# Synchrotron X-ray Irradiation of a Rat's Head Model: Monte Carlo Study of Chromatic Gel Dosimetry
## 3 Results  
The use of PDA-based gel dosimeters as a clinical reference system in synchrotron irradiations was examined using the EGS5 Monte Carlo code system. In the simulation, the percentage depth dose (PDD; the dose distribution as a function of the depth in the tissue) in a rat's brain tumor was compared with the PDD of two types of PDA-gel dosimeters: PDA-Phydagel and PDA-RTV. For both dosimeters, the absorbed spectra demonstrated similar trends to the spectrum in the brain tumor. Figure 6 shows that the PDA-RTV dosimeter accords more closely with the brain's spectrum relative to the PDA-Phydagel. This results from the molecular composition of the gel. The silicone atoms present in the PDA-RTV gel interact similarly to the elements present in the brain and skull tissue (i.e., sodium, magnesium, and calcium) since their elemental atomic numbers are similar. Additionally, the RTV matrix contains a larger amount of carbon than the PhytagelTM (two orders of magnitude greater than the amount of carbon in the brain). Accordingly, the molecular composition dictates the dose deposition nature of the tissue. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Body Zone** & **Radius (cm)** & **Length (cm)** & **Thickness (cm)** \\ \hline Brain & 0.84 & 2.00 & 0.84 \\ Skull & 0.88 & 2.08 & 0.04 \\ Tumor/dosimeter & 0.20 & 2.00 & 0.20 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Dimensions of the bodies in the geometry setup for the simulations. Figure 5: The initial synchrotron X-ray spectrum that was used as the energy distribution function for random initial photon energy sampling. Furthermore, the PDD curves obtained in the simulation showed similar results. PDD curves are used by radiation physicists as a tool to improve understanding of the dose distribution in the irradiated organ prior to the radiation treatment. Thus, damage caused by the radiation is decreased. Calculating PDD curves of the dosimeters and comparing them with the PDD curve of the brain demonstrated the differences in the radiation behavior of each material. Figure 7 shows the PDD curve obtained for the rat's brain, and the following figures, Figures 8 and 9, show the PDD curves for the PDA-Phydtagl and PDA-RTV dosimeters, respectively. The statistical error varies between 2 and 3% for all the PDD data. For each PDD curve, the doses were calculated for slices of 0.05 cm thickness up to 2 cm depth, using the volume of each slice, the density of the material, and the absorbed energy distribution per incident photon that was obtained via the simulation. The dose-difference plot between the brain and the PDA-Phydtagl dosimeter is shown in Figure 10. The PDD curve of each dosimeter and the brain PDD curve were compared by subtraction of the two curves to obtain the dose corrections formula from the linear fit. Figure 6: Absorbed photons spectra of the simulated brain (dashed blue line), the PDA-Phydtagl dosimeter (black line), and the PDA-RTV dosimeter (red line). Figure 7: The simulation results for the PDD curve in the rat’s brain. Figure 8: The simulation results for the PDD curve of the PDA-Phydagel dosimeter. Figure 10: Dose-difference (Gy per incident photons) plot between the brain and the PDA-Phydagel dosimeter achieved by the subtraction of both curves. Figure 9: The simulation results for the PDD curve of the PDA-RTV dosimeter. The dose difference linear fitting of the PDA-Phydtagel-Brain is acceptable (R\({}^{2}\) = 0.923), but it is relatively low when compared with the linear fitting dose of the PDA-RTV-Brain (R\({}^{2}\) = 0.979), as shown in Figure 11. For both dose-difference plots, the difference between the PDD curves decreases as the depth of the material increases. This difference is due to the photoelectric effect and Compton scattering that are dominant in the energy range of the synchrotron irradiation, both of which are affected by the different atomic number of the materials comprising the Phytagel dosimeter and the brain (e.g., the difference in Z contributes to the different attenuation in each material).