# Synchrotron X-ray Irradiation of a Rat's Head Model: Monte Carlo Study of Chromatic Gel Dosimetry
## 1 Introduction  
Three-dimensional (3D) dosimetry is increasingly used in radiation therapy because it can expand our knowledge regarding the dose distribution of high-energy radiation in the human body. Such information can help improve treatment planning accuracy and decrease the damage to the healthy cells surrounding a tumor. For this reason, scholars and practitioners are devoting increasing attention to polymer gel dosimeters, which have the unique ability to record in 3D irradiated doses in the energy range of medical treatments. Polymer gel dosimeters are based on crosslinking monomers in a gel matrix via radiation-induced polymerization, forming a polymer with different physical or chemical properties in the irradiated area, thereby recording the dose distribution in that area [1]. Various polymer gel-type dosimeters are being studied and developed, including the standard polyacrylamide-based dosimeters (PAG, or the commercially available name BANG(r)[2, 3]). These dosimeters consist of the co-monomers acrylamide and N,N'-methylene-bis-acrylamide (Bis) dispersed in an aqueous gelatin matrix. The main disadvantage of PAG dosimeters is the need to avoid oxygen contamination of the solution during their preparation, i.e., they cannot be prepared in normal atmospheric conditions [4]. Another disadvantage of PAG dosimeters and others is that their analysis requires MRI imaging. This has led to the development of chromatic gel dosimeters that are prepared easily and can be analyzed using optical techniques. In aprevious study, a polydiacetylene-based gel dosimeter was characterized and examined as a tool for dose calculations in medical treatment planning [5]. The polydiacetylene-based gel dosimeter was examined using a 160 kV X-ray beam [5]. The colorless monomeric diacetylenes polymerize via a radiation-induced topotactic 1,4-addition mechanism to form the conjugated, linear-backbone, blue polydiacetylene (PDA), which consists of alternating triple and double bonds (yne-ene motif, -C=C-C\(\equiv\)C-) [6]. Polydiacetylene exists in two phases--the blue phase and the auto-fluorescent (emission peak around 650 nm) red phase that can be achieved with further stimulation such as heat, excess radiation, changes in the chemical environment, etc. A schematic drawing of the three phases of PDA is shown in Figure 1. The two absorbance peaks for the blue phase are positioned at 590 nm (vibronic) and 650 nm (excitonic), and for the red phase the peaks are positioned at 500 nm (vibronic) and 550 nm (excitonic) [7]. The intensity of the absorbance is proportional to the irradiated dose. This unique property of PDA enables analysis of the absorbed dose using absorbance and fluorescence techniques, as opposed to with other dosimeters. Diacetylenes embedded in a water-equivalent gel matrix can then show the dose distribution in 3D as an equivalent for soft-tissue materials. Monte Carlo (MC) simulations can further verify this distribution. This technique aims to simulate the dose distribution in matter using individual interaction probabilities [8]. Radiation transport of an individual particle inside a defined medium MC simulation is based on random variables and the tracking of that particle with a large number of histories. MC methods provide knowledge about the physics and the interaction of particles (including photons) in the matter [9]. A Monte Carlo simulation code system contains four main components: (1) The cross-section data for all the processes that are being considered in the simulation, (2) the algorithm used for the transport of the particles, (3) a definition of the geometry of the problem and its physical parameters, and (4) the analysis of the output of the simulation [10].