PHWR Public Health Weekly Report

Radiation is an essential component of modern medical diagnostics and its importance continues to grow. However, radiation can have harmful effects on the human body, which necessitates the use of various radiation shielding facilities. These facilities, which are designed to protect patients and medical staff from radiation exposure, include shielding walls, ceilings, floors, observation windows, and lead aprons made primarily of concrete, lead, and lead glass [1,2].

According to Article 37 of the Medical Service Act of Republic of Korea, Korean medical institutions must establish radiation-shielding facilities to use radiation. The safety management rules for radiation-generating devices outlined in Annex 2 guide the inspection of these facilities, with further details provided in related guidelines [3]. The thickness of the radiation-shielding materials is determined based on their half-value layer and the peak tube voltage of the radiation-generating device. However, the current guidelines suggest uniform thicknesses that do not reflect these variables. For instance, the Medical Service Act mandates that the lead equivalence for shielding walls should be at least 1.5 mm for tube voltages above 100 kV, and 1 mm for tube voltages below 100 kV.

Lee et al. [4] used Monte Carlo N-Particle6 (MCNP6) to calculate the lead thickness required to achieve 50%, 70%, 90%, and 95% shielding at peak tube voltages of 80, 100, 120, and 140 kVp; their results revealed thicknesses of 0.33, 0.5, 0.56, and 0.61 mm, respectively. Experimental validation showed that these thicknesses provided shielding comparable to the simulations, highlighting significant discrepancies with the 1 mm lead equivalence stipulated by current regulations.

In another study, Lee et al. [5] assessed the impact of aging on the shielding efficiency of radiation-shielding facilities by simulating changes in material composition and the development of cracks. They found that increased oxidation of lead led to the formation of lead oxide, and larger cracks in the lead plates significantly reduced the shielding efficiency. Similarly, a decrease in the lead oxide content in lead glass also resulted in a lower shielding performance. These findings underscore the substantial impact of aging on shielding efficiency.

This study aimed to extend previous research using MCNP6 to simulate the actual structure of radiation-shielding walls and to calculate the optimal shielding thickness. Additionally, the impact of aging on the shielding efficiency of these walls was analyzed, providing a basis for periodic inspections of radiation shielding facilities.

1. Monte Carlo Simulation and Energy Spectrum

The Monte Carlo simulation is a technique that uses repeated random sampling to probabilistically estimate uncertain outcomes. The radiation effects, from their source and their impact on the human body, are inherently probabilistic, making Monte Carlo simulations essential. Globally, many research teams actively use Monte Carlo simulations for this purpose [6]. This study employed MCNP6, a well-known Monte Carlo simulation tool.

The experimental setup for MCNP6 is shown in Figure 1. The distance from the radiation source to the shielding wall was set to 100 cm, with a field size of 10×10 cm². The attenuation rate was calculated using F4 Tally, which measures the ratio of transmitted to incident X-rays at various tube voltages.

Figure 1. The experimental setup simulated within the Monte Carlo simulation
Source-to-radiation shielding wall distance of 100 cm, a field size of 10×10 cm², and a three-layer structure composed of concrete-lead-concrete. SSD=source to surface distance.

To define the source term in the Monte Carlo simulation, peak tube voltages commonly used in clinical settings (80, 100, 120, and 140 kVp) were selected. The source term includes information about the number of particles to be transported, energy spectrum, and physical model. The energy spectra for these voltages were generated using the SRS-78 program (Institute of Medical Physics and Engineering) which is widely cited for its reliability [7]. As shown in Figure 2, the energy spectrum for each peak tube voltage was calculated, and the source term was accordingly defined in MCNP6.

Figure 2. Energy spectra for 80, 100, 120, and 140 kVp calculated using the SRS-78 program
(A) Energy spectrum at peak tube voltage of 80 kVp. (B) Energy spectrum at peak tube voltage of 100 kVp. (C) Energy spectrum at peak tube voltage of 120 kVp. (D) Energy spectrum at peak tube voltage of 140 kVp.

