Impact of Metallic Implants on Dose Distribution in Radiotherapy with Electrons, Photons, Protons, and Very-High-Energy Beams

Abstract

Metallic implants in radiotherapy patients alter dose distributions due to their high density and unique composition, potentially compromising treatment precision. This study evaluates the effects of three metallic materials, Co-Cr-Mo alloy, titanium alloy, and stainless steel, on dose distribution across four radiotherapy modalities: 6 MV photons, 15 MeV electrons, 170 MeV protons, and very-high-energy electrons (100 and 150 MeV). Monte Carlo simulations in the TOol for PArticle Simulations Monte Carlo (TOPAS MC) generated percentage depth dose curves and dose profiles, with dosage data standardized to a reference point and uncertainties addressed via error propagation. Results revealed that the Co-Cr-Mo alloy produced the most significant alterations. For instance, at 100 MeV Very High Electron Energy (VHEE), the dose at a 15 cm depth was 34.57% lower than in water; 6 MV photons showed a 15.16% reduction, and the proton Bragg peak shifted 9.5 cm closer to the source. These pronounced changes along the central beam axis affected dose distributions anterior and posterior to the metal. A prostate cancer simulation further demonstrated considerable dose reduction with deeply embedded metallic implants. The findings underscore the critical impact of implant properties on radiotherapy dose distributions, emphasizing the need to integrate these factors into clinical protocols to improve dosimetric accuracy and treatment safety.

1. Introduction

Metallic implants are becoming increasingly popular in medical operations. Many patients receiving joint replacements or cardiac stents also receive radiotherapy (RT). This raises questions about possible patient risks and the interactions between these implants and the ionizing radiation used in radiotherapy. However, metallic implants have the potential to drastically change the intended radiation dose distribution, possibly resulting in over-irradiation of healthy tissues or under-treatment of the tumor [1]. This may lessen the effectiveness of treatment and raise the possibility of side effects. It is essential to comprehend these relationships. Various factors influence radiation beam dispersion, absorption, and attenuation, including the implant’s location, size, and composition. Localized dose changes that arise from this may impact treatment outcomes [2].

Researchers are examining how metallic implants affect radiotherapy to optimize treatment and reduce risk [3]. Effects are significant for recently developed radiation methods like FLASH radiotherapy with very high electron energy (VHEE), which may interact differently with metallic implants than more conventional methods. VHEE is a new modality with unique physical diametric properties that is promising for dose delivery. If we compare VHEE with other modalities, they show lower lateral scattering than photon beams [4]. They are more insensitive to inhomogeneities than other charged particle therapies [5]. VHEE may also have the advantage of ultra-high dose rate (UHDR) capability [6]. The combination of VHEE and UHDR contributes to faster and, therefore, more precise radiation treatment of the patient and limits the occurrence of inhomogeneities associated with patient movement or patient organ movement.

Metal prostheses are widely employed in orthopedics to substitute damaged joints and bones. There is a growing trend among patients to choose surgical procedures, such as metal prostheses, due to the increasing prevalence of arthritis, osteoporosis, and other degenerative joint disorders. In the case of metastatic bone disease in cancer patients, metal plates and screws are used for stabilization. Orthopedics practitioners commonly employ knee and hip joint replacements because of the high incidence of degenerative alterations that affect these weight-bearing articulations [7]. The occurrence of degenerative alterations frequently necessitates joint replacement procedures to restore functionality and reduce the accompanying pain. Patients’ prosthesis replacement procedures do not consider radiotherapy treatment; thus, problematic situations may arise in the future during the treatment of oncological diseases. In contemporary orthopedics, various materials for fabricating metallic prostheses are being used. Each of these materials has significant implications for their application in the human body. Stainless steel has become known for its excellent robustness and corrosion resistance [8]. This characteristic renders it a preferred option for a wide range of joint replacements. In recent times, titanium and its alloys have garnered significant attention as formidable candidates due to their remarkable biocompatibility and low mass [9]. The attributes above contribute to mitigating stress shielding and enhancing implant durability, particularly in hip and knee replacement procedures. The utilization of cobalt-chromium alloys in joint replacements is prevalent due to their exceptional mechanical strength and resistance to wear [10].

