Development of a Customized Three-Dimensional Bolus Using Transparent Gel Wax and Its Application in Electron Beam Therapy

Development of a Customized Three-Dimensional Bolus Using Transparent Gel Wax and Its Application in Electron Beam Therapy | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Development of a Customized Three-Dimensional Bolus Using Transparent Gel Wax and Its Application in Electron Beam Therapy Young Jin Won, Jung Ju JO, Su Ah Yu, SU Jung Shim, Kum Bae Kim, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7174847/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 6 You are reading this latest preprint version Abstract Purpose: This study aimed to develop a customized bolus using commercially available transparent gel wax (TGW) without requiring 3D printing technology and to evaluate its applicability in radiotherapy, including its physical and dosimetric characteristics. Methods: A dental alginate impression was taken from the curved surface of a mastectomy phantom to replicate the patient's skin contour. Based on this, a silicone mold was fabricated, into which TGW was poured to form the customized bolus. The fabricated bolus was evaluated for its physical properties (density, electron density, homogeneity) and bolus conformity index (BCI), treatment planning dosimetric analysis, and actual dose measurements using optically stimulated luminescent (OSL) dosimeters, in comparison with a conventional vinyl gel sheet bolus (SuperFlex). Results: The TGW bolus demonstrated high transparency and excellent homogeneity (standard deviation of internal HU: ±5.1), and its BCI was 0.03, indicating a very high level of conformity to the virtual bolus. Under 6 MeV and 8 MeV electron beam conditions, the TGW bolus showed mean dose errors of − 0.2%±1.3% and 0.5%±1.2%, respectively, which were more consistent and lower than those of the SuperFlex bolus (0.6%±1.8% and − 2.1%±1.4%, respectively). In particular, the TGW bolus showed superior conformity and reproducibility in regions with high surface curvature, effectively reducing dose loss caused by air gaps. Conclusion: TGW is a low-cost, transparent, and flexible material that enables rapid fabrication (within one day) of a customized bolus through a simple molding process. Its superior physical stability and dosimetric performance compared to existing products suggest strong potential for clinical application in radiotherapy. Gel wax Bolus Electron beam therapy OSL dosimeter Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1. Introduction Postmastectomy radiation therapy significantly improves survival rates in high-risk breast cancer patients [ 1 , 2 ]. In treatment planning and delivery, the clinical goal is to ensure accurate and homogeneous dose coverage of the target volume extending to the skin surface [ 3 ]. To achieve sufficient dose to the superficial tissue, boluses are often applied to eliminate surface irregularities and increase the skin dose [ 4 ]. Additionally, virtual boluses or pseudo skin flash techniques are used to compensate for breast deformation during treatment, thereby improving plan quality [ 5 – 7 ]. An ideal bolus should be easy to manufacture, non-toxic, stable throughout the treatment course, flexible for placement beneath the skin, capable of providing a uniform dose distribution to the target, and have tissue-equivalent physical properties [ 8 ]. Among various materials, vinyl gel sheet boluses (e.g., SuperFlex bolus) have been most commonly used due to their convenience and effectiveness. However, SuperFlex boluses often fail to conform perfectly to the patient’s skin, resulting in air gaps that can cause dose discrepancies of up to 10% when a 10 mm gap is present [ 9 ]. Efforts to reduce air gaps and improve conformity have been partially addressed through the use of 3D printing technologies [ 10 – 12 ]. However, commonly used 3D printer materials such as polylactic acid (PLA) are non-flexible. Thermoplastic polyurethane (TPU), a more flexible alternative, offers better conformity to the patient’s skin contour, but remains opaque and requires prolonged printing time [ 13 ]. To overcome these limitations, molded materials such as silicone, hydrogel, and silica gel have been employed [ 14 , 15 ]. Although these materials offer greater flexibility, they lack transparency. Transparent boluses are advantageous for confirming skin contact and visualizing skin markings during setup. To date, only a few bolus products are both transparent and flexible—namely, the silicone-based bolus (PapricaLab Co. Ltd., CLEANBOLUS) and the water-clear polymeric gel bolus (CLEARSIGHT RT, ClearSight bolus) [ 16 – 18 ]. Notably, polymeric gel materials allow customization of hardness, elasticity, transparency, and density, while maintaining mechanical stability for extended periods—ranging from 16 months up to 4 years without deformation. Furthermore, they have a relatively low curing temperature (~ 70°C), enabling fast curing at room temperature without tackiness [ 17 – 21 ]. Water-clear polymeric gels have previously been fabricated as standardized boluses in thicknesses of 3, 5, and 10 mm, similar to commercial products such as the SuperFlex bolus [ 17 , 18 , 22 ]. In addition, patient-customized boluses using hydrogel and silica gel have been developed through 3D printing technology [ 14 , 15 ]. We aim to fabricate a customized bolus using Transparent gel wax (TGW), a material similar to water-clear polymeric gels. Although TGW can be produced through 3D printer molding methods like those used for hydrogel or silica gel, such 3D printing processes require lengthy fabrication times. To reduce production time, we propose a non-3D printing fabrication approach for TGW boluses using alginate. Alginate rapidly sets within 2 to 15 minutes after mixing with water and accurately replicates complex structures, which is why it is commonly used as an impression material in dentistry [ 23 ]. However, alginate undergoes shrinkage due to water evaporation after fabrication, making it unsuitable for prolonged radiation therapy. Using a 3D-printed breast phantom, we aim to replicate the phantom without alginate shrinkage and produce a customized TGW bolus within one day. The outcomes of this fabrication method will be comparatively analyzed. 2. Methods and Materials 2.1 Physical Property Evaluation To measure the physical density (PD) and relative electron density (RED) of Alginate (GC, AFP; Aroma Fine Plus, Japan), SuperFlex bolus (Action Products, Inc., Bolx-II, USA), and TGW bolus (Candlebada, gel wax, Korea), samples were prepared by curing each material in acrylic molds sized 8 cm × 3 cm × 4 cm. Computed tomography (CT; Siemens, Somatom Confidence, Germany) scans were performed using the following parameters: 120 kVp, 15 mAs, 40 cm field of view (FOV), and 2 mm slice thickness. Images were analyzed within a region of interest (ROI) with a 2 cm diameter. The density and relative electron density of each bolus were estimated using the CT to electron density (ED) conversion curve based on Hounsfield units (HU). Homogeneity was evaluated by the standard deviation (SD) of HU values within the ROI. According to vendor specifications, the physical densities of SuperFlex and TGW were 1.00 g/cm³ and 0.87 g/cm³, respectively. 