2. Determining Optimal Shielding Thickness Using Monte Carlo Simulation

In clinical settings, radiation-shielding walls typically have a three-layer concrete-lead-concrete structure. This study aimed to determine the optimal shielding thickness for such a configuration. Specifications for Portland cement (a common type of concrete) and pure lead were sourced from the Pacific Northwest National Laboratory and input into MCNP6, including the density and elemental composition (Table 1) [8]. The structure of the radiation-shielding wall is shown in Figure 1. The optimal shielding thickness was calculated considering both conservative estimates and weight.

Table 1 Specifications for Portland cement and lead provided by PNNL

Material specifications	Lead		Portland cement
Components and ratio	Pb	1	H	0.010000
			C	0.001000
			O	0.529107
			Na	0.016000
			Mg	0.002000
			Al	0.033872
			Si	0.337021
			K	0.013000
			Ca	0.044000
			Fe	0.014000
Density (g/cm3)	11.34		2.3

PNNL=Pacific Northwest National Laboratory..

3. Impact of Deterioration on Attenuation Rate

Concrete and lead, the primary materials of radiation-shielding walls, can deteriorate due to impacts, leakage, and other factors, leading to cracks, weathering, delamination, and spalling. This study categorized the deterioration scenarios into three types.

Concrete layer thickness reduction: Over time, impacts, leakage, and corrosion can cause spalling and delamination, thereby reducing the thickness of the concrete layers. The attenuation rate was calculated by reducing the optimal shielding thickness by a certain value.

Lead-plate thickness reduction: Lead plates can oxidize and become thin over time owing to impacts, leakage, and corrosion. Similar to the concrete layer, the attenuation rate efficiency was recalculated by reducing the optimal lead plate thickness.

Crack formation in shielding walls: Cracks in the concrete layer can be categorized by size into micro, intermediate, and large cracks. The crack was modeled as a wedge shape, with the length of the base incrementally increased. The shielding rate was then calculated for each peak tube voltage based on crack size.

1. Optimal Shielding Thickness for Simulated Radiation-shielding Walls Using Monte Carlo Simulation

The optimal shielding thickness for each peak tube voltage was calculated to satisfy a 99% shielding rate while considering the thickness and weight of the radiation protection barrier, ensuring a conservative evaluation of its shielding performance (Table 2). For a three-layer concrete-lead-concrete structure, the results showed that at a peak tube voltage of 80 kVp, the optimal shielding thickness was 1.4 cm of concrete and 0.04 mm of lead. For a peak tube voltage of 100 kVp, the thickness required was 1.6 cm of concrete and 0.06 mm of lead. At 120 kVp, the necessary thickness was 1.2 cm of concrete and 0.08 mm of lead. Finally, at 140 kVp, the optimal thickness was 1.4 cm of concrete and 0.08 mm of lead. The shielding thickness satisfying a 99% shielding rate for all peak tube voltages was determined to be 1.4 cm of concrete and 0.08 mm of lead for 140 kVp. The total thickness of the radiation protection barrier was confirmed to be 2.808 cm, approximately 3 cm, which was the sum of two concrete layers and one lead layer.

Table 2 Optimal shielding thickness for the concrete-lead-concrete three-layer radiation shielding wall at various peak tube voltages

Materials	Tube voltage (kVp)
	80	100	120	140
Lead (mm)	0.04	0.06	0.08	0.10
Concrete (cm)	1.4	1.4	1.4	1.4

2. Attenuation Rate Calculation Based on Assumed Deterioration of Radiation-shielding Walls

The deterioration of the radiation-shielding wall was simulated based on the optimal thickness of 1.4 cm for each concrete layer and 0.08 mm for the lead layer. For each peak tube voltage (80, 100, 120, and 140 kVp), the attenuation rates were calculated as the thickness of the concrete and lead layers decreased by 0.2 cm and 0.02 mm increments, respectively, until fully depleted (Table 3). When the concrete thickness decreased, the attenuation rates at 80, 100, 120, and 140 kVp decreased from 99.8%, 99.3%, 99.1%, and 98.9% to 99.6%, 98.7%, 98.3%, and 98%, respectively. In contrast, a reduction in the thickness of the lead layer resulted in a significant decrease in the attenuation rate at all tube voltages, from a maximum of 98.9% to a minimum of 74.4%.

