Physical characteristics of STR
STR is a rubber material with no cross-linking polymerization. After heating the STR wrapped in clingfilm with a 700-W microwave oven for 1 min [22], it was rolled out to a sheet of uniform thickness by hand or using a stick such as a rolling pin between 1 mm-thick acrylic plates. That was repeated until the STR was stretched to a thickness of 1 mm. The elemental ratio of the STR (wt%) is C: 5.5%, H: 0.9%, O: 1.4%, and W: 92.2% [20]. The STR bolus can be formed to fit the contours of the face in real time, as shown in Figure 1.
The hardness of the STR was measured using a type-A durometer according to Japanese Industrial Standard (JIS) K6253 [24]. The hardness was defined as the resistance force of a material when it was pressed at a single point with the indentor [9]. The durometer readings (hardness values of 0, 50, and 100) indicated resistance forces of 550, 4300, and 8050 mN, respectively. A larger value for hardness indicated greater rigidity. The tensile strength and elongation were determined by the relationship between the tensile stress and elongation rate according to JIS K6251 [25]. The elongation rate was obtained by stretching the test piece of STR (2.0 ± 0.2-mm thickness) with each tensile stress. The tensile strength and elongation were defined as maximum tensile stress and maximum elongation rate at breaking point of the test piece of STR, respectively. Each test was performed at simulated room and body temperatures (20°C and 36°C, respectively). The hardness, tensile strength, and elongation were compared with the corresponding values of silicone rubber bolus [9].
Experimental setup of dosimetric characteristics
The irradiated field was 10 × 10 cm2 at a distance of 100 cm from the source to surface of the water-equivalent phantom (Tough Water; Kyoto Kagaku, Co., Ltd., Kyoto, Japan) (Figure 2(a) and (b)). The thicknesses of the STR boluses were 1-, 2-, and 3-mm, and we used STR boluses of size 150 (W) × 150 (L) mm2 supplied by Hayakawa Rubber Co. to minimize the uncertainty of the thickness. The dimensions of the gel bolus (MTCB455S and MTCB450S; CIVCO, IA, USA) were 150 (W) × 150 (L) × 5 or 10 (T) mm3. The STR bolus or gel bolus was placed on the water-equivalent phantom, and 200 monitor units were irradiated using a linear accelerator (Elekta synergy; Elekta AB, Sweden).
Dosimetric characteristics of the STR bolus with 6-MV photon beams
The percentage depth dose (PDD) was measured with a parallel plate ionization chamber (PPC40; IBA Dosimetry, Germany) for the 6-MV photon beam (Figure 2(a)). The characteristics of the PDDs were evaluated according to the depth of maximum dose (dmax) and the relative doses at 1 mm, 50 mm, and 100 mm depth (D1mm, D50mm, and D100mm, respectively). The water-equivalent thickness of the STR bolus was determined by shifting the distance of the build-up curve in the PDD until it overlaid that for the no bolus [10].
The lateral dose profiles were measured with radiochromic film (Gafchromic EBT3; Ashland, Inc., Covington, KY, USA). The film was set up perpendicular to the beam axis at the surface and at depths of 50 mm and 100 mm (d50mm and d100mm, respectively) in the water-equivalent phantom (Figure 2(b)). The irradiated films were scanned by a digital scanner (ES-10000G; Epson Corp., Nagano, Japan) at least 24 hours after irradiation [26]. The scanned film was analyzed with a DD-System (version 10.3, R-TECH, Inc., Tokyo, Japan) using the 16-bit red channel and 150-dots-per-inch resolution. The lateral dose profile was normalized to the dose at center of the beam axis, and the penumbra size (as the width of the off-axis distance from 80% to 20% dose levels, P80-20) and 50% dose width (W50) were evaluated at the surface, d50mm, and d100mm.