2.2 Breast Phantom Design and Structure Definition To replicate the curved skin surface of a patient who had undergone total mastectomy, the surface contours were artificially modeled in MIM software (MIM Software Inc., v7.1.10, USA) using the structure tool, and converted to a stereolithography (STL) file format as shown in Fig. 1. The model was then smoothed using Laplacian smoothing in Meshlab software (Meshlab_64bit v1.3.4 Beta) and converted to G-Code using 3D printing software (Simplify3D, v4.1.1, USA). Subsequently, the breast phantom was printed using a 3D printer (Creality3D, CR-10 Max, Shenzhen, China). A virtual bolus structure (15 cm × 15 cm × 1 cm) was created 1 cm above the breast phantom surface using MIM software. For electron beam therapy planning, a planning target volume (PTV) for 6 MeV electrons (11 cm × 11 cm × 1.5 cm) was defined extending 1.5 cm deep from the phantom surface. Similarly, the PTV for 8 MeV electrons (11 cm × 11 cm × 2.1 cm) was defined extending 2.1 cm deep from the phantom surface. These structures were exported to the treatment planning system (TPS; Elekta Oncology Systems, Monaco, v5.51.10, Sweden) for further planning. 2.3 Fabrication of a Customized TGW Bolus To replicate the surface of the breast phantom, alginate (GC Corporation, Aroma Fine Plus Normal Set, Tokyo, Japan) was mixed with distilled water at a ratio of 0.42:1 and poured onto the phantom surface as shown in Fig. 2. The alginate was allowed to set for 10 minutes, after which it was carefully removed from the phantom surface. A mold for the TGW bolus was then prepared using silicone (PONY, Moldmaster Ultra, China), by mixing parts A (base) and B (curing agent) at a 1:1 ratio. The cured alginate was placed into an acrylic frame, and the mixed silicone was poured into the frame and left to cure for 6 hours. Once the silicone mold was set, the alginate was removed. To enhance the optical transparency of the TGW bolus, the surface of the silicone mold was coated with epoxy resin (Smooth-On Inc., XTC-3D, USA). TGW, composed of white mineral oil and a copolymer (SEBS; styrene-ethylene/butylene-styrene), was heated to 140°C until it became liquid. To remove bubbles, the liquid TGW was degassed for 5 minutes under vacuum using a defoaming chamber (HISCO, CL1.5, China). The degassed liquid TGW was then poured into the silicone mold and allowed to solidify for 30 minutes. The solidified customized TGW bolus was removed from the silicone mold and stored on a polyethylene terephthalate (PET) film. 2.4 Evaluation of Customized TGW Bolus Conformity The positional conformity between the virtual bolus created in the TPS and the fabricated bolus on the corresponding CT images was evaluated using the Bolus Conformity Index (BCI), calculated as follows: Here, the BCI quantifies how well the customized bolus conforms to the virtual bolus, with values closer to 0 indicating higher spatial agreement. VBV represents the volume of the virtual bolus, VBV ∪ BV denotes the union volume of the virtual bolus and the fabricated bolus, and VBV ∩ BV represents the intersection volume of the two. The numerator thus reflects the non-overlapping volume portion between the two boluses, normalized by the volume of the virtual bolus. 2.5 Treatment Planning and Evaluation A virtual bolus was placed on the surface of the breast phantom, and a prescription dose of 10 Gy in five fractions was applied to the PTV. In the TPS, the RED values were set to 0.976 for the SuperFlex bolus and 0.823 for the TGW bolus. Electron beams of 6 MeV and 8 MeV were calculated, respectively. The treatment planning objective for the 6 MeV beam was to deliver 80% of the prescribed dose to the PTV located up to 1.3 cm beneath the skin surface. For the 8 MeV beam, the goal was to deliver 80% of the dose to the PTV located up to 2.1 cm deep from the surface. All treatment plans were normalized such that 40% of the PTV received 10 Gy to allow for comparative evaluation. Dosimetric evaluation of the treatment plans included analysis of the homogeneity index (HI), conformity index (CI), surface dose (Point 1), D max , D mean , D min of the PTV, and the mean dose to the body minus PTV (BMP mean ). Planning was performed using the Monaco TPS. The HI was defined as (D 2% –D 98% )/D median , and the CI was defined as TV/PTV, where TV represents the treated volume enclosed by the 80% isodose line [ 24 ]. Dose–volume histograms (DVHs) were used to evaluate dose distributions to the PTV and organs at risk (OARs). The technical parameters for plan calculation included: field size of 15 cm × 15 cm cone, source-to-skin distance (SSD) of 100 cm, grid size of 2 mm, Monte Carlo algorithm with 700,000 histories per cm². 2.6 Dose Assurance Optically stimulated luminescence (OSL) nanoDot dosimeters (Landauer, Inc., Greenwood, IL) were calibrated using a 2 Gy irradiation from a linear accelerator (Linac; Elekta, VersaHD, Sweden) in accordance with the IAEA TRS-398 calibration protocol [ 25 , 26 ]. After setting up the breast phantom as shown in Fig. 3 within the radiotherapy treatment room, dose verification was performed using 6 and 8 MeV electron beams with either SuperFlex or TGW bolus materials, depending on the treatment plan. For each plan, a total of four OSL dosimeters were placed—one at the center and three at peripheral positions. The measured doses were then compared to the planned doses, and the differences were analyzed. 3. Results 3.1 Physical Property Evaluation The HU, SD, PD, and RED of the SuperFlex bolus and TGW bolus used in this study are summarized in Table 1. Among the materials tested, the SuperFlex bolus exhibited HU values most similar to those of water. In terms of homogeneity, the TGW bolus demonstrated the best performance with a standard deviation of ± 5.1, followed closely by the SuperFlex bolus with ± 5.3, indicating both materials are highly homogeneous. 3.2 Bolus Conformity Index Evaluation The volume of the virtual bolus was measured to be 217.7 cc, while the SuperFlex and TGW bolus volumes were 169.2 cc and 221.8 cc, respectively. The non-overlapping volume between the virtual bolus and the SuperFlex bolus was 49.7 cc, whereas the non-overlapping volume between the virtual bolus and the TGW bolus was only 6.4 cc. The BCI for the SuperFlex bolus was 0.23, whereas the TGW bolus demonstrated a significantly lower BCI of 0.03, indicating a much higher volumetric conformity to the virtual bolus (Fig. 4). 3.3 Plan Evaluation The dosimetric results for the PTV calculated in the TPS are presented in Table 2. For the 6 MeV electron beam plan, where the PTV was located at a depth of 1.3 cm, the CI for the SuperFlex bolus plan was 1.38, suggesting adequate coverage of the PTV with minimal unnecessary dose to surrounding tissue. The surface dose in the SuperFlex plan was also the most consistent with the prescribed dose, delivering 10.03 Gy. In the TGW bolus plan, the R 80% depth was measured at 1.5 cm, indicating a slight overdose of approximately 0.2 cm beyond the intended depth (Fig. 5). This slight overdose at the distal end may provide beneficial skin sparing while still ensuring adequate coverage of the superficial target volume. Such a dose distribution is clinically acceptable, especially in postmastectomy patients where homogeneous dose to the chest wall is essential and minor variations within subcentimeter range are not considered clinically significant. 3.4 Dose Assurance Under the same conditions as the treatment plans, electron beams were delivered to the breast phantom in the radiotherapy suite, and dose measurements were obtained using optically stimulated luminescence (OSL) dosimeters. The results are summarized in Table 3. The average error rate ± SD for the SuperFlex bolus was 0.6%±1.8% for the 6 MeV plan and − 2.1%±1.4% for the 8 MeV plan. For the TGW bolus, the corresponding values were − 0.2%±1.3% at 6 MeV and 0.5%±1.2% at 8 MeV, indicating that the TGW bolus provided dose delivery closer to the treatment plan. In terms of maximum absolute error at measurement points, the SuperFlex bolus exhibited errors of up to 3.1% at 6 MeV and − 4.1% at 8 MeV, whereas the TGW bolus showed smaller maximum deviations of − 1.8% at 6 MeV and − 2.1% at 8 MeV. These findings suggest improved dosimetric accuracy and consistency when using TGW bolus. From a clinical perspective, this improved accuracy translates into better reproducibility of dose delivery across treatment sessions. Especially in cases involving highly contoured surfaces such as the chest wall or head and neck regions, minimizing dose deviations due to air gaps can directly impact local control and treatment outcomes. 