Metal implants in radiation oncology pose ongoing issues for doctors and medical physicists. They impact the precision and efficacy of treatment. Prostheses with high proton number Z mainly cause the complexity of image processing and dose calculations for the specified volume. These prostheses have a dosimetric impact on the treatment results, particularly for patients with pelvic area malignancies. Comprehending and resolving the difficulties that arise from this problem is crucial in guaranteeing secure and efficient radiation treatment. Recent research has highlighted the significance of reducing dosimetric inaccuracies, as demonstrated in the study by James D. Rijken [11], which emphasizes the importance of optimizing the density of the metal implant in treatment planning systems (TPSs). Researchers have observed a dosimetric inaccuracy of up to 10% in the planning tumor volume (PTV) areas for the rays passing through the implant [12]. A thermoluminescent detector (TLD), an anthropomorphic human pelvic phantom, was used to investigate spot doses in the PTV areas. The findings revealed notable fluctuations in administered doses, which were influenced by the presence of a metal prosthesis.

Why is VHEE interesting to us? Precisely because FLASH radiotherapy has the potential of highly advantageous oncological treatment. Developments in this field have been advancing significantly over the last decade. Experts in radiotherapy have focused on the possibility that damage to surrounding healthy tumor tissue is minimized when using an ultra-high dose rate (UHDR ≥ 40 Gy/s) and have called this the FLASH effect [13]. Hart et al. [14] irradiated Drosophila melanogaster larvae with X-ray UHDR compared to conventional radiotherapy (CONV RT), thus setting a possible limit to observe the FLASH effect. Based on their results, it is possible to increase the therapeutic window by using UHDR RT for some intermediate doses, focusing on protecting healthy tissue. Bazalova-Carter et al. [15] performed treatment planning for RT with VHEE and compared this with VMAT plans. The research group showed in three cases that treatment with VHEE may have a dosimetric advantage over photon VMAT plans, so it should be investigated further as a treatment method. This was followed up by Fisher et al. [16], who point out the possibility of using Spatially Fractionated Radiotherapy (SFRT) in combination with VHEE. One of the main factors in FLASH RT with VHEE is dosimetry, which, according to Vanreusel et al. [17], can be performed using point scintillation detectors. Hart et al. [18] performed dosimetric measurements using a plastic scintillator. The research groups emphasize the importance of developing suitable dosimeters, exhibiting dose rate independence and sufficient radiation hardness. Studies have shown the possibility of using VHEE for FLASH RT and deeply oriented tumors. This is why it is also necessary to focus on how VHEE reacts with metal implants in the human body.

This work aims to investigate the effect of metal implants on dose distribution in radiotherapy by using Monte Carlo simulations that demonstrate the ideal case of a metal box interacting with ionizing radiation. We used different energies in radiotherapy as ionizing radiation: photons with energy 6 MV, electrons with energy 15 MV, protons with energy 170 MeV, and experimental VHEE with 100 MeV and 150 MeV. Finally, we simulated the case of a patient with two stainless steel femoral metal replacements. The simulation was a single direct field using VHEE, showing the appearance of attenuation behind the metal replacement in the computed tomography (CT) image.

2. Materials and Methods

2.1. Experimental Setup

The simulations included in the study were conducted using the TOol for PArticle Simulations Monte Carlo (TOPAS MC). We prepared an experimental setup utilizing a water phantom. We constructed a water phantom of 50 × 50 × 50 cm³ in the simulations. At a depth of d_box = 5 cm, the water phantom houses a metal box of 5 × 5 × 2 cm³, which is the most frequently utilized material in implant orthopedics. The source-to-surface distance (SSD) is 100 cm, and the scoring grid dimensions are 20 × 20 × 50 cm³, with bins defined as 40 × 40 × 500 cm³. We specified a radiation field size of 10 × 10 cm², as it is the most often utilized field size for irradiating prostate tumors in patients with a metallic femoral joint replacement. The dose is administered at the isocenter at a depth of 6 cm within a phantom using various energies. The area measures 10 × 10 cm² using SSD technology. The isocenter is located at the center of the metal box (5 × 5 × 2 cm³). The materials utilized for the metal box are those most employed in orthopedic replacements, specifically cobalt-chromium-molybdenum (Co-Cr-Mo) alloy, titanium alloy, and stainless steel [19]. The properties of the various metallic materials utilized in our simulations are presented in the table depicted in Figure 1, showing the experimental water phantom setup.