Table 3 Attenuation rate based on the reduction in thickness of the concrete or lead layer in the radiation shielding wall at different peak tube voltages

Type	Thickness (cm)	Tube Voltage (kVp)
		80	100	120	140
Concrete layer	1.4	99.80	99.38	99.15	98.94
	1.2	99.79	99.32	99.07	98.85
	1	99.77	99.25	98.99	98.76
	0.8	99.75	99.17	98.90	98.65
	0.6	99.72	99.09	98.79	98.53
	0.4	99.69	98.99	98.66	98.40
	0.2	99.66	98.88	98.52	98.23
	0	99.62	98.75	98.35	98.03
Lead layer	0.08	99.80	99.38	99.15	98.94
	0.06	99.56	98.74	98.34	97.95
	0.04	98.90	97.41	96.54	95.80
	0.02	96.78	93.94	92.06	90.47
	0	86.60	81.20	77.30	74.40

Unit: %..

The impact of cracks on the attenuation rate was also analyzed (Figure 3). Crack sizes increased by an order of magnitude from 0.1 mm to a maximum of 10 cm. At 80 kVp, the attenuation rate decreased from 96.8% to 51.1%. At a peak tube voltage of 100 kVp, the attenuation rate decreased from 94% to 50.1%. Furthermore, at 120 kVp, the attenuation rate decreased from 92.2% to 49.4%, and at 140 kVp, it decreased from 90.6% to 48.9%. These results demonstrate that while the deterioration of concrete has a relatively minor impact on the attenuation rates, the deterioration of the lead layer and the formation of cracks significantly reduce shielding effectiveness. This underscores the importance of periodic inspections and maintenance of radiation-shielding facilities to ensure continued protection against radiation exposure.

Figure 3. The shielding rate of the radiation shielding wall as a function of the increase in the crack base length for each peak tube voltage

In this study, we used a Monte Carlo simulation to calculate the optimal shielding thickness for a concrete-lead-concrete three-layer radiation-shielding wall constructed similarly to those in actual clinical settings. The goal was to achieve an attenuation rate of 99% while considering the size and weight of the shielding wall. Additionally, we evaluated the impact of wall deterioration on attenuation performance.

The optimal shielding thicknesses for the three-layer structure were determined for all peak tube voltages resulting in thicknesses of 1.4 cm and 0.08 mm for concrete and lead, respectively. Notably, thicknesses of 1.4 cm for concrete and 0.1 mm for lead satisfied all tube voltages. Using these values, we assessed the impact of deterioration on shielding performance. A decrease in the thickness of the concrete layers had minimal impact on the shielding performance at each tube voltage. However, a decrease in the thickness of the lead layer significantly changed the shielding performance across all tube voltages, confirming the critical role of lead in radiation shielding. Additionally, the presence of cracks in the radiation-shielding wall led to an average decrease of approximately 19% in shielding performance across all tube voltages.

This study successfully determined the optimal shielding thickness for a radiation-shielding wall similar to that used in actual clinical settings. The results showed that efficient shielding could be achieved with the calculated thickness, which is thinner than that suggested by current medical regulations. Furthermore, the study highlighted that the deterioration of the lead layer has a more substantial impact on attenuation performance than the deterioration of the concrete layer, and that cracks significantly compromise shielding effectiveness. However, the aging scenarios defined in this study had limitations in reflecting the actual aging process over time.

The findings of this study provide a basis for the periodic inspection of radiation-shielding facilities. Future research will extend the evaluation of shielding performance to various types of radiation-shielding facilities.

Ethics Statement: Not applicable.

Funding Source: This study was supported by the 2023 project from the Korea Disease Control and Prevention Agency (KDCA) for investigating the current status and developing improvement plans for radiation shielding facilities (Policy research, 202303840001).

Acknowledgments: None.

Conflict of Interest: The authors have no conflicts of interest to declare.

Author Contributions: Conceptualization: KYL. Data curation: KYL, KHJ. Formal analysis: KYL, KHJ. Funding acquisition: CHB. Investigation: KYL, KHJ, JOK, CHB. Methodology: KYL, CHB. Project administration: CHB. Supervision: CHB. Validation: JOK, CHB. Visualization: KYL, KHJ. Writing – original draft: KYL. Writing – review & editing: JOK, JWG, YJM, CHB.