4. Discussion A bolus is a tissue-equivalent material widely used in cases such as postmastectomy or cutaneous malignancies, with the purpose of compensating for irregular patient contours, enhancing surface dose, or attenuating excess dose in certain regions [ 4 , 8 ]. In this study, the use of a TGW bolus offered three distinct advantages or options. The first advantage lies in the favorable physical properties of the TGW bolus, including transparency and stability. Existing transparent boluses include silicone-based CLEANBOLUS and polymeric gel-based ClearSight bolus [ 16 – 18 ]. TGW can be easily purchased from online candle supply retailers at very low cost and is categorized into CLP (Candle gel low density), CMP (Candle gel medium density), and CHP (Candle gel high density) types. These types vary in the ratio of mineral oil to copolymer, with increased copolymer content yielding higher tensile strength but lower flexibility. Although the exact composition is not disclosed, the CHP used in this study is estimated to contain 12–15% copolymer, which is comparable to the material properties of ClearSight bolus. According to Maneas et al., TGW exhibits high optical transparency and stable characteristics in scattering and absorption, with a well-defined absorption spectrum in the visible range of 400–900 nm, enabling accurate differentiation of transmitted colors [ 27 ]. Due to its low optical diffusion, the shape of light transmitted through TGW remains well-defined. This allows clear visualization of the patient’s skin and ink markings through the bolus without distortion. Additionally, the TGW maintains its elastic and soft properties over time without degradation. As a thermoplastic material, it melts into a liquid form at 75 ~ 85°C and can be reshaped and reused. These properties have primarily been explored in the context of ultrasound studies [ 19 – 21 , 27 – 29 ]. The second advantage is that this study represents the first attempt to fabricate a customized transparent bolus. Standard boluses such as SuperFlex may create air gaps at the patient’s skin surface, potentially reducing the surface dose [ 9 ]. Furthermore, Anderson et al. reported that using a standard bolus in postmastectomy patients resulted in a 24% complication rate, which could be reduced to 9% with the use of a customized bolus—strongly supporting its clinical recommendation [ 30 ]. We successfully developed a customized TGW bolus using an alginate-based casting technique that does not require 3D printing. The customized TGW bolus demonstrated excellent conformity; however, minor surface irregularities were observed. This issue arose from uneven application of the surface coating agent on the silicone mold and was mitigated by performing a secondary coating. Furthermore, the alginate-based casting method enables faster clinical application compared to 3D printing. As the print size increases, the printing time with a 3D printer becomes longer, and producing a bolus of sufficient size for clinical use typically takes at least 2ཞ3 days. In contrast, although the alginate casting technique requires more manual effort to fabricate the alginate mold and subsequent silicone mold, the total fabrication time for the TGW bolus remains under one day, allowing rapid deployment in clinical settings. When choosing an alginate casting method for clinical use, one should also consider that alginate hardens quickly—typically within 2ཞ5 minutes—depending on the product type, which can make mold formation challenging [ 23 ]. Therefore, using commercially available alginate products with a slower setting time of 10–15 minutes allows for easier fabrication of immobilization structures directly within the CT simulation room. This non-3D-printed alginate casting approach can be extended to other anatomical regions. For example, it may be used as a substitute for gingival or tongue bite immobilization in the oral cavity. Traditional putty materials have been used to stabilize oral structures but are often avoided due to their high density. However, by casting putty in a similar way to alginate, one can replicate the geometry and subsequently mold a custom oral immobilization device using silicone and TGW instead of a bolus [ 31 ]. The advantage of customized TGW bolus is its ability to accurately reproduce the planned dose distribution. This was validated using optically stimulated luminescent (OSL) dosimeters. As shown in Fig. 4, the customized TGW bolus achieved excellent conformity to the curved surface of the breast phantom, and as demonstrated in Table 3, it resulted in dose measurements that were more consistent with the treatment plan. In particular, except for measurement point 1, which was located on a flat surface, points 2, 3, and 4 on the curved regions showed notable discrepancies when using the SuperFlex bolus—up to 3.1% for 6 MeV and − 4.1% for 8 MeV. The maximum air gap between the SuperFlex bolus and the phantom surface was measured at 6 mm. In clinical situations, even greater air gaps may occur, potentially leading to higher dose errors [ 4 , 9 , 30 ]. In contrast, when using the customized TGW bolus, the dose errors were reduced to − 1.8% at most for 6 MeV and − 2.1% for 8 MeV. Moreover, the SD of ± 1.3% and mean absolute deviation (MAD) of ± 0.9% indicate lower variation in dose depending on measurement position. This suggests that the customized TGW bolus offers more consistent dose delivery across different regions and aligns with previous research demonstrating that customized boluses provide more stable and accurate dose distributions in radiotherapy [ 10 – 15 , 32 , 33 ]. A third advantage is the ability to utilize different electron beam energies depending on the bolus density. It is not strictly necessary for a bolus to have a physical density exactly equal to 1.0 g/cm³; rather, it should effectively smooth out surface irregularities and enhance surface dose [ 16 – 18 ]. Figure 5A shows the planned dose distribution using 6 MeV electron beams when the PTV is located at a depth of 1.3 cm. This corresponds to measurement point 1 in Table 3. If the PTV were instead located at 1.5 cm depth, 8 MeV beams would typically be required to deliver 80% of the prescribed dose, as seen in Fig. 5C. However, by using a lower-density bolus such as TGW, it becomes feasible to achieve the same coverage at 1.5 cm depth using 6 MeV, as demonstrated in Fig. 5B. This approach requires overriding the RED value of the bolus in the treatment planning system, which can be a minor inconvenience. Nevertheless, similar adjustments are already made in clinical practice to improve treatment outcomes—for example, using virtual boluses for plan optimization or employing pseudo skin flash techniques [ 5 – 7 ]. When these modifications provide dosimetric benefits, they are considered acceptable and necessary parts of the planning workflow. Despite these options, the density of TGW—a mineral oil and copolymer-based material—can potentially be increased closer to 1.0 g/cm³ by adding agents such as silica or titanium dioxide (TiO₂). However, such additions significantly reduce the optical transparency of the material [ 17 , 28 ]. The TGW bolus with a physical density of 0.87 g/cm³ may serve as a viable option in its current form, but further research is warranted to identify new compounds that can increase the density while preserving transparency. At our institution, customized TGW boluses have already been applied clinically in patient treatments. However, further studies are needed to evaluate treatment outcomes and long-term prognoses. 