In radiotherapy, the SSD parameter is essential to achieving optimal dose distribution and to minimizing the dose burden on healthy tissues. The most used SSD for electrons and photons is 100 cm, which balances beam penetration and homogeneous coverage of the target area. For electrons, an SSD of 100 cm provides a uniform distribution over the surface due to the limited depth penetration of the electrons. For VHEE, where the energy is greater than 50 MeV, the SSD is often increased to 150 cm, allowing irradiation of deeper tumors with better homogeneity. A more comprehensive range of SSDs is used for proton-specific Bragg peaks, depending on the technology (fixed scattered beam, pencil beam scanning). Commonly, SSDs of 150 to 250 cm are used for protons. Our simulations aimed to compare the effect of the metal box on the dose distribution in the water phantom for different energies. We chose the same SSD, namely 100 cm for all energy. This should provide consistent conditions for a clear comparison of the influence of the metal box on the dose distribution.

2.2. Monte Carlo Simulations

For the simulations, we used the TOPAS MC Version 3.9 software environment [20,21]. The following physics modules were used: “g4em-Livermore”, “g4h-phy\_QGSP\_BIC\_HP”, “g4decay”, “g4ion-binary cascade”, “g4h-elastic\_HP”, and “g4stopping”. The cut-off resistance, whose value we set to 5 × 10³ cm, was used for detailed simulations. It is beneficial in capturing small details in the batch distribution. We left the other parameters at the default setting.

The MC model consisted of a 50 × 50 × 50 cm³ water phantom, a 20 × 20 × 50 cm³ scoring grid along the beam central axis, and a 5 × 5 × 2 cm³ metal box. SSD was defined to be 100 cm from the water source in the water phantom. The scored volume consisted of 0.5 × 0.5 × 0.1 cm³ voxels. Energies were set up based on the typically used photons (6 MV), electrons (15 MeV), and protons (170 MeV) in radiation therapy. Figure 2 shows an experimental model set in TOPAS MC.

Based on the simulations, we evaluated the changes in percentual depth dose (PDD) and dose profile (DP) versus water for three metallic materials. In evaluating PDD, we focused on the dose drop at 1, 2, 3, 5, 10, and 15 cm depths. We observed the changes upstream and downstream of the metal box due to backscattering and bremsstrahlung. We then evaluated inline and crossline dose profiles, where we equally focused on the decrease in absolute dose behind the metal box. We viewed 2D dose profiles, showing how ionizing radiation reacts with the metallic elements in its radiation center.

2.3. Prostate Cancer Basic Treatment Simulation

We prepared a single-fraction treatment plan with one direct field via a metal substitute for a patient with prostate cancer. Although this treatment approach is not routinely used for patients with metal substitutes, we sought to highlight how the metal substitute affects the distributed dose in the patient’s body CT. We focused on a patient with a metal replacement in the pelvic region precisely because the femoral head is one of the thickest possible metal parts implanted in the human body. We used CT images in Digital Imagining and Communications in Medicine (DICOM) and TOPAS MC for calculation. We used only VHEE to develop the treatment plan to focus on the issue of metal replacement for a new treatment modality. We used VHEE energies of 100 MeV in our planning. The CT dataset used for simulation was anonymized and used solely for research purposes, with no human subjects directly involved, and thus did not require ethical approval.

3. Results

The experimental results section provides a thorough comparative examination of dose distributions across different energies and materials, emphasizing the quantification of the influence of metallic implants on dose attenuation, which is essential for the accuracy and effectiveness of radiation. Considering the essential function of radiotherapy in cancer management, these findings highlight the imperative for careful treatment planning when high-density implants are present, as they might substantially modify dose distributions and influence therapeutic results.

3.1. 2D Dose Distribution

Figure 3 presents a comparative analysis of the 2D dose distribution for four different energies. The color scale shows the percentage dose distribution, which converts the dose to the maximum value. All simulations show the inhomogeneity due to the metal box, highlighting the need to consider metallic materials when planning RT treatments. The dose distribution for 15 MeV shows a pronounced dose deposition near the surface, with rapid decay. A pronounced shadowing effect is observed due to a metal box positioned at a depth of 1 cm. Protons with an energy of 170 MeV show a characteristic Bragg peak, which can be observed at a depth of 10 cm, i.e., it is significantly offset in range compared to the Bragg peak without the metal box. The VHEE dose distribution in the region of the metal box produces a clear attenuation zone with a sharp dose drop behind the box. The VHEEs show the least lateral scatter. In the case of photon beams, the metal box also causes a pronounced dose attenuation. The effect of the dose increases at a depth of 5 cm, and the beginning of the metal box is visible. The dose increase near the metal box is due to backscattering and bremsstrahlung, which are characteristic of particle interactions with high-density material.

