3.1. Radiochromic Gel Fabrication
At onset of the study, the water equivalent radiochromic gel dosimeter was fabricated. The procedure consisted of mixing CCl4 (Merck, Keinlworth, United States) and leuco-malachite green (LMG) (Sigma Aldrich, St Louis, MO, United States). Then a polyurethane resin (Crystal Clear 2006, Smooth-On, Easton, PA, USA) which was supplied in two parts (part A and part B) were mixed together by ratio of 100 to 90 to afford optically clear polyurethane resins that form the matrix of the radiochromic gel dosimeter. The solutions prepared were combined together and thoroughly mixed. Then the prepared mixture was poured into polyspectrophotometer cuvettes and cylindrical plastic containers. The ﬁlled cuvettes and containers were kept in a pressure pot (60 psi) for 5 days to minimize out gassing (
3). Table 1 shows the percentage composition of the prepared radiochromic gel. By using mayneord formula ( 4) and considering the elemental compositions of radiochromic gel, the effective atomic number of the fabricated dosimeter would be 7.8 which is almost water equivalent.
Table 1. Percentage of Chemical Composition of Radiochromic Gel
Components Polyurethane LMG CCl 4 Percentage (%) 94 1 5
LMG, leuco-malachite green.
3.2. Absorption Spectrum
The maximum absorptions of cuvettes were determined using UV-Vis spectrophotometer (Varian, Palo Alto, California, USA).
The next step was calibration of the dosimeters against various dose levels. Varian Clinac 2100c linear accelerator was used for calibration. Various levels of absorbed doses were delivered to cuvettes. For each dose step, three cuvettes were used. All samples were kept in a dark and cold environment to prevent any accidental absorption changes and 48 hours after irradiation, again by using UV-Vis spectrophotometer, the maximum absorption of each cuvette was obtained. In order to ensure that the changes in light absorbance versus radiation dose are only due to the radiation doses absorbed by the dosimeter, the absorption values were obtained by subtracting the relevant values of pre-irradiation absorbance from that of the irradiated cuvettes (
5). Therefore, the dose response changes and post-irradiation are solely due to the absorbed radiation doses. Finally, a calibration curve, which is the variations of optical density changes against absorbed doses, was plotted. 3.4. Irradiating Cylindrical Radiochromic Gel Dosimeter
After calibration and obtaining the optical density changes corresponding to each step of the absorbed dose, the irradiation of cylindrical radiochromic gel dosimeter was performed. In this section, the radiochromic gel was placed in a water phantom to prepare the electronic equilibrium condition and irradiated by three radiation fields, depicted schematically in
Figure 1. Schematic view of irradiation directions
3.5. Designing Novel Optical Computed Tomography
Initially, absorption of the radiochromic gel against various wavelengths was needed. Wavelength of the light in which maximum absorption of radiochromic gel occurred proposed the emitting light source of the novel OCT. Since maximum absorption of this gel appears at 633 nm, which is related to red color, a 3 Watt (W) red light-emitting diode (LED) was selected as the light source. Therefore, the principle idea of this novel OCT was to measure the absorbance of the LED light that passed through the whole cylindrical radiochromic gel. The amount of absorption corresponded to the absorbed dose. In order to obtain accurate results, some other accessories were needed that are described as follows.
For such set-up, it was mandatory to make sure that all parts of the cylindrical radiochromic gel received the light of LED. Therefore, the LED was followed by a lens as optical diffuser.
Then, the cylindrical radiochromic gel was placed in a flat optical glass known as aquarium containing a matching liquid in terms of refractive index (methyl salicylate) and pinned to the step motor, providing rotation of radiochromic gel with 1.8° intervals. Light emitted from the LED was collimated to a parallel beam of diameter 25 cm by the initial lens followed by the light source. The beam was then transmitted through the radiochromic gel, remained parallel due to existence of the matching fluid. Then a telecentric lens (TC-23-240, Opto Engineering, Italy) collected light representing attenuation line integrals onto a 12-bit charge-coupled device (CCD) array of 1040 × 1392 (a102f, Basler, Germany) while it rejected most of the scattered light that originated within the sample as the lens had a manufacturer speciﬁed acceptance angle less than 0.1.
In fact, telecentric optics provides a means to acquire orthographic projections; all projection lines are approximately parallel to the optical axis, creating an ideal for parallel geometry computed tomography. To acquire a complete scan, projections are acquired at multiple angles as the radiochromic gel is rotated around 360° (by the step motor).
3.6. Dose Distribution
In order to obtain dose distribution, multiple light beam images were taken from different directions while rotating the gel around 360. The optical data were then fed into a tomographic reconstruction algorithm and processed by a computer. The deduction of the radiochromic gel structure from incident beam was done using a technique first obtained in 1917 by Johann Radon who was studying mathematical properties of X-ray in the tissue which is now known as Radon transform. Radon transform is an integral transform which is an extension of Beer Lambert’s law whose inverse is used to reconstruct images from OCT scans. Then, conversion of optical density changes to absorbed dose is achieved through the calibration curve.
3.7. Testing the Accuracy of the Results
In order to verify the accuracy of the results of novel OCT, a validation test was done using a pinpoint ion chamber with an effective volume of 0.015 cc (PTW, Freiburg, Germany) and the results were then compared to those of novel OCT. In this section, the percentage depth dose and beam profile of a 3 × 3 cm
2 field size irradiating by 6MV photon beams were obtained by ion chamber. Then a cylindrical sample of radiochromic gel with 10 cm diameter was selected and irradiated to extract the percentage depth dose and beam profile.