Advancements in high-energy radiation therapy, particularly intensity-modulated radiation therapy, have significantly facilitated the optimization of beam paths to minimize radiation exposure to the surrounding organs at risk (OARs) [1,2]. However, when radiation beams pass in close to critical OARs, such as during treatments of the pelvis or lower extremities in male patients, additional shielding is often required to protect sensitive reproductive tissues [3,4]. Particularly, radiation exposure to the testes can substantially hinder spermatogenesis, potentially leading to temporary or permanent infertility [5,6]. Despite advances in beam modulation, conventional shielding methods are inadequate in providing patient-specific comfort and protection.

Conventional lead shields for protecting the testes from radiation exposure come in standardized sizes and shapes, which might not conform to individual anatomical variations, leading to patient discomfort [7,8]. To overcome this limitation, we employed thermoplastic materials capable of being molded to the body surface and quickly cooling to a stable shape at body temperature, providing a viable solution for patient-specific shielding. In this study, we evaluated the use of these sheets as patient-specific gonadal shields during the computed tomography (CT) simulation process, aiming to improve patient comfort and protective efficacy compared to traditional methods.

1. Patient-specific gonadal shield

The thermoplastic sheet (Paprica Lab Co., Ltd.) has a physical density of 0.969 g/cm³, a Shore hardness (Shore A) of 36, making it softer than a rubber eraser (approximately Shore A 40), and a melting point of 49°C. The sheets were heated in an Digital dry-heat convection oven (Orfit) at 75°C for 5 minutes to achieve sufficient pliability for molding. Fig. 1 summarizes the entire process of creating a patient-specific gonadal shield using this sheet. During the CT simulation, the sheet was molded precisely to the testicular contours of the patient, which served as the basis for treatment planning. A contour was created to assign a Hounsfield unit (HU) value corresponding to the lead in the gonadal shield, facilitating the treatment process. After the simulation, the shape was retained to create a final shield using a lead sheet molded to the thermoplastic form. Considering the high rigidity and malleability of lead, we manually fabricated the shield by layering multiple 1 mm thick lead sheets and bending them by hand. The shield exhibited consistent conformity even with repeated use.

Figure 1. Step-by-step creation of a patient-specific gonadal shield. CT, computed tomography.

2. Measurement of the shielding dose

The shielding dose was measured using 38 optically stimulated luminescent dosimeters (OSLDs or nano-dot dosimeters, Landauer, Inc.). Each measurement was repeated thrice to obtain an average value. The OSLDs were placed on both the inner and outer surfaces of the lead shield, ensuring identical positioning on both sides. Dosimeters were attached around the testicular area to assess the shielding efficacy.

1. Patient-specific gonadal shield

In our CT simulation, we used a 5-mm thick thermoplastic sheet to rapidly and effectively create an alternative gonadal shield. Traditional lead shields are difficult to conform to individual patient contours because of their high density and rigidity. Moreover, the toxicity of lead poses a significant risk of heavy metal poisoning, limiting its direct applications in clinical settings. In contrast, the developed thermoplastic sheet accurately conformed to the anatomy of the patient, minimizing air gaps between the sheet and the body (Fig. 2). The CT number of the thermoplastic sheet was approximately –80 HUs. To establish an accurate treatment plan, this value was reassigned to match 6,000 HU of lead, which was then applied in the treatment process. Intermittent air pockets, each measuring less than 1 mm in diameter, were observed between the thermoplastic sheet and the body, resulting in a cumulative air gap volume of 8.4 cm³. This minimal gap was deemed negligible, exerting little to no effect on the overall dose reduction efficacy.

Figure 2. (a) Overall, (b) axial, and (c) sagittal views of a computed tomography image obtained using the thermo plastic sheet during the radiation treatment simulation. The air gaps between the body and the sheet were minimal.

The ability of the thermoplastic sheet to retain its shape after cooling allowed for precise crafting of the lead shield. As lead is less elastic and flexible than the thermoplastic sheet, the lead shield was made slightly larger to ensure patient comfort. Consequently, we created a patient-specific lead shield that matched the contour of the thermoplastic sheet (Fig. 3).

Figure 3. A patient-specific gonad shield created using a thermoplastic sheet.

The shielding dose around the gonadal area was measured using OSLD for the point dose assessment. As shown in Fig. 4, identical OSLD positions were selected inside and outside of both shields for measurement. The treatment target was the pubic region, where the patient received a single fraction dose of 1,800 cGy using a 10× flattening filter-free beam. Tables 1 and 2 summarize the results of the point dose measurements. The patient-specific shield comprised anterior and posterior panels that could be opened and closed for easier setup. By averaging the OSLD values within the same row, we observed a maximum dose reduction of 56.1% in the posterior shield area. However, in the anterior shield, the dose tended to increase, up to 1.97 times, in the inferior direction and some parts of the superior direction. This may be due to the small gaps on the inferior side of the fabricated lead shield. Additionally, in the 360-degree Volumetric Modulated Arc Therapy plan, the couch attenuates the beam in the posterior direction, whereas in the anterior direction, higher-energy beams interact with the high-Z material (lead), possibly increasing scatter effects.

Table 1 . Point dose measurement results for the anterior patient-specific shield.

OSLD number				Measured dose (cGy)			OSLD position
Outside	Inside	Outside		Mean	Inside	Mean
6	11	2.30		2.30	3.06	3.06	Patient-Inferior-Anterior
10	44	2.89		2.82±0.06	5.36	5.55±0.15
12	4	2.74			5.72
41	26	2.83			5.58
3	15	4.01		4.15±0.26	7.88	8.18±0.32	Patient-Superior-Anterior
18	45	4.52			8.63
13	9	3.92			8.04
2	29	18.13		18.33±1.52	11.85	13.92±1.46
50	46	20.28			14.96
5	25	16.58			14.95

Table 2 . Point dose measurement results for the posterior patient-specific shield.