3.2. Percentual Depth Dose and Dose Profiles

The subsequent figures present the examination of PDD and Dose Profiles for four distinct materials: Water, Co-Cr-Mo Alloy, Ti Alloy, and Stainless Steel. Our objective is to evaluate the impact of metallic materials on dose distribution, particularly in the regions adjacent to and behind the box, where attenuation and dispersion occur. In the PDD, we concentrated on depth values of 1, 2, 3, 5, 10, and 15 cm, while for the dose profiles, we recorded the dose values at the origin of the x-axis. Our reference was the dose distribution values for water absent from the metal box.

In Figure 4a, the metal box is situated at a depth of 5 cm, with an energy of 6 MV for photons. The dose distributions with the metal box exhibit significant dose attenuation attributable to the metal box itself. The Co-Cr-Mo alloy exhibits the most significant dose reduction, with a decrease of 15.16% at d₁₅. A notable reduction in dose is evident in the center plane of plot b for all metallic materials: Co-Cr-Mo alloy at 67.87%, stainless steel at 77.47%, and titanium at 88.43%. Dose values are normalized to the maximum dose in water (100%). Values exceeding 100% represent localized build-up effects and should be interpreted as relative, not absolute, increases.

Simulations of the dose distributions for 15 MeV (Figure 5) for electrons show significant differences in the dose distributions, depending on the presence of the metal box and the materials used. In water, the dose rate reaches 88.63% at a depth of 1 cm, with a gradual decrease in dose consistent with the expectation for the electron beam. The metallic materials cause a significant attenuation, with the Co-Cr-Mo alloy showing the largest decrease in dose, with values of 82.88% at 1 cm depth and 38.75% at 5 cm depth. Titanium and stainless steel show similar behavior, with a more moderate attenuation compared to Co-Cr-Mo, with dose rates of 82.7% and 80.92% at 1 cm depth and values of around 39.00% at 5 cm depth. The crossline profile in water reaches 99.30%, while it drops to 4.00% for Co-Cr-Mo, demonstrating a significant lateral attenuation. Titanium has a crossline dose of 18.59%, and steel reaches 74.40%. The inline profile in water shows a maximum dose of 100% but drops to 4.04% for the Co-Cr-Mo alloy, with titanium and steel reaching 18.67% and 74.86%, respectively. The tabular data supports these observations, where Co-Cr-Mo causes the most significant attenuation at depths of 1 cm to 5 cm, while titanium and steel show moderate attenuation.

Simulations of the dose distributions for 100 MeV VHEE (Figure 6) reveal distinct differences in dose behavior contingent upon the material and the presence of the metal box. At a depth of 1 cm in water, the dose attains 88.00%, which is analogous to the measurements for titanium alloy (88.16%), stainless steel (88.10%), and Co-Cr-Mo alloy (88.00%). The disparities in batch behavior are increasingly evident at increased depths. At a depth of 5 cm, the dose in water attains 95.45%, whereas the Co-Cr-Mo alloy exhibits 85.76%, indicating the most significant attenuation. Titanium and steel exhibit moderate attenuation, with doses of 86.00%. At a depth of 15 cm, the dose in water is 97.79%, whereas for the Co-Cr-Mo alloy, it decreases to 63.22%, demonstrating its substantial impact on dose attenuation. The crossline profiles indicate a substantial reduction in dose for metallic materials, with water achieving 100% and Co-Cr-Mo diminishing to 43.32%.

Titanium attains 77.01%, whereas stainless steel achieves 52.18%. The inline profiles exhibit analogous behavior, with Co-Cr-Mo decreasing to 43.71% compared to 98.61% in water. Titanium and steel exhibit a mild attenuation, with inline doses of 77.36% and 53.09%, respectively. The tabulated data indicate that the Co-Cr-Mo alloy significantly influences dose attenuation at 100 MeV VHEE. In contrast, titanium and steel demonstrate a very low attenuation relative to water.