5. Conclusion We evaluated the physical suitability of commercially available, inexpensive, and readily accessible TGW for use as a radiotherapy bolus material. Using a non-3D printing fabrication method with alginate casting, we were able to reduce production time and rapidly apply the bolus to patients. The customized TGW bolus conformed perfectly to the patient’s body surface, exhibited excellent flexibility and high transparency, and was easy to reproduce in the radiotherapy setting. Moreover, in terms of dose delivery, it performed as well as or better than conventional products. These findings suggest strong potential for broader clinical adoption of customized TGW boluses in the future. Declarations Acknowledgements This research was supported by the National Research Council of Science & Technology (NST) grant by the Korea government (MSIT) (No. CAP22042-300) & Korea Institute of Radiological & Medical Sciences (KIRAMS) grant funded by the Korea government (Ministry of Sciences and ICT) (No. 50572 − 2025) & the Nuclear Safety Research Program through the Korea Foundation of Nuclear Safety (KoFONS) using the financial resource granted by the Nuclear Safety and Security Commission (NSSC) of the Republic of Korea(RS-2022-KN071220). Conflict of Interest The authors declare that they have no conflict of interest. References Overgaard M, Hansen PS, Overgaard J, Rose C, Andersson M, Bach F, Kjaer M, Gadeberg C, Mouridsen HT, Jensen MB, Zedeler K (1997) Postoperative radiotherapy in high-risk premenopausal women with breast cancer who receive adjuvant chemotherapy: Danish Breast Cancer Cooperative Group 82b Trial. 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Biomed Opt Express 9(8):3495–3502. https://doi.org/10.1364/BOE.9.003495 Maneas E, Xia W, Nikitichev DI, Daher B, Manimaran M, Wong RY, Chang CW, Rahmani B, Capelli C, Schievano S, Burriesci G (2018) Anatomically realistic ultrasound phantoms using gel wax with 3D printed moulds. Phys Med Biol 63(1):015033. https://doi.org/10.1088/1361-6560/aa9e2c Anderson PR, Hanlon AL, McNeeley SW, Freedman GM (2004) Low complication rates are achievable after postmastectomy breast reconstruction and radiation therapy. Int J Radiat Oncol Biol Phys 59(4):1080–1087. https://doi.org/10.1016/j.ijrobp.2003.12.036 Kim J, Moon JY, Park RH, Shin HB, Shin SJ, Chang JS (2022) Feasibility of using dental putty-based custom molds for high-dose-rate brachytherapy of oral mucosal melanoma. Phys Med 103:119–126. https://doi.org/10.1016/j.ejmp.2022.10.010 Park SY, Choi CH, Park JM, Chun M, Han JH, Kim JI (2016) A patient-specific polylactic acid bolus made by a 3D printer for breast cancer radiation therapy. PLoS ONE 11(12):e0168063. https://doi.org/10.1371/journal.pone.0168063 Wang KM, Rickards AJ, Bingham T, Tward JD, Price RG (2022) Evaluation of a silicone-based custom bolus for radiation therapy of a superficial pelvic tumor. J Appl Clin Med Phys 23(4):e13538 Tables Tables 1 to 3 are available in the Supplementary Files section. Supplementary Files Tablefin.pdf Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Minor revisions 01 Mar, 2026 Reviewers agreed at journal 09 Dec, 2025 Reviewers invited by journal 09 Dec, 2025 Editor invited by journal 28 Jul, 2025 Editor assigned by journal 28 Jul, 2025 First submitted to journal 27 Jul, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7174847","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":557840758,"identity":"9d3f8591-ee51-4ad9-ad32-6d84b3c2fc83","order_by":0,"name":"Young Jin Won","email":"","orcid":"","institution":"UEMC: Eulji University Uijeongbu Eulji Medical Center","correspondingAuthor":false,"prefix":"","firstName":"Young","middleName":"Jin","lastName":"Won","suffix":""},{"id":557840759,"identity":"e62e3b90-feb7-4fdf-8174-7d13c8f5adf3","order_by":1,"name":"Jung Ju JO","email":"","orcid":"","institution":"Korea Institute of Radiological \u0026 Medical Sciences","correspondingAuthor":false,"prefix":"","firstName":"Jung","middleName":"Ju","lastName":"JO","suffix":""},{"id":557840760,"identity":"26025824-9532-4e23-af37-937b06d09b0a","order_by":2,"name":"Su Ah Yu","email":"","orcid":"","institution":"Eulji University Uijeongbu Eulji Medical Center","correspondingAuthor":false,"prefix":"","firstName":"Su","middleName":"Ah","lastName":"Yu","suffix":""},{"id":557840761,"identity":"77cc3e79-27e0-4e7a-9869-51bc6069f851","order_by":3,"name":"SU Jung Shim","email":"","orcid":"","institution":"Eulji University Uijeongbu Eulji Medical Center","correspondingAuthor":false,"prefix":"","firstName":"SU","middleName":"Jung","lastName":"Shim","suffix":""},{"id":557840762,"identity":"fe9a9b84-6879-4c0b-b1c5-9257dad35bda","order_by":4,"name":"Kum Bae Kim","email":"","orcid":"","institution":"Korea Institute of Radiological \u0026 Medical 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4","display":"","copyAsset":false,"role":"figure","size":52187,"visible":true,"origin":"","legend":"\u003cp\u003eSee image above for figure legend.\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7174847/v1/9f35d97542d0857a542a19db.jpg"},{"id":98245586,"identity":"b74c8286-14e7-407c-9f5b-78ce39533dcd","added_by":"auto","created_at":"2025-12-15 16:18:08","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":95263,"visible":true,"origin":"","legend":"\u003cp\u003eSee image above for figure legend.\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7174847/v1/9736bc474f31a54922f58216.jpg"},{"id":98245500,"identity":"81ae3643-f64e-465d-8748-7982827f6956","added_by":"auto","created_at":"2025-12-15 16:17:59","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":148358,"visible":true,"origin":"","legend":"\u003cp\u003eSee image above for figure legend.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-7174847/v1/7503b9dda9bcbe4c44a462c7.png"},{"id":98246268,"identity":"6dd6a7f2-c4a8-4cad-bd3d-388567c941e4","added_by":"auto","created_at":"2025-12-15 16:18:55","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1101864,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7174847/v1/ab175099-9d04-4d28-ac2b-ccbef1cb9a2c.pdf"},{"id":98245523,"identity":"09bec53d-3b36-46ee-a7b0-c7ebfaeb35a8","added_by":"auto","created_at":"2025-12-15 16:18:01","extension":"pdf","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":138805,"visible":true,"origin":"","legend":"","description":"","filename":"Tablefin.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7174847/v1/fc1f2a898f2859efef718d76.pdf"}],"financialInterests":"","formattedTitle":"Development of a Customized Three-Dimensional Bolus Using Transparent Gel Wax and Its Application in Electron Beam Therapy","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003ePostmastectomy radiation therapy significantly improves survival rates in high-risk breast cancer patients [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. In treatment planning and delivery, the clinical goal is to ensure accurate and homogeneous dose coverage of the target volume extending to the skin surface [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. To achieve sufficient dose to the superficial tissue, boluses are often applied to eliminate surface irregularities and increase the skin dose [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Additionally, virtual boluses or pseudo skin flash techniques are used to compensate for breast deformation during treatment, thereby improving plan quality [\u003cspan additionalcitationids=\"CR6\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eAn ideal bolus should be easy to manufacture, non-toxic, stable throughout the treatment course, flexible for