Condensation silicone elastomer material with the commercial name ProtesilTM (Vannini Dental Industry, Florence, Italy) is used in dentistry as a common material to shape a surface mold. These materials are classified as a group of elastomeric impressions with suitable physical and chemical properties such as biocompatibility, sufficient working time, room temperature working condition, dimensional stability, plasticity, non-toxicity to tissue, and electron density .
The TPS measurement was performed based on the dose calculation extensively detailed at AAPM Task Group 43. HDR plus 3 (Eckert & Ziegler BEBIG Gmbh, Berlin, Germany) as a brachytherapy TPS, MultiSourceTM (Eckert & Ziegler BEBIG Gmbh, Berlin, Germany) as HDR-BT afterloader treatment unit and a 60Co radioactive source (model: Co0.A86, EZAG BEBIG, Berlin, Germany) as an HDR-BT source, was applied. Dose verification measurements were performed using Gafchromic EBT3 film (Ashland Specialty Ingredients, NJ, USA lot#09071703 and #04171901) and three MOSFETTM detectors to be described in the following section. The feasibility and efficiency of MOSFET detectors were confirmed in real-time in-vivo dosimetry for brachytherapy by previous researchers [20,21].
2-1: Mold construction
Four blocks of mold material were constructed in 5, 10, 15, and 20 mm thickness and 100×100 mm2 area by a plastic cast. The mold was removed from the cast after 15 to 20 minutes. Three plastic catheters (Flexible Catheter Single Leader, 1.65 mm diameter, Eckert & Ziegler BEBIG, Gmbh, Berlin, Germany) were fixed on top of each mold block via adhesive tape and with 10 mm parallel spacing between the catheters.
2-2: MOSFET calibration
In the current study, the MOSFET dosimeters (Best Medical Canada LTD model TN-502RD-H) with a sensitive volume of less than 4*10−5 mm3 and a physical volume of 4 mm3 were utilized. The MOSFET Calibration Jig (TN-RD-57-30, Best Medical Canada) was applied to the facile and reproducible placement of the MOSFETs through calibration and measurement. The jig is an accessory of the mobile MOSFET wireless patient dosimetry system (TN-RD-70-W, Best Medical Canada) . MOSFET detectors were placed on the surface of the jig, with the flat surface facing the beam during all the experimental measurements.
PMMA slabs were also used in the calibration step; 10 cm of slabs were located under the detectors for providing the backscatter condition, and 2 cm of slabs were placed on the MOSFET for buildup effect. The calibration was carried out for a field size of 10 ×10 cm2 with an SSD of 98 cm (100 cm at the MOSFET plane). The MOSFETs were then irradiated with 100 cGy by a 6 MV photon beam with a mono-energy medical linear accelerator (Elekta,Compact), and the corresponding readings were tabulated in table1.
MOSFETs were initially positioned in the jig channels, such as all of them were placed into a 100 × 100 mm2 central square. The first detector was in the center (red arrow), the second was in the upper right (10 mm to the right and 10 mm up from the center; yellow arrow), and the third one was in the lower left (10 mm to the left and 10 mm down from the center; blue arrow); as illustrated in Figure 1.
2-3: CT set up, and dose calculations
A 64-slice CT scanner (General Electric Medical Systems, USA) was also applied to acquire tomographic imaging to provide data required to be imported to the TPS. CTIs were acquired with 1 mm slice thickness. Metallic x-ray markers were also placed inside the catheters during scanning.
Two configurations of source loading for comparing the TPS data with MOSFET dose calculations were performed. Details of the procedure are briefly explained in the sections below.
2-3-1: The central point of the central BT catheter located just above the central MOSFET detector at the middle of the jig, shown as red MOSFET in Figure 1, was activated. The planning aim was to deliver 3 Gy to 10 mm under the phantom surface, the XZ plane of the central point. Eventually, four treatment plans were performed for four mold thicknesses, with the identical prescribed dose to the same point of interest.
24 set up configurations were created using a combination of 30cm PMMA slabs. The radiation doses were then calculated for 6 points at different depths for four different mold thicknesses. More details of the configurations are shown in Figure 2. At this step, 72 dose points (24 setups × 3 MOSFET) were compared for various mold thicknesses by the MOSFET.
2-3-2: A treatment plan was developed to deliver a 3 Gy radiation dose to all three MOSFETs. Radiation doses into a point located at 10 mm under the skin for different mold thickness were measured using MOSFET detectors. All measurements were repeated three times for all configurations explained at 2-3-1. The processed results were then compared with those calculated by TPS, as explained at 2-3-1.
The Source to Detector Distance (SDD) was defined as follows:
Where tcatheter, tmold, tslab, and tMOSFET are the measured thicknesses of the catheters, the mold, the PMMA slab, and the MOSFET detector, respectively. The SDD was used to calculate the depth of the intended plane in TPS-calculated 3D dose distribution and coinciding with the plane of the MOSFETs and to define the exact position of control points in TPS.
It should be emphasized that to prevent the angular dependency of the MOSFET detector, the direction of entrance radiation to the MOSFET at all of the calculations set up were set to be the same as the calibration condition.
A calibration curve was obtained with eight pieces of 2×2 cm2 GAFCHROMICTM EBT3 films, shown in Figure 3, to verify the film response to irradiation with the dose range from 1 to 9 Gy. For calibration curve measurements, buildup conditions and background radiation were also taken into considerations. Film dosimetry was performed for all four mold thicknesses explained at 2-3-1. An EBT3 film (5×10 mm) slice was also positioned under the mold surface.