Figure 7 presents simulations of the dose distributions for 150 MeV VHEE in addition to their analysis. At a depth of 1 cm in water, the dose attains 82.89%, which is analogous to the values for Co-Cr-Mo alloy (82.95%), titanium (82.96%), and steel (82.80%). At increasing depths, the disparities become more evident; at a depth of 5 cm, water attains 90.95%, whilst Co-Cr-Mo declines to 81.54%. Titanium shows higher values (81.66%), but steel attains 82.1%, signifying a marginal reduction relative to Co-Cr-Mo alloy. At a depth of 15 cm, the dose in water is 99.85%, whereas Co-Cr-Mo diminishes to 80.59%, further demonstrating its considerable attenuation. Crossline profiles exhibit the same behavior, with the dose attaining 100% in water but decreasing to 61.12% for Co-Cr-Mo. Titanium exhibits a high value of 93.76%, whereas steel attains 75.51%. The inline profiles exhibit a consistent pattern, indicating that water maintains a value of 100%, while Co-Cr-Mo substantially decreases to 60.67%, titanium registers at 93.5%, and steel at 74.16%. The results indicate that the Co-Cr-Mo alloy significantly influences dose attenuation at 150 MeV VHEE. However, titanium and steel exhibit a mild attenuation relative to water, as evidenced by the lower discrepancies among values at all depths and profiles.

In the case of 170 MeV protons (Figure 8), the range of the Bragg peak in water is a benchmark for comparing the effect of different metallic materials on the shift and dose reduction. In the simulations, the Bragg peak for water was set to a depth of 19.4 cm, with metallic materials inducing significant shifts of this peak towards the surface, characteristic of protons’ interaction with dense materials.

The Co-Cr-Mo alloy caused the most significant shift of the Bragg peak, namely to 9.9 cm, which implies a shortening of the rank by almost 9.5 cm relative to water. This shift is directly related to the high density of Co-Cr-Mo, which causes intense proton energy loss in the surface layers. This phenomenon is significant in treating patients with Co-Cr-Mo implants, as there is a sharp reduction in the depth near the implant where the maximum dose is achieved.

For the titanium alloy, the Bragg peak shifted to 15 cm, representing a more moderate shift of approximately 4.4 cm relative to water. As a lower-density material compared to Co-Cr-Mo, titanium causes less of a shift, meaning its effect on proton therapy is less dramatic but still significant. Stainless steel caused a shift of the Bragg peak to 12.4 cm, which is intermediate between Co-Cr-Mo and titanium, with a shift of 7 cm from the reference value for water. The steel also exhibits a high interaction with protons, leading to a significant shortening of the rank.

3.3. Prostate Cancer Treatment

In the case of direct field simulation in the CT of a patient with prostate cancer, we can observe how the metal implant affects the decrease in dose distribution. We used the direct VHEE field (100 MeV) for irradiation. The purpose is to demonstrate attenuation characteristics of VHEEs. We compare the scenario with a similar CT scan where the metal prostheses are replaced with bone. The comparative analysis reveals a noticeable attenuation of the dose distribution curve when VHEEs interact with the metal prostheses. This observation underscores the significant impact of material composition on the attenuation properties of VHEEs (Figure 9). Conducting fundamental direct-field simulations is crucial, as they elucidate phenomena such as bremsstrahlung production and the formation of localized hotspots or cool spots in dose distribution during radiation interaction with high-density metallic implants. These effects can result in considerable dosage heterogeneities, potentially jeopardizing therapeutic safety and efficacy. Ongoing research in this domain is essential, particularly as VHEE technology advances towards clinical use. Precise simulation protocols and dosage calculation methods must be developed to assist medical physicists in treatment planning. These guidelines will be essential for guaranteeing the secure and efficient incorporation of VHEEs into standard radiation for patients with metallic implants.

4. Discussion

We focused on a comprehensive investigation of the impact of metal prostheses (metal boxes), primarily on dose profiles and PDDs. We used TOPAS MC simulations. We dedicated ourselves to the study by focusing on professional practice in Slovakia’s hospital setting. The AAPM RT TG 63 [3] report discusses the problem of a high atomic number of hip prostheses in patients receiving radiotherapy treatment in the pelvic region. It is noted that prostheses can lead to differences in the dose, consequently impacting the treatment’s results. The research also highlights the need for more scientific comprehension and methodology in clinically measuring radiation doses for these individuals. Nevertheless, numerous other studies have been released that specifically examine this matter.

