# The Performance of LiF:Mg-Ti for Proton Dosimetry within the Framework of the MoVe IT Project
## 1 Introduction  
Proton beam therapy (PBT) represents the election therapy for the treatment of many solid tumors [1, 2]. Charged particles (protons or ions) show physical properties that give a more localized energy deposition with reference to conventional MV X-ray therapy. PBT treatment plans manage to maximize the tumor control probability while minimizing the damage to surrounding healthy tissues. The theoretical advantage over photon radiation lies within the depth-dose profile, referred to as the Bragg curve, culminating with a sharp decrease beyond the so-called Bragg peak, especially for protons. Consequently, the deposited dose increases steeply within a short distance nearing the end of the protons' range, resulting in high conformity to the tumor. In order to account for the difference in biological response between high linear energy transfer (LET) radiation and low LET radiation, the relative biological effectiveness (RBE) parameter was introduced. RBE is defined as the ratio of the absorbed dose of a usually low LET reference radiation (such as X-rays or Co-60 gamma rays) to the absorbed dose of an ionizing particle that produces the same biological effect. The RBE is a modifying factor that converts the physical dose into a biological equivalent dose. For protons, the International Commission on RadiationUnits [3] recommended that one assume the value of the RBE as being equal to 1.1, meaning that protons are 10% more effective than photons regarding the defined biological response; however, the use of a constant RBE of 1.1 by the proton therapy community is currently under debate [4]. Beyond the biological endpoint, RBE depends on many factors [5; 6], such as the deposited physical dose, radiation quality (LET, type, energy), radiosensitivity of the irradiated tissue and cell type, dose rate, and oxygen concentration. Interestingly, the RBE of protons increases close to the field edges and beyond the distal Bragg peak, as LET increases moderately through the spread-out Bragg peak (SOBP) [5; 7; 8; 9; 10; 11]. The spatial variation in RBE along the proton beam may have important clinical impacts regarding potential increases in treatment complications, because the distal edge of the SOBP could be situated within the surrounding normal tissue [12]; that is, an increase in RBE at the end of the proton range should be accounted for in treatment planning [13] in order to gain the optimum therapeutic advantage. The knowledge of proton radiobiology is based on important information extracted from in vitro experiments using cell cultures [14; 15; 16; 17; 18; 19]. New studies regarding the relative radiobiological effectiveness of proton irradiation should be carried out from in vivo, through which the generated hypotheses could be formed for the far more complex biology of patients; however, due to the associated costs and time required, few studies assessing the radiation effects of protons on in vivo models are available in the literature [20; 21; 22]. The scarcity of such in vivo experiments means the RBE issue remains unsolved. Advances in particle therapy have meant that models are required for biologically optimized treatment planning systems (TPS) in ion beam therapy, as well as dedicated devices for plan verification that can account for a wide range of complex physical and biological effects. The present work lies within the framework of the MoVe IT project, aiming to exploit both modeling and verification methods for ion beam TPS, with a main focus on RBE, the biological impacts of nuclear fragmentation, and intra-tumor heterogeneity. This project involves interdisciplinary research, in which physical and radiobiological expertise are managed in 4 scientific work packages (WP). The results of this paper are gained from experimental activities included within WP3, which is targeted toward biological dosimetry.