placement beneath the skin, capable of providing a uniform dose distribution to the target, and have tissue-equivalent physical properties [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Among various materials, vinyl gel sheet boluses (e.g., SuperFlex bolus) have been most commonly used due to their convenience and effectiveness. However, SuperFlex boluses often fail to conform perfectly to the patient\u0026rsquo;s skin, resulting in air gaps that can cause dose discrepancies of up to 10% when a 10 mm gap is present [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eEfforts to reduce air gaps and improve conformity have been partially addressed through the use of 3D printing technologies [\u003cspan additionalcitationids=\"CR11\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. However, commonly used 3D printer materials such as polylactic acid (PLA) are non-flexible. Thermoplastic polyurethane (TPU), a more flexible alternative, offers better conformity to the patient\u0026rsquo;s skin contour, but remains opaque and requires prolonged printing time [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. To overcome these limitations, molded materials such as silicone, hydrogel, and silica gel have been employed [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Although these materials offer greater flexibility, they lack transparency.\u003c/p\u003e\u003cp\u003eTransparent boluses are advantageous for confirming skin contact and visualizing skin markings during setup. To date, only a few bolus products are both transparent and flexible\u0026mdash;namely, the silicone-based bolus (PapricaLab Co. Ltd., CLEANBOLUS) and the water-clear polymeric gel bolus (CLEARSIGHT RT, ClearSight bolus) [\u003cspan additionalcitationids=\"CR17\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Notably, polymeric gel materials allow customization of hardness, elasticity, transparency, and density, while maintaining mechanical stability for extended periods\u0026mdash;ranging from 16 months up to 4 years without deformation. Furthermore, they have a relatively low curing temperature (~\u0026thinsp;70\u0026deg;C), enabling fast curing at room temperature without tackiness [\u003cspan additionalcitationids=\"CR18 CR19 CR20\" citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Water-clear polymeric gels have previously been fabricated as standardized boluses in thicknesses of 3, 5, and 10 mm, similar to commercial products such as the SuperFlex bolus [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. In addition, patient-customized boluses using hydrogel and silica gel have been developed through 3D printing technology [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eWe aim to fabricate a customized bolus using Transparent gel wax (TGW), a material similar to water-clear polymeric gels. Although TGW can be produced through 3D printer molding methods like those used for hydrogel or silica gel, such 3D printing processes require lengthy fabrication times. To reduce production time, we propose a non-3D printing fabrication approach for TGW boluses using alginate.\u003c/p\u003e\u003cp\u003eAlginate rapidly sets within 2 to 15 minutes after mixing with water and accurately replicates complex structures, which is why it is commonly used as an impression material in dentistry [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. However, alginate undergoes shrinkage due to water evaporation after fabrication, making it unsuitable for prolonged radiation therapy. Using a 3D-printed breast phantom, we aim to replicate the phantom without alginate shrinkage and produce a customized TGW bolus within one day. The outcomes of this fabrication method will be comparatively analyzed.\u003c/p\u003e"},{"header":"2. Methods and Materials","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Physical Property Evaluation\u003c/h2\u003e\u003cp\u003eTo measure the physical density (PD) and relative electron density (RED) of Alginate (GC, AFP; Aroma Fine Plus, Japan), SuperFlex bolus (Action Products, Inc., Bolx-II, USA), and TGW bolus (Candlebada, gel wax, Korea), samples were prepared by curing each material in acrylic molds sized 8 cm \u0026times; 3 cm \u0026times; 4 cm. Computed tomography (CT; Siemens, Somatom Confidence, Germany) scans were performed using the following parameters: 120 kVp, 15 mAs, 40 cm field of view (FOV), and 2 mm slice thickness. Images were analyzed within a region of interest (ROI) with a 2 cm diameter.\u003c/p\u003e\u003cp\u003eThe density and relative electron density of each bolus were estimated using the CT to electron density (ED) conversion curve based on Hounsfield units (HU). Homogeneity was evaluated by the standard deviation (SD) of HU values within the ROI. According to vendor specifications, the physical densities of SuperFlex and TGW were 1.00 g/cm\u0026sup3; and 0.87 g/cm\u0026sup3;, respectively.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Breast Phantom Design and Structure Definition\u003c/h2\u003e\u003cp\u003eTo replicate the curved skin surface of a patient who had undergone total mastectomy, the surface contours were artificially modeled in MIM software (MIM Software Inc., v7.1.10, USA) using the structure tool, and converted to a stereolithography (STL) file format as shown in Fig.\u0026nbsp;1. The model was then smoothed using Laplacian smoothing in Meshlab software (Meshlab_64bit v1.3.4 Beta) and converted to G-Code using 3D printing software (Simplify3D, v4.1.1, USA). Subsequently, the breast phantom was printed using a 3D printer (Creality3D, CR-10 Max, Shenzhen, China).\u003c/p\u003e\u003cp\u003eA virtual bolus structure (15 cm \u0026times; 15 cm \u0026times; 1 cm) was created 1 cm above the breast phantom surface using MIM software. For electron beam therapy planning, a planning target volume (PTV) for 6 MeV electrons (11 cm \u0026times; 11 cm \u0026times; 1.5 cm) was defined extending 1.5 cm deep from the phantom surface. Similarly, the PTV for 8 MeV electrons (11 cm \u0026times; 11 cm \u0026times; 2.1 cm) was defined extending 2.1 cm deep from the phantom surface. These structures were exported to the treatment planning system (TPS; Elekta Oncology Systems, Monaco, v5.51.10, Sweden) for further planning.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3 Fabrication of a Customized TGW Bolus\u003c/h2\u003e\u003cp\u003eTo replicate the surface of the breast phantom, alginate (GC Corporation, Aroma Fine Plus Normal Set, Tokyo, Japan) was mixed with distilled water at a ratio of 0.42:1 and poured onto the phantom surface as shown in Fig.\u0026nbsp;2. The alginate was allowed to set for 10 minutes, after which it was carefully removed from the phantom surface. A mold for the TGW bolus was then prepared using silicone (PONY, Moldmaster Ultra, China), by mixing parts A (base) and B (curing agent) at a 1:1 ratio. The cured alginate was placed into an acrylic frame, and the mixed silicone was poured into the frame and left to cure for 6 hours. Once the silicone mold was set, the alginate was removed. To enhance the optical transparency of the TGW bolus, the surface of the silicone mold was coated with epoxy resin (Smooth-On Inc., XTC-3D, USA).\u003c/p\u003e\u003cp\u003eTGW, composed of white mineral oil and a copolymer (SEBS; styrene-ethylene/butylene-styrene), was heated to 140\u0026deg;C until it became liquid. To remove bubbles, the liquid TGW was degassed for 5 minutes under vacuum using a defoaming chamber (HISCO, CL1.5, China). The degassed liquid TGW was then poured into the silicone mold and allowed to solidify for 30 minutes. The solidified customized TGW bolus was removed from the silicone mold and stored on a polyethylene terephthalate (PET) film.