High-energy radiation therapy aims to provide the maximum radiation to lesions while protecting normal organs. Radiation therapy methods for treating lesions include intensity-modulated radiation, image-guided radiation, and stereotactic radiation therapies [1]. Furthermore, radiosurgery using a linear accelerator equipped with an image guiding device has been developed in various forms and applied for clinical use. Furthermore, gamma knife radiosurgery using ⁶⁰Co, a traditionally used sealed source, is continuously being upgraded.

In recent years, gamma knife radiosurgery has evolved into a form that is applicable for the effective treatment of head/neck and brain lesions. Moreover, it is useful in the treatment of multiple metastatic brain tumors caused by breast and lung cancers [2-8].

Generally, a gamma knife uses a ⁶⁰Co source, which has a half-life of 5.27 y. The ⁶⁰Co source is a nuclide emitting two gamma rays and one beta ray. It exhibits most peaks at 1.17 and 1.33 MeV in gamma rays depending on energy spectrum analysis [9-11].

The ⁶⁰Co source has been shown to be a non-invasive method that focuses on high-dose radiation on lesions. However, to accurately localize lesions, magnetic resonance imaging and computed tomography images are required to delineate anatomical organ volumes and treatment targets for treatment planning [12]. At the beginning of image acquisition, a patient must be fixed invasively with screws of the Leksell frame. Additionally, gamma knife radiosurgery based on a fixed frame has proven effectiveness as a stereotactic operation that can maintain an error of less than 1 mm while the patient’s posture is being fixed [13].

However, the patient may not only experience pain and inconvenience caused by a head screw inserted inside the human body, but also dangers associated with normal tissues being exposed to radiation [14].

Hwang et al. [15] reported that nanoparticle doses of various materials inserted for radiation therapy can be up to 3.11 and 1.04 times more in kilovolt and megavolt X-rays, respectively. Furthermore, it was demonstrated that larger gold nanoparticles and lower incident energies are more closely related to secondary electron generation [16]. Mesbahi et al. [17] stated that the insertion of gold nanoparticles increased dose enhancement. Furthermore, Cheung et al. [18] reported a dose enhancement of up to 10% near a platinum implant.

In this study, we evaluated the additional exposure caused by the interaction of ⁶⁰Co gamma rays with each screw inserted in the human body and thermoplastic mask for fixing the patient using the Monte Carlo method.

The Monte Carlo N-Particle code version 2.7.0 (Los Alamos National Laboratory, USA) based on the Monte Carlo technique was used. Fixation screws and a thermoplastic mask were simplified to cylindrical and rectangular shapes based on schematics from the data sheet (see Fig. 1). Monte carlo simulations for screws were performed in a water phantom at 5 mm intervals from 45 to 65 mm. Calculations were performed starting from the build-up point of 5 mm with/without a thermoplastic mask measuring 2.4 and 3.2 mm (Fig. 2). The point of interest (Tally) was assigned with the screws and thermoplastic mask.

The composition was simulated with titanium alloy (4.54 g/cm²), aluminum alloy (2.85 g/cm²), and thermoplastic mask (1.1 g/cm²), separately. The energy was defined as a source of 1.17 and 1.33 MeV photons and ⁶⁰Co energy peaks were considered [19]. The distance of the source was specified at 165 mm based on the schematics of the Leksell knife. Additionally, the distance between the source and phantom was composed of air defined according to an International Commission on Radiation Units and Measurements report 44 [20].

An F4 tally was performed as specified by photon particle transport to count the electron fluence per unit area (particle/mm²) [21-24]. To ensure that the statistical uncertainty remained within 5% in the simulation, the number of repetitions was performed 1×10⁸ times (Fig. 3) [25].

