Silicon microdosimetry in hadron therapy fields

Hadron therapy is one of the most sophisticated methods of radiation therapy that has been constantly evolving during the past decades. The use of hadron beams for cancer treatment can be more effective in comparison to the conventional radiotherapy, due to the high ballistic precision and the high biological effectiveness of the particles. The implementation of the hadron beams in cancer therapy raised the need of establishing protocols for the dosimetric characterization of the beams for therapeutic precision and radiation protection. Several attempts to provide standards and protocols for hadron therapy based on the conventional dosimetric approach were proved to be insufficient, since an average quantity such as the absorbed dose cannot provide information on the biological effects of the hadron beams that are strictly related to the local distribution of the energy deposited at micrometric scale. The microdosimetric approach of the characterization of hadron beams intends to cover this gap and provide information of all beam properties, both physical and biological. Tissue–Equivalent Proportional Counters (TEPCs) are the main detectors used to perform microdosimetry for assessing the beam quality in hadron therapy. However, there are several problems and limitations in the use of TEPCs, such as distortions of microdosimetric distributions due to wall effects and paralyzation of the detector at high flux fields because of pile-up effects associated to high count rates. These drawbacks in addition to the lack of transportability and ease of use, mainly due to the need of a continuous tissue–equivalent gas flow system, encourage the seeking for alternative methods, such as silicon microdosimetry. A silicon microdosimeter, based on the monolithic silicon technology, was proposed during the past decade, by the Laboratory of Nuclear Measurements of “Politecnico di Milano” for hadron therapy applications. The device was irradiated with a 62 MeV clinical proton beam at the “Centro di AdroTerapia e Applicazioni Nucleari Avanzate” (CATANA) facility of the “Istituto Nazionale di Fisica Nucleare” (INFN) – “Laboratori Nazionali del Sud” (LNS) (Catania, Italy) and a 100 MeV pulsed proton beam at the Loma Linda University Medical Centre (California, USA). The results of these measurements confirmed the detector’s capability of characterizing a therapeutic proton beam. Preliminary measurements with a 62 AMeV carbon ion beam were also performed at the CATANA facility. The aim of this thesis was to investigate the capability of characterizing a heavy ion beam with a silicon prototype device and its’ geometrically varying versions. Supplementary measurements were carried out at the CATANA facility with a 62 AMeV carbon ion beam, under the same experimental conditions with the preliminary measurements performed in the past, in order to ensure reproducibility. Previous results were used to compare and confirm the consistency of the new results and additional sets of measurements completed the characterization of this field. The results of this experimental campaign for some common points were compared with the ones of the preliminary measurements performed in the past and found in agreement. Finally, the microdosimetric profile and the characterization of the irradiation field were completed by summarizing all sets of measurements. A comparison of the numerical and experimental study carried out to characterize a 290 AMeV carbon ion beam at the Heavy Ion Medical Accelerator in Chiba (HIMAC) facility (Chiba, Japan). A comparison between the response of two different versions of the silicon device to the same field and a comparison of the detector response to two different fields (monoenergetic and clinical) of the same energy are also included. The numerical results were compared and found in agreement with the experimental data, confirming the consistency of the results and enhancing the confidence on the detectors’ performance in high energy and flux hadrontherapy fields. Preliminary measurements aiming to demonstrate the silicon detector capability of characterizing a therapeutic carbon ion beam in comparison to the one of a TEPC were performed at the “Centro Nazionale di Adroterapia Oncologica” (CNAO) facility (Pavia, Italy) with a 362 AMeV clinical carbon ion beam. Measurements were carried out with a mini – TEPC by INFN – “Laboratori Nazionali di Legnaro” (LNL), under the same experimental conditions, enabling the direct comparison of the results. These measurements are among the first microdosimetric measurements performed in therapeutic carbon ion field and the first that are carried out together with the reference detection system. The results of the comparison between the microdosimetric spectra derived with the two detection systems were considered to be satisfactory and the detector capability is confirmed. Minor deviations that occurred could be due to uncertainty in the precision of positioning of the two detectors (their dimensions are of different order of magnitude with the silicon detector being in μm while mini TEPC in mm) and to uncertainties induced by possible geometrical differences related to the chord length distribution in the sensitive volumes. Also, for this particular set of measurements the superiority of the TEPC concerning the minimum detectable energy does not seem to affect the final result. Due to the small number of measuring positions though, it is not safe to draw any conclusions concerning this issue and therefore supplementary measurements are recommended. In conclusion, the capability of the silicon detectors to acquire microdosimetric spectra similar to those obtained with a reference microdosimeter has been confirmed, especially with the experimental campaign at the CNAO facility where a direct comparison was made. However, all results (including the ones of the mini-TEPC) were carried out at beam currents about two orders of magnitude lower than clinical ones, due to signal saturation and pile-up effects. Still, the irradiations in such high energy and flux fields provided useful information on the detector behavior that concerns the charge collection by the pixels guards. These indications require further investigation and could be the subject of future research. In seek of other potential microdosimetric applications of the silicon microdosimeter, its latest version was irradiated with a 70 MeV carbon ion beam in vacuum at the Heavy Ion Accelerator Facility of the Australian National University. The device was irradiated for the first time in the context of an experiment aiming at better understanding the radiobiological effectiveness of a therapeutic carbon ion beam in the distal part of the Bragg peak and estimating the quality factor of carbon ions only with minimal fragment contribution. The outcome of this primary test, based on the successful execution of the experimental procedure and to the good experimental results was considered to be satisfactory. Nevertheless, a feasibility study is necessary to be conducted in the future and additional irradiations are recommended for a more detailed analysis of the new detector’s behavior and performance, especially focusing on the possible impact of the modifications of the new version device might have.