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e2.4 Evaluation of Customized TGW Bolus Conformity\u003c/h2\u003e\u003cp\u003eThe positional conformity between the virtual bolus created in the TPS and the fabricated bolus on the corresponding CT images was evaluated using the Bolus Conformity Index (BCI), calculated as follows:\u003c/p\u003e\u003cp\u003e\u003cimg src=\"https://myfiles.space/user_files/69519_bce2c0439cd956a6/69519_custom_files/img1765800933.png\"\u003e\u003c/p\u003e\u003cp\u003eHere, the BCI quantifies how well the customized bolus conforms to the virtual bolus, with values closer to 0 indicating higher spatial agreement. VBV represents the volume of the virtual bolus, VBV \u0026cup; BV denotes the union volume of the virtual bolus and the fabricated bolus, and VBV \u0026cap; BV represents the intersection volume of the two. The numerator thus reflects the non-overlapping volume portion between the two boluses, normalized by the volume of the virtual bolus.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e\u003ch2\u003e2.5 Treatment Planning and Evaluation\u003c/h2\u003e\u003cp\u003eA virtual bolus was placed on the surface of the breast phantom, and a prescription dose of 10 Gy in five fractions was applied to the PTV. In the TPS, the RED values were set to 0.976 for the SuperFlex bolus and 0.823 for the TGW bolus. Electron beams of 6 MeV and 8 MeV were calculated, respectively. The treatment planning objective for the 6 MeV beam was to deliver 80% of the prescribed dose to the PTV located up to 1.3 cm beneath the skin surface. For the 8 MeV beam, the goal was to deliver 80% of the dose to the PTV located up to 2.1 cm deep from the surface. All treatment plans were normalized such that 40% of the PTV received 10 Gy to allow for comparative evaluation.\u003c/p\u003e\u003cp\u003eDosimetric evaluation of the treatment plans included analysis of the homogeneity index (HI), conformity index (CI), surface dose (Point 1), D\u003csub\u003emax\u003c/sub\u003e, D\u003csub\u003emean\u003c/sub\u003e, D\u003csub\u003emin\u003c/sub\u003e of the PTV, and the mean dose to the body minus PTV (BMP\u003csub\u003emean\u003c/sub\u003e). Planning was performed using the Monaco TPS. The HI was defined as (D\u003csub\u003e2%\u003c/sub\u003e\u0026ndash;D\u003csub\u003e98%\u003c/sub\u003e)/D\u003csub\u003emedian\u003c/sub\u003e, and the CI was defined as TV/PTV, where TV represents the treated volume enclosed by the 80% isodose line [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. Dose\u0026ndash;volume histograms (DVHs) were used to evaluate dose distributions to the PTV and organs at risk (OARs).\u003c/p\u003e\u003cp\u003eThe technical parameters for plan calculation included: field size of 15 cm \u0026times; 15 cm cone, source-to-skin distance (SSD) of 100 cm, grid size of 2 mm, Monte Carlo algorithm with 700,000 histories per cm\u0026sup2;.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e2.6 Dose Assurance\u003c/h2\u003e\u003cp\u003eOptically stimulated luminescence (OSL) nanoDot dosimeters (Landauer, Inc., Greenwood, IL) were calibrated using a 2 Gy irradiation from a linear accelerator (Linac; Elekta, VersaHD, Sweden) in accordance with the IAEA TRS-398 calibration protocol [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. After setting up the breast phantom as shown in Fig.\u0026nbsp;3 within the radiotherapy treatment room, dose verification was performed using 6 and 8 MeV electron beams with either SuperFlex or TGW bolus materials, depending on the treatment plan. For each plan, a total of four OSL dosimeters were placed\u0026mdash;one at the center and three at peripheral positions. The measured doses were then compared to the planned doses, and the differences were analyzed.\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e3.1 Physical Property Evaluation\u003c/h2\u003e\u003cp\u003eThe HU, SD, PD, and RED of the SuperFlex bolus and TGW bolus used in this study are summarized in Table\u0026nbsp;1. Among the materials tested, the SuperFlex bolus exhibited HU values most similar to those of water. In terms of homogeneity, the TGW bolus demonstrated the best performance with a standard deviation of \u0026plusmn;\u0026thinsp;5.1, followed closely by the SuperFlex bolus with \u0026plusmn;\u0026thinsp;5.3, indicating both materials are highly homogeneous.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Bolus Conformity Index Evaluation\u003c/h2\u003e\u003cp\u003eThe volume of the virtual bolus was measured to be 217.7 cc, while the SuperFlex and TGW bolus volumes were 169.2 cc and 221.8 cc, respectively. The non-overlapping volume between the virtual bolus and the SuperFlex bolus was 49.7 cc, whereas the non-overlapping volume between the virtual bolus and the TGW bolus was only 6.4 cc. The BCI for the SuperFlex bolus was 0.23, whereas the TGW bolus demonstrated a significantly lower BCI of 0.03, indicating a much higher volumetric conformity to the virtual bolus (Fig.\u0026nbsp;4).\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Plan Evaluation\u003c/h2\u003e\u003cp\u003eThe dosimetric results for the PTV calculated in the TPS are presented in Table\u0026nbsp;2. For the 6 MeV electron beam plan, where the PTV was located at a depth of 1.3 cm, the CI for the SuperFlex bolus plan was 1.38, suggesting adequate coverage of the PTV with minimal unnecessary dose to surrounding tissue. The surface dose in the SuperFlex plan was also the most consistent with the prescribed dose, delivering 10.03 Gy. In the TGW bolus plan, the R\u003csub\u003e80%\u003c/sub\u003e depth was measured at 1.5 cm, indicating a slight overdose of approximately 0.2 cm beyond the intended depth (Fig.\u0026nbsp;5).\u003c/p\u003e\u003cp\u003eThis slight overdose at the distal end may provide beneficial skin sparing while still ensuring adequate coverage of the superficial target volume. Such a dose distribution is clinically acceptable, especially in postmastectomy patients where homogeneous dose to the chest wall is essential and minor variations within subcentimeter range are not considered clinically significant.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\u003ch2\u003e3.4 Dose Assurance\u003c/h2\u003e\u003cp\u003eUnder the same conditions as the treatment plans, electron beams were delivered to the breast phantom in the radiotherapy suite, and dose measurements were obtained using optically stimulated luminescence (OSL) dosimeters. The results are summarized in Table\u0026nbsp;3. The average error rate\u0026thinsp;\u0026plusmn;\u0026thinsp;SD for the SuperFlex bolus was 0.6%\u0026plusmn;1.8% for the 6 MeV plan and \u0026minus;\u0026thinsp;2.1%\u0026plusmn;1.4% for the 8 MeV plan. For the TGW bolus, the corresponding values were \u0026minus;\u0026thinsp;0.2%\u0026plusmn;1.3% at 6 MeV and 0.5%\u0026plusmn;1.2% at 8 MeV, indicating that the TGW bolus provided dose delivery closer to the treatment plan.\u003c/p\u003e\u003cp\u003eIn terms of maximum absolute error at measurement points, the SuperFlex bolus exhibited errors of up to 3.1% at 6 MeV and \u0026minus;\u0026thinsp;4.1% at 8 MeV, whereas the TGW bolus showed smaller maximum deviations of \u0026minus;\u0026thinsp;1.8% at 6 MeV and \u0026minus;\u0026thinsp;2.1% at 8 MeV. These findings suggest improved dosimetric accuracy and consistency when using TGW bolus. From a clinical perspective, this improved accuracy translates into better reproducibility of dose delivery across treatment sessions. Especially in cases involving highly contoured surfaces such as the chest wall or head and neck regions, minimizing dose deviations due to air gaps can directly impact local control and treatment outcomes.