle treatment monitoring
• PET imaging
• photodetector
• Silicon Photomultiplier
• PET-scanner
• INSIDE project
• Coincidence Time Resolution

In the last decades, the interest in charged hadrontherapy (also known as particles therapy) as cancer treatment has greatly increased, leading to a rapid growth of dedicated facilities. Currently there are 55 operating structures and 35 more are under construction. In most of these facilities hadrontherapy is performed using proton beams and can be also referred to as proton therapy. Typical energy values for therapeutical protons beams are in the interval 3-300 MeV, corresponding to a range in water from 0.014 to 51.45 cm.
With respect to standard radiotherapy employing photons in the gamma-rays energy range, hadrontherapy shows a superior physical selectivity, which allows to deliver a radiation dose more specifically to the tumour volume, while less damaging the surrounding healthy tissues. At the same time this kind of therapy requires a precise monitoring of the location of the delivered dose, both to ensure an efficient irradiation of the tumour and to safeguard patients health, avoiding the development of severe side-effect due to the overdosing of radiosensitive organs.

Over the years, a number of particles range verification methods have been proposed: the most consolidated and mature for the clinical application is the one based on the Positron Emission Tomography (PET).
When passing through tissues, impinging particles may be subjected to nuclear interactions with atomic nuclei, which result in the creation of positron emitting isotopes all along the beam path. The detection by a PET-scanner of the back-to-back photons, coming from the annihilation of the positron with an atomic electron, allows obtaining the spatial distribution of the activity induced in the irradiated tissues. The activation profile is characterised by a constant or slowly rising trend with depth, followed by a sudden drop to zero few millimeters in front of the Bragg peak. The particles range is usually determined through a comparison of the experimental activity profile with the one calculated with a Monte Carlo simulation.

Among the proposed methods for the particles treatment monitoring, one of the most promising, even though still in development, is the one based on the detection of charged particles or prompt-gammas emitted during the irradiation. The particles range is then estimated by exploiting its correlation with the prompt-particles emission point.

This thesis has been carried out in the framework of the INSIDE project, which aims at developing a multimodal system for the on-line monitoring of the dose delivered to a patient during an hadrontherapy treatment, both in case of proton and carbon ion beams. The system will be installed at the the National Centre of Oncological Hadrontherapy (CNAO) in Pavia, a clinical facility specifically dedicated to hadrontherapy treatments, and it will be composed of a dual head PET scanner and a charged particle tracker for the detection of prompt secondary particles. In particular, the main subject of this work is the characterization of a prototype of the INSIDE PET-scanner.

The document is organised as follows: chapter 1 is dedicated to a general description of particles therapy and, in particular, of the clinical particles beams characteristics. Moreover, the National Centre of Oncological Hadrontherapy in Pavia, the particles therapy-dedicated facility where the INSIDE system is going to be installed, is also described.

Chapter 2 is dedicated to a review of the existing techniques for the particles range verification, paying particular attention to the PET one.

The third chapter presents the main characteristics of the INSIDE system, with the PET-scanner described with more details. The PET scanner will feature two planar heads (10x5 cm^2 each) constituted by 5x2 matrices of pixelated LFS scintillating crystals. Each pixel is coupled one to one with a Silicon Photomultiplier, which represents the latest technology of the solid state photo-detectors. The front-end electronics is constituted by 64 channels Integrated Circuits, with timing resolution at the state of the art. The final goal of the PET scanner is to provide a verification of the particels range with a spatial resolution of about 1 mm in less than 5 minutes after the begininng of irradiation, using only a fraction of the total dose.

Chapter 4 is devoted to the characterization of a prototype of the PET scanner, made up by two LFS matrices facing each other. Specifically, it has been verified the correct functioning of the different SiPMs and their gain uniformity. Moreover it has been monitored the system performances stability over time.

In May of this year, the prototypal PET system has been moved to CNAO for a test beam session. During the test, a PMMA phantom had been irradiated with proton beams at four different energies (68, 72, 84 and 100 MeV). Chapter 5 reports the analysis of the data acquired during the test and a discussion on the achieved results. In particular, it has been verified the capability of the system to detect the coincidence signals both during inter-spill and in-spill phases of the irradiation and it has been measured the achieved timing resolution. The standard deviation of the coincidence distribution or Coincidence Time Resolution (CTR) is 620 ps in the inter-spills and 700 ps in the in-spill one.

Finally, the image reconstruction algorithm employed is described and the images of the activity distribution obtained are presented.