Considering theoretical assumptions and investigations in metal implants, it is imperative to prioritize locations where metal replacements are present. Designing the radiation therapy treatment for patients with metal implants is a complex task, as the presence of the implant affects how the radiation dose is distributed. The issue’s historical origins can be traced back to 1984, when Hudson et al. [22] documented notable alterations in the dose distribution with exposure to metal restorations. Specific implants induced the alterations using 8 MV gamma beams (Cobalt) in the treatment. Biggs and Russell [23] found that 25 MV X-ray beams had an average dose decrease of 2%, whereas Cobalt had a 5% reduction. The magnitude of the disturbance is contingent upon the specific model and composition of the implant, as well as the energy level of the radiation beam being used. Hence, it is imperative to possess knowledge of a particular implant, precisely the technique employed for dose calculation.

Accuracy is crucial in radiation therapy when calculating doses using algorithms. The uniformity of the surroundings significantly influences the estimation of the dose. When there is a disruption, the significance of algorithms in the computation becomes even more crucial, particularly when it comes to metal implants. The accuracy of the algorithms used to calculate the dose in the field of hip prostheses is directly related to the complexity of the physical modeling of the processes. One notable limitation of the superposition method occurs at interfaces where there are distinct atomic numbers. This limitation is supported by our experimental findings, which align with the results reported in the study conducted by Keall et al. [24]. One possible option to address these restrictions is to introduce nuclei produced in other media [23] or to apply the modifications proposed by [25,26]. Using a Monte Carlo approach, Bazalova et al. [27] examined the impact of artefacts produced by metallic materials from three commonly used prosthetic materials (Ti alloys, stainless steel, and Co-Cr-Mo) on dose estimation. During the calculation, they utilized the sinogram interpolation approach to rectify metal artefacts and subsequently assessed their influence. They acquired data demonstrating 11% differences in the prostate region when using bilateral hip endoprosthesis made from Co-Cr-Mo material. For patients with one-sided endoprosthesis, it is advisable to refrain from threading the bundle across the specified region. For bilateral endoprosthesis cases, employing the sinogram interpolation correction algorithm is advisable to compensate for artefacts in the beam going through the affected area. If any bundles traverse the region impacted by the artefact, the correction method should be applied, even for individuals with unilateral endoprosthesis.

Four fundamental strategies are employed in the realm of radiation therapy for patient exposure using photon beams: 3D Conformal Radiation Therapy (3D CRT), forward Intensity Modulated Radiation Therapy (fIMRT), Intensity Modulated Radiation Therapy (IMRT), and Volumetric Modulated Arc Therapy (VMAT). Techniques are selected according to the extent of the exposed area. The writers in the field of research address the problem of mitigating the adverse impacts of metal implants on dose distribution by utilizing an appropriately selected calculating technique. VMAT technology offers exceptional precision and flexibility in preventing irradiation of metal implants, hence assuring a more uniform distribution of doses. Most studies concentrate on the pelvic region, specifically the site of hip replacements. To et al. [28] examined the planning approach of VMAT at LU for prostate patients with bilateral hip arthroplasties. They focused on ensuring the plan’s quality and feasibility while minimizing the radiation exposure to the prosthesis. Significant disparities were observed in fulfilling these goals across the three retrospectively constructed VMAT plans for 20 patients. The PA designs comprised six half-arcs that circumvented the need to enter each prosthesis.

Nevertheless, it was demonstrated that these plans were suboptimal compared to plans that employed two complete arcs with optimized maximum dose (MD) and two complete arcs with maximized DVH constraint (MDVH). The MD and MDVH plan achieved comparable dosimetric quality assurance and demonstrated considerable enhancements in rectal and bladder DVH measures compared to the PA plan. The findings of this study unequivocally demonstrate that MD and MDVH plans surpass PA plans in terms of dosimetric QA and feasibility. Simultaneously, these two strategies efficiently minimized the input doses to the prosthesis, reaching a mere 1% of the authorized dose. The study emphasized the significance of utilizing the VMAT technique in bilateral hip joint endoprosthesis, which allowed for the testing of dosimetric values using a human pelvic phantom. There is much focus on selecting the appropriate exposure approach in ongoing research. The VMAT approach has great potential in minimizing irradiation of the metal implant region and achieving improved dose distribution. A study by To et al. [28] showcased the efficacy of this strategy and yielded significantly enhanced outcomes compared to conventional methods. The findings of the studies highlight the significance of selecting an appropriate radiation therapy planning approach and the necessity for ongoing research to enhance the treatment of patients with metal implants.