OSLD number				Measured dose (cGy)			OSLD position
Outside	Inside	Outside		Mean	Inside	Mean
22	16	5.22		5.22	2.29	2.29	Patient-Inferior-Posterior
27	21	7.81		8.34±0.40	3.79	3.70±0.21
48	36	8.76			3.90
23	17	8.45			3.40
43	33	13.61		14.12±0.51	5.65	6.29±0.64	Patient-Superior-Posterior
39	1	14.63			6.93
49	32	3.52		3.52	6.23	6.23
38	37	20.39		24.19±3.80	14.91	12.54±2.37
24	34	27.99			10.17

Figure 4. Images showing the placement of the optically stimulated luminescent dosimeters (OSLDs): behind the (a) posterior and (b) anterior shields. (c) Positions of the OSLDs used for dose measurement.

Radiation exposure to the testes can impair spermatogenesis, significantly increasing the risk of infertility [4,9]. Even with fractionated radiation doses, the semen volume and sperm count can reduce significantly, and the recovery period depends on the radiation dose received. For instance, it may take approximately 9 to 18 months for recovery from doses below 1 Gy, approximately 30 months for doses between 2 and 3 Gy, and over 5 years for doses between 4 and 6 Gy [5,10]. Although treatment plans can be optimized to minimize radiation exposure to the testes, repeated fractionated treatments can result in cumulative radiation doses that pose a risk of infertility.

Conventional gonadal lead shields are manufactured in standard sizes and shapes that do not sufficiently conform to the individual contours of the patient, often resulting in discomfort. Additionally, lead is a high-density material of 11.34 g/cm³, and if not customized properly, its weight can exert pressure or pinch the skin, making the patient uncomfortable. This issue is particularly problematic for the testes, which are highly sensitive to pain. However, molding lead shields directly on the gonadal area during the CT simulation is not suitable due to health concerns, and even thin lead sheets are difficult to shape quickly because of their rigidity.

To address these challenges, we used the thermoplastic sheet as a template for fabricating patient-specific lead shields. Thermoplastic materials have recently gained attention in radiation therapy because they can be easily molded to the body surface with minimal heat and rapidly cool to a stable configuration at body temperature [11,12]. Previously, thermoplastic materials have been primarily used for head and neck immobilization or in stereotactic body radiotherapy to enhance repositioning accuracy [13,14]. In this study, we used a thermoplastic sheet during CT simulation to create a gonadal shield that conformed to the anatomy of the patient, facilitating the fabrication of a customized lead shield with greater precision. The thermoplastic sheet retained its shape well after cooling, further improving the preciseness of the fabrication of the lead shield.

Using the patient-specific gonadal shield, we measured the shielding dose and found that the dose in the inferior direction of the anterior shield was higher than that before shielding. This may be attributed to secondary electron scattering due to the incomplete closure of the lead shield, which was manually bent and could not be fully sealed. This issue represents a limitation of our study, highlighting the need to further improve the replication of the shape using lead sheets. Some alternatives to this include using more flexible lead alloys or composite materials. These materials maintain high radiation shielding performance while being easier to mold, allowing for a better fit to the contours of the patient. Second, 3D printing technology can be used to produce highly precise, customized shields tailored to the anatomical structure of each patient. In all other areas, the surface dose of testis was effectively reduced from 28 cGy to below 15 cGy. This indicates that although there may be a temporary reduction in sperm count, it is expected to fully recover within nine months. Moreover, the improved comfort will significantly enhance the compliance of the patients for the treatment.

We effectively employed a thermoplastic sheet to rapidly and precisely model the anatomical contours of the patient to create a patient-specific lead shield with enhanced radiation protection and comfort during CT simulations. Future research should focus on validating the efficacy of these shields using larger patient cohorts or exploring the automation of lead shield fabrication to improve consistency and efficiency. Furthermore, as these shields can be rapidly molded on-site, the thermoplastic sheet shows significant potential for broader clinical applications, including the precise fabrication of boluses, immobilization devices, and patient-conforming masks.

This work was supported by Korea Institute of Energy Technology Evalutaion and Planning (KETEP) grant funded by the Korea government (MOTIE) (20227410100040, Development of patch-type flexible personal dosimeter and real-time remote monitoring system using high-performance inorganic perovskite).

Thermoplastic sheet was provided by Paprica Lab Co., Ltd. Chang Heon Choi and Jin Jegal are members of the editorial board of the Progress in Medical Physics, but have no role in the decision to publish this article. The other authors declare no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

All relevant data are within the paper and its Supporting Information files.

Conceptualization: Jung-in Kim. Data curation: Jin Jegal, Hyojun Park, Jung-in Kim. Formal analysis: Seonghee Kang, Chang Heon Choi, Jung-in Kim. Funding acquisition: Seonghee Kang. Investigation: Jin Jegal, Hyojun Park, Jung-in Kim. Project administration: Seonghee Kang, Chang Heon Choi, Jung-in Kim. Visualization: Jin Jegal. Writing – original draft: Jin Jegal. Writing – review & editing: Seonghee Kang, Chang Heon Choi, Jung-in Kim.

The study was approved by the Institutional Review Board of Seoul National University Hospital (IRB approval number; 2411-147- 1594).