# Feasibility of a Patient-Specific Bolus Using the Life-Casting Method for Radiation Therapy
## 2 Materials and Methods  
### Fabrication of a Patient-Specific Bolus Using the LC Method  
Figure 1 shows the fabrication procedure for a patient-specific bolus constructed using the LC method for the breast region. The bolus material used was EcoFlex(tm) 0020 (Durometer Shore 0.2), which is a silicone rubber compound supplied by Smooth-On, EcoFlex(tm) 0020. It is composed of a resin and curing agent with a mixing ratio of 1:1. The curing time is 4 h, and the shrinkage ratio is less than 0.001. EcoFlex(tm) 0020 exhibits advantageous characteristics for casting, including ease of mold filling, flexibility, and slight tackiness.  
Moreover, its density is similar to that of water [14]. The mass density and effective atomic number of EcoFlex(tm) are 1.07 g/cm3 and 11.66, respectively. The elemental composition (atomic weight, %) of EcoFlex(tm) 0020 is as follows: H (7.3), C (28.9), O (4.7), Na (0.2), and Si (58.9) [15; 16]. In the study by [PERSON] et al., dosimetric tests were conducted using the EcoFlex(tm) 0020. According to their results, the depth of the maximum dose was sufficiently similar to that of water. The percent depth dose (PDD) graph input into the treatment planning system (TPS) is measured in water. For materials similar to water, it is straightforward to determine the depth of the maximum dose based on the thickness [14]. Silicone rubber has demonstrated biocompatibility in bolus applications and has been utilized in various instances within this field [14; 15; 16; 17; 18; 19; 20]. EcoFlex(tm) 0020 is marketed by the vendor as a \"skin-safe\" product [14]. The 3D printing method requires acquiring computed tomography (CT) scans of the patient or phantom, importing the CT images into a TPS to generate the bolus structure, converting the structure into an STL file, and performing the postprocessing procedures [21]. This procedure, along with the mold printing, is time-intensive. Furthermore, owing to the need for CT scans using the fabricated bolus, there is an increase in the exposure to radiation. In this study, the LC method was employed,which enabled a considerably more streamlined fabrication of the bolus compared with the currently available 3D printing techniques.  
The fabrication procedure is illustrated in Figure 1. First, wet Nok-San Cast gypsum sheets (Nok-San, Seoul, Republic of Korea) were affixed to an in-house phantom or to the patient's skin. The tissues, bones, and lungs in the in-house phantom were simulated using silicone rubber (EcoFlexTM 0020), gypsum sheets (Nok-San Cast), and air, respectively. Once dried, rapid-curing silicone rubber was applied, and another gypsum sheet was attached to the top and bottom of the mold. To prevent distortion of both surfaces, holes were introduced, and support rods were used for stabilization. Subsequently, a support layer with the desired thickness was applied between the two surfaces. The support layer was fabricated using acrylic, and the thickness of the support layer was 1 cm. The surfaces were then covered with gypsum sheets, excluding the area designated as EcoFlexTM 0020. Demolding was performed after pouring and curing with EcoFlexTM 0020.