Experimental study on carbon fragmentation for hadrontherapy

Abstract

Nowadays, one of the most advanced methods for solid tumors treatment is represented by hadrontherapy, a radiotherapy technique which applies collimated beams of protons or heavier ions for the sterilization of tumor cells. Due to their very favorable profile of the released dose in tissue, the charged hadron beams can be very effective in destroying the tumor and sparing the adjacent healthy tissue in comparison to the standard X-ray based treatments. The most important difference between protons and heavier ions is the increased biological effectiveness of the latter, i.e. a lower physical dose is needed with ions to obtain a given biological effect. For carbon ions, which are considered the optimal choice, the effect of the favourable absorbed dose distribution, which is highly localized, is enhanced by the large relative biological effectiveness (RBE) towards the end of the particle range. Nevertheless, when the carbon beam penetrates matter, the primary ions can be fragmented as a result of the collisions with the tissue atomic nuclei. The collisions along the carbon path lead to the attenuation of the primary beam intensity and the production of secondary fragments. These lighter particles have longer ranges and wider energy distributions with respect to the primary particles and give rise to a characteristic dose tail behind the Bragg peak. As far as the biological effect of ion radiation is dependent on the particle field composition, a detailed knowledge of the fragmentation process is essential in order to guarantee the appropriate treatment accuracy. Currently, the Monte Carlo codes are the most powerful tools able to precisely compute the biological dose to be delivered. However, the accuracy of a Monte Carlo simulation is associated to the reliability of the physical processes implemented in the code which, for a realistic estimation of fragmentation products, have to be validated versus experimental data, which are still a small amount in the literature. As a consequence, the main goal of the present work consist in estimating the double-differential fragmentation cross sections in the energy range of interest for hadrontherapy. After an overview of the historical development of hadrontherapy, the physical and biological rationale of carbon ions application in tumor treatments are dealt with in Chapter 1, focusing particularly on the fragmentation issue. In order to extract fragmentation cross sections in different experimental conditions, two measurements were performed at intermediate energies on both a thin carbon target and different tissue-equivalent targets, and a third one were done in the relativistic energy domain on a thicker carbon target. In Chapter 2 the experimental devices used in order to perform the three experiments have been described. The first two experiments were carried out at the LNS-INFN in Catania with a beam of carbon ions at 62 AMeV. The cross sections angular and energy distributions were obtained for the thin carbon target analysis and also a comparison with those extracted by means of the GEANT4 Monte Carlo code were performed. The results are shown and discussed in Chapter 3 together with those associated to the second experiment, done at the same energy but on thick tissue-equivalent targets. In Chapter 4 the preliminary results of the measurement performed at the GSI laboratory (Darmstadt, Germany) with a 400 AMeV carbon beam and the comparison with those obtained with the FLUKA Monte Carlo code are presented and discussed. In the end, the conclusions of the whole work done are drawn and the future perspectives are outlined.