The results showed a relative increase in the number of secondary electrons based on the absence of fixed screws at a depth of 5 mm. The larger the size and energy of the fixing screw in each energy point, the more secondary electrons appeared through the interaction with the radiation and screws. As shown in Table 1 for the titanium alloy, 278.39%, 298.11%, 315.66%, 331.25%, and 344.86% more electrons were generated at lengths of 45, 50, 55, 60, and 65 mm, respectively, in 1.17 MeV. For the case of 1.33 MeV, 279.61%, 299.48%, 317.57%, 333.65%, and 347.53% more electrons were generated at lengths of 45, 50, 55, 60, and 65 mm, respectively. Furthermore, as shown in Table 2 for the aluminum alloy, 309.77%, 334.18%, 356.21%, 376.18%, and 394.26% electrons were generated at lengths of 45, 50, 55, 60, and 65 mm in 1.17 MeV, respectively; and 310.13%, 335.1%, 357.44%, 378.06%, and 396.27% more electrons were generated at lengths of 45, 50, 55, 60, and 65 mm in 1.33 MeV, respectively. Furthermore, the occurrence of secondary electrons of aluminum alloy was more extensive than that of titanium alloy.

Electrons generated by the interaction between the radiation and the thermoplastic mask were measured. As shown in Table 3, electron generation increased by 264.69% and 391.25% for the 2.4 and 3.2 mm thermoplastic masks in 1.17 MeV, respectively. For 1.33 MeV, electron generation increased by 262.79% and 388.75% for the 2.4 and 3.2 mm thermoplastic masks, respectively.

According to the physical mechanism, the photoelectric effect is dominant in diagnostic X-rays using lower incident energies. Additionally, more secondary electrons are generated as the energy decreases and the atomic number of the medium increases. Furthermore, in high-energy radiation therapy, energy transfer occurs primarily through Compton scattering, which produces fewer secondary electrons than low-energy regions, such as diagnostics, because of its low dependence on the atomic number. However, it was demonstrated that the generation of reduced secondary electrons increased again when pair production began [26].

We performed a simulation based on the Monte Carlo method to evaluate the number of secondary electrons generated by fixed screws inserted into the human body during gamma knife treatment. In the study, as the size and energy of the screws used for fixation increased, the generation of secondary electrons increased up to 394.26% and 396.27% at 1.17 and 1.33 MeV, respectively.

Although differences in measured values may arise because of the build-up region and geometry structure, we identified concerns regarding further exposure due to materials inserted into the human body.

The results show that aluminum alloy yielded a more considerable dosage increase than titanium alloy because some of the scattered radiation was absorbed by the titanium alloy, which was higher in density (Fig. 4). Furthermore, in other gamma knife equipment using a thermoplastic mask, interactions between the source and mask may result in additional secondary electrons, which may contribute to additional exposure to normal tissues [27-30].

In this study, it is meaningful to obtain useful information regarding the increase in dose that could not be measured during radiation surgery; this can be performed by confirming the generation of secondary electrons according to the size and energy of the fixing screw inserted into the human body and the generation of secondary electrons according to each thickness of thermoplastic mask through simulations.

We attempted to confirm the trend of secondary electrons generated from a single source. From the trend, we can predict that more secondary electrons occur when the 201 sources of the gamma knife are used.

It was discovered in this study that the larger size and energy of fixing screws for fixing a patient’s posture, the more secondary electrons appeared during gamma knife treatment. In addition, the appropriate methods and materials for each treatment should facilitate the reduction in radiation exposure to normal tissues [31,32], e.g., by moving the head screw fixation point and removing part of the thermoplastic mask.

Furthermore, carbon fiber fixing screws, such as polyaxial screws, are mainly used to fix bones in orthopedic and neurosurgical operations [33,34]. The number of electrons produced due to interactions with radiation must be evaluated using a fixing screw made of carbon fiber. In the future, electron generation in aluminum, titanium alloy, and carbon fiber screws must be compared to ascertain the material that can reduce normal tissue exposure. Furthermore, effective methods can be determined by comparing various types of radiation therapy.