\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Discussion","content":"\u003cp\u003eA bolus is a tissue-equivalent material widely used in cases such as postmastectomy or cutaneous malignancies, with the purpose of compensating for irregular patient contours, enhancing surface dose, or attenuating excess dose in certain regions [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. In this study, the use of a TGW bolus offered three distinct advantages or options.\u003c/p\u003e\u003cp\u003eThe first advantage lies in the favorable physical properties of the TGW bolus, including transparency and stability. Existing transparent boluses include silicone-based CLEANBOLUS and polymeric gel-based ClearSight bolus [\u003cspan additionalcitationids=\"CR17\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. TGW can be easily purchased from online candle supply retailers at very low cost and is categorized into CLP (Candle gel low density), CMP (Candle gel medium density), and CHP (Candle gel high density) types. These types vary in the ratio of mineral oil to copolymer, with increased copolymer content yielding higher tensile strength but lower flexibility. Although the exact composition is not disclosed, the CHP used in this study is estimated to contain 12\u0026ndash;15% copolymer, which is comparable to the material properties of ClearSight bolus.\u003c/p\u003e\u003cp\u003eAccording to Maneas et al., TGW exhibits high optical transparency and stable characteristics in scattering and absorption, with a well-defined absorption spectrum in the visible range of 400\u0026ndash;900 nm, enabling accurate differentiation of transmitted colors [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. Due to its low optical diffusion, the shape of light transmitted through TGW remains well-defined. This allows clear visualization of the patient\u0026rsquo;s skin and ink markings through the bolus without distortion. Additionally, the TGW maintains its elastic and soft properties over time without degradation. As a thermoplastic material, it melts into a liquid form at 75\u0026thinsp;~\u0026thinsp;85\u0026deg;C and can be reshaped and reused. These properties have primarily been explored in the context of ultrasound studies [\u003cspan additionalcitationids=\"CR20\" citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan additionalcitationids=\"CR28\" citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe second advantage is that this study represents the first attempt to fabricate a customized transparent bolus. Standard boluses such as SuperFlex may create air gaps at the patient\u0026rsquo;s skin surface, potentially reducing the surface dose [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. Furthermore, Anderson et al. reported that using a standard bolus in postmastectomy patients resulted in a 24% complication rate, which could be reduced to 9% with the use of a customized bolus\u0026mdash;strongly supporting its clinical recommendation [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eWe successfully developed a customized TGW bolus using an alginate-based casting technique that does not require 3D printing. The customized TGW bolus demonstrated excellent conformity; however, minor surface irregularities were observed. This issue arose from uneven application of the surface coating agent on the silicone mold and was mitigated by performing a secondary coating.\u003c/p\u003e\u003cp\u003eFurthermore, the alginate-based casting method enables faster clinical application compared to 3D printing. As the print size increases, the printing time with a 3D printer becomes longer, and producing a bolus of sufficient size for clinical use typically takes at least 2ཞ3 days. In contrast, although the alginate casting technique requires more manual effort to fabricate the alginate mold and subsequent silicone mold, the total fabrication time for the TGW bolus remains under one day, allowing rapid deployment in clinical settings.\u003c/p\u003e\u003cp\u003eWhen choosing an alginate casting method for clinical use, one should also consider that alginate hardens quickly\u0026mdash;typically within 2ཞ5 minutes\u0026mdash;depending on the product type, which can make mold formation challenging [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. Therefore, using commercially available alginate products with a slower setting time of 10\u0026ndash;15 minutes allows for easier fabrication of immobilization structures directly within the CT simulation room.\u003c/p\u003e\u003cp\u003eThis non-3D-printed alginate casting approach can be extended to other anatomical regions. For example, it may be used as a substitute for gingival or tongue bite immobilization in the oral cavity. Traditional putty materials have been used to stabilize oral structures but are often avoided due to their high density. However, by casting putty in a similar way to alginate, one can replicate the geometry and subsequently mold a custom oral immobilization device using silicone and TGW instead of a bolus [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe advantage of customized TGW bolus is its ability to accurately reproduce the planned dose distribution. This was validated using optically stimulated luminescent (OSL) dosimeters. As shown in Fig.\u0026nbsp;4, the customized TGW bolus achieved excellent conformity to the curved surface of the breast phantom, and as demonstrated in Table\u0026nbsp;3, it resulted in dose measurements that were more consistent with the treatment plan. In particular, except for measurement point 1, which was located on a flat surface, points 2, 3, and 4 on the curved regions showed notable discrepancies when using the SuperFlex bolus\u0026mdash;up to 3.1% for 6 MeV and \u0026minus;\u0026thinsp;4.1% for 8 MeV. The maximum air gap between the SuperFlex bolus and the phantom surface was measured at 6 mm. In clinical situations, even greater air gaps may occur, potentially leading to higher dose errors [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eIn contrast, when using the customized TGW bolus, the dose errors were reduced to \u0026minus;\u0026thinsp;1.8% at most for 6 MeV and \u0026minus;\u0026thinsp;2.1% for 8 MeV. Moreover, the SD of \u0026plusmn;\u0026thinsp;1.3% and mean absolute deviation (MAD) of \u0026plusmn;\u0026thinsp;0.9% indicate lower variation in dose depending on measurement position. This suggests that the customized TGW bolus offers more consistent dose delivery across different regions and aligns with previous research demonstrating that customized boluses provide more stable and accurate dose distributions in radiotherapy [\u003cspan additionalcitationids=\"CR11 CR12 CR13 CR14\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eA third advantage is the ability to utilize different electron beam energies depending on the bolus density. It is not strictly necessary for a bolus to have a physical density exactly equal to 1.0 g/cm\u0026sup3;; rather, it should effectively smooth out surface irregularities and enhance surface dose [\u003cspan additionalcitationids=\"CR17\" citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Figure\u0026nbsp;5A shows the planned dose distribution using 6 MeV electron beams when the PTV is located at a depth of 1.3 cm. This corresponds to measurement point 1 in Table\u0026nbsp;3. If the PTV were instead located at 1.5 cm depth, 8 MeV beams would typically be required to deliver 80% of the prescribed dose, as seen in Fig.\u0026nbsp;5C. However, by using a lower-density bolus such as TGW, it becomes feasible to achieve the same coverage at 1.5 cm depth using 6 MeV, as demonstrated in Fig.\u0026nbsp;5B.\u003c/p\u003e\u003cp\u003eThis approach requires overriding the RED value of the bolus in the treatment planning system, which can be a minor inconvenience. Nevertheless, similar adjustments are already made in clinical practice to improve treatment outcomes\u0026mdash;for example, using virtual boluses for plan optimization or employing pseudo skin flash techniques [\u003cspan additionalcitationids=\"CR6\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. When these modifications provide dosimetric benefits, they are considered acceptable and necessary parts of the planning workflow.