# Feasibility of a Patient-Specific Bolus Using the Life-Casting Method for Radiation Therapy
###### Abstract  
Radiation therapy for treating shallow tumors is challenging, necessitating the use of boluses. This study introduces the first application of the life-casting method to fabricate patient-specific bolus molds from gypsum sheets, comparing them with commercial boluses. Our developed boluses reduced the air gap between the skin and bolus by 77.62% compared to that of commercial boluses. In vivo dosimetry using the patient-specific bolus demonstrated better results compared to using a commercial bolus. When using the commercial bolus, the mean %Diff and max %Diff were 1.10 \(\pm\) 0.61%, respectively, and 2.00% for three-dimensional conformal radiation therapy (3D-CRT) and 7.19 \(\pm\) 1.90% and 10.14% for volumetric modulated are therapy (VMAT), respectively. Contrastingly, our developed bolus demonstrated more accurate dose delivery with a mean %Diff and max %Diff of 0.82 \(\pm\) 0.61% and 1.69% for 3D-CRT and 3.42 \(\pm\) 1.01% and 5.03% for VMAT, respectively. Furthermore, the standard deviation between the measurements was more than 50% lower when using a patient-specific bolus than when using a commercial bolus. These results show that our bolus reduces air gaps, improves the accuracy of bolus positioning, and enhances the precision of dose delivery compared with the performance of commercial boluses. Therefore, the developed bolus is expected to be valuable in clinical applications.  
life-casting; patient-specific bolus; silicone rubber; radiation therapy; air gap +
Footnote †: journal: Medical Imaging  
0000-0002-0002-3313-3313-3133-31  
Three-dimensional (3D) printing technology facilitates the production of diverse geometries. Currently, 3D printing is used in various medical fields, including patient-specific implants and external prostheses [9]. Moreover, 3D printing technology has been effectively utilized to produce patient-specific boluses for radiation therapy [1; 3; 7; 9; 10]. Compared with conventional boluses, 3D-printed boluses have a good ability to closely conform to a patient's skin surface. Patient-specific boluses produced through 3D printing have demonstrated excellent efficacy in radiotherapy because they closely adhere to the skin. However, their practical application has been hindered by certain limitations, such as the immaturity of printed materials, inconvenience, time-consuming preparation, and additional dosing [3]. Moreover, they require costly supplementary equipment, including 3D printers and accessories, which limit their clinical applicability.  
Recently, devices capable of simultaneously performing dose measurements and bolus effects using gel dosimeters have been developed. [PERSON] et al. developed and evaluated a dual-application polyvinyl alcohol glutaraldehyde methyl thymol blue Fricke hydrogel (PVA-GTA-MTB) gel dosimeter. The PVA-GTA-MTB gel dosimeter can be easily fabricated, is suitable for dose assessment, is minimally toxic, and has a smaller air gap than does the SuperFlab bolus. This can be highly beneficial in clinical settings. However, this study did not conduct airgap quantification, and it appears that airgap issues still persist in clinical practice [11]. Furthermore, in gel dosimetry, the density can change during radiation exposure, and such changes are likely to occur with each fraction of the radiation treatment [12]. This could potentially lead to alterations in the delivered dose within a specific range.  
The life-casting (LC) method is a technique that allows for the direct utilization of a living body as the primary model. LC refers to the 3D portrayal technology of a living object using casting materials and techniques [13]. In the present study, we developed bolus molds using easily accessible and cost-effective gypsum sheets and employed the LC method for their fabrication. Subsequently, we assessed the performance of the patient-specific boluses produced using these molds.