\u003c/p\u003e\u003cp\u003eDespite these options, the density of TGW\u0026mdash;a mineral oil and copolymer-based material\u0026mdash;can potentially be increased closer to 1.0 g/cm\u0026sup3; by adding agents such as silica or titanium dioxide (TiO₂). However, such additions significantly reduce the optical transparency of the material [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. The TGW bolus with a physical density of 0.87 g/cm\u0026sup3; may serve as a viable option in its current form, but further research is warranted to identify new compounds that can increase the density while preserving transparency.\u003c/p\u003e\u003cp\u003eAt our institution, customized TGW boluses have already been applied clinically in patient treatments. However, further studies are needed to evaluate treatment outcomes and long-term prognoses.\u003c/p\u003e"},{"header":"5. Conclusion","content":"\u003cp\u003eWe evaluated the physical suitability of commercially available, inexpensive, and readily accessible TGW for use as a radiotherapy bolus material. Using a non-3D printing fabrication method with alginate casting, we were able to reduce production time and rapidly apply the bolus to patients.\u003c/p\u003e\u003cp\u003eThe customized TGW bolus conformed perfectly to the patient\u0026rsquo;s body surface, exhibited excellent flexibility and high transparency, and was easy to reproduce in the radiotherapy setting. Moreover, in terms of dose delivery, it performed as well as or better than conventional products. These findings suggest strong potential for broader clinical adoption of customized TGW boluses in the future.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAcknowledgements\u003c/h2\u003e\u003cp\u003eThis research was supported by the National Research Council of Science \u0026amp; Technology (NST) grant by the Korea government (MSIT) (No. CAP22042-300) \u0026amp; Korea Institute of Radiological \u0026amp; Medical Sciences (KIRAMS) grant funded by the Korea government (Ministry of Sciences and ICT) (No. 50572\u0026thinsp;\u0026minus;\u0026thinsp;2025) \u0026amp; the Nuclear Safety Research Program through the Korea Foundation of Nuclear Safety (KoFONS) using the financial resource granted by the Nuclear Safety and Security Commission (NSSC) of the Republic of Korea(RS-2022-KN071220).\u003c/p\u003e\u003cp\u003eConflict of Interest\u003c/p\u003e\u003cp\u003eThe authors declare that they have no conflict of interest.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eOvergaard M, Hansen PS, Overgaard J, Rose C, Andersson M, Bach F, Kjaer M, Gadeberg C, Mouridsen HT, Jensen MB, Zedeler K (1997) Postoperative radiotherapy in high-risk premenopausal women with breast cancer who receive adjuvant chemotherapy: Danish Breast Cancer Cooperative Group 82b Trial. 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J Appl Clin Med Phys 23(4):e13538\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"},{"header":"Tables","content":"\u003cp\u003eTables 1 to 3 are available in the Supplementary Files section.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"physical-and-engineering-sciences-in-medicine","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"apes","sideBox":"Learn more about [Physical and Engineering Sciences in Medicine](http://link.springer.com/journal/13246)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/apes/default.aspx","title":"Physical and Engineering Sciences in Medicine","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Gel wax, Bolus, Electron beam therapy, OSL dosimeter","lastPublishedDoi":"10.21203/rs.3.rs-7174847/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7174847/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003ch2\u003ePurpose:\u003c/h2\u003e\u003cp\u003eThis study aimed to develop a customized bolus using commercially available transparent gel wax (TGW) without requiring 3D printing technology and to evaluate its applicability in radiotherapy, including its physical and dosimetric characteristics.\u003c/p\u003e\u003ch2\u003eMethods:\u003c/h2\u003e\u003cp\u003eA dental alginate impression was taken from the curved surface of a mastectomy phantom to replicate the patient's skin contour. Based on this, a silicone mold was fabricated, into which TGW was poured to form the customized bolus. The fabricated bolus was evaluated for its physical properties (density, electron density, homogeneity) and bolus conformity index (BCI), treatment planning dosimetric analysis, and actual dose measurements using optically stimulated luminescent (OSL) dosimeters, in comparison with a conventional vinyl gel sheet bolus (SuperFlex).\u003c/p\u003e\u003ch2\u003eResults:\u003c/h2\u003e\u003cp\u003eThe TGW bolus demonstrated high transparency and excellent homogeneity (standard deviation of internal HU: \u0026plusmn;5.1), and its BCI was 0.03, indicating a very high level of conformity to the virtual bolus. Under 6 MeV and 8 MeV electron beam conditions, the TGW bolus showed mean dose errors of \u0026minus;\u0026thinsp;0.2%\u0026plusmn;1.3% and 0.5%\u0026plusmn;1.2%, respectively, which were more consistent and lower than those of the SuperFlex bolus (0.6%\u0026plusmn;1.8% and \u0026minus;\u0026thinsp;2.1%\u0026plusmn;1.4%, respectively). In particular, the TGW bolus showed superior conformity and reproducibility in regions with high surface curvature, effectively reducing dose loss caused by air gaps.\u003c/p\u003e\u003ch2\u003eConclusion:\u003c/h2\u003e\u003cp\u003eTGW is a low-cost, transparent, and flexible material that enables rapid fabrication (within one day) of a customized bolus through a simple molding process. Its superior physical stability and dosimetric performance compared to existing products suggest strong potential for clinical application in radiotherapy.\u003c/p\u003e","manuscriptTitle":"Development of a Customized Three-Dimensional Bolus Using Transparent Gel Wax and Its Application in Electron Beam Therapy","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-15 16:11:04","doi":"10.21203/rs.3.rs-7174847/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Minor revisions","date":"2026-03-01T07:12:13+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2025-12-09T23:13:51+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-12-09T20:16:01+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Physical and Engineering Sciences in Medicine","date":"2025-07-28T23:16:53+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-07-28T15:03:01+00:00","index":"","fulltext":""},{"type":"submitted","content":"Physical and Engineering Sciences in Medicine","date":"2025-07-27T22:47:07+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"physical-and-engineering-sciences-in-medicine","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"apes","sideBox":"Learn more about [Physical and Engineering Sciences in Medicine](http://link.springer.com/journal/13246)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/apes/default.aspx","title":"Physical and Engineering Sciences in Medicine","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"35442edf-4ff9-4a23-91ee-ca4eccee111b","owner":[],"postedDate":"December 15th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-03-18T09:00:09+00:00","versionOfRecord":[],"versionCreatedAt":"2025-12-15 16:11:04","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7174847","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7174847","identity":"rs-7174847","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}