The results of our study obtained for different energy modalities (electrons, VHEE and protons) show the consistent and significant effects of metallic materials on the dose distribution and the extent of energy deposition in RT scenarios. Metallic implants, such as Co-Cr-Mo alloys, titanium, and stainless steel, greatly influence the physical processes of particle–tissue interaction, manifested in dose reduction and a shift in the depth of maximum deposition (Bragg peak for protons).

Simulations for 15 MeV electrons show that metallic implants, especially Co-Cr-Mo alloy, cause a dramatic dose attenuation behind the implant. At a depth of 1 cm, the dose for Co-Cr-Mo alloy was more than 6% lower than for water. This difference increased with increasing depth, with a significant dose decrease behind the implant, resulting in a shadowing zone with low energy deposition. Lower-density alloys, such as titanium and steel, showed less but still significant attenuation. This phenomenon is a direct result of the high density of the metals causing intense scattering and absorption of electrons, leading to a dose reduction in the target region behind the implant. Electrons have a significantly limited range compared to VHEE or protons, meaning their interaction with metallic materials can be limited to the surface irradiation areas. Clinical implications include potential under-dosing in target areas, particularly behind metal implants, critical for near-surface irradiation. Thus, for patients with metal implants, it is essential to consider modification of the radiation schedule or a change in modality. While adapting the radiation modality or modifying the treatment schedule may be clinically desirable in patients with metallic implants, the feasibility of such changes can be limited by institutional resources and infrastructure. Not all treatment centers have access to advanced modalities such as VHEE or proton therapy, and even within a center, re-planning may depend on the availability of trained personnel and appropriate imaging and verification systems. Therefore, any proposed change must be evaluated within the context of the specific clinical environment, and decisions should be based on a multidisciplinary assessment involving medical physicists, radiation oncologists, and treatment planning staff.

To improve the consistency and safety of patient care across different institutions, it is essential to establish and implement clear, accessible, and standardized clinical guidelines for managing radiotherapy in patients with metallic implants. These guidelines should be readily available and applicable in daily clinical practice, supporting harmonized treatment planning workflows across centers worldwide. Simulations for VHEE (100 MeV and 150 MeV) showed a more modest dose attenuation than electrons but still significant differences between materials. For 100 MeV VHEE, the Co-Cr-Mo alloy proved to be the most influential on the dose profile, with a decrease in maximum dose of up to 30% compared to water, with titanium and stainless steel causing a more minor but still significant decrease. At 150 MeV VHEE, this effect was even more pronounced, with Co-Cr-Mo causing a shift in the depth of the maximum dose and a decrease in its value, indicating the high ability of metallic materials to affect energy deposition even at high energies.

VHEEs are an exciting alternative for deep irradiation because they have significant penetration compared to lower electron energies. However, interaction with metal implants remains a significant challenge. Although lower-density alloys such as titanium show less dose reduction, there is still a shift of maximum energy deposition towards the surface, which could lead to underdosing in deeper target tissues. This effect could be addressed by adjusting the beam energy or optimizing the irradiation angle.

Protons pose a specific challenge in the presence of metal implants due to the shift of the Bragg peak. The results for 170 MeV protons showed a dramatic shift of the Bragg peak due to the presence of metal, particularly Co-Cr-Mo alloy. In water, the Bragg peak was at a depth of 19.4 cm, while for Co-Cr-Mo, this peak shifted to 9.9 cm, indicating a nearly 50% reduction in rank. Titanium and steel also affected the shift of the Bragg peak, but to a lesser extent, with values of 15 cm and 12.4 cm, respectively. In addition to the shift of the Bragg peak, a significant reduction of the maximum dose in metals was also observed. The maximum dose reached 100% in water, while Co-Cr-Mo reduced this value to 70.87%, a more than 29.00% reduction. Titanium and steel also reduced the maximum dose, with titanium reaching 73.07% and steel reaching 71.36%. These results are significant for proton treatment, where precise planning depends on the exact location of the Bragg peak. Metal implants can significantly shorten the proton range and reduce treatment efficacy if these factors are not considered.

The results obtained for all modalities demonstrate that metal implants significantly impact distribution and depth of maximum energy deposition. High-density metallic materials, such as Co-Cr-Mo, have the greatest impact, leading to a significant shift of the Bragg peak for protons and a sharp drop in dose behind the implant for electrons and VHEE. On the other hand, lower-density materials, such as titanium, have a more moderate effect but still cause measurable changes in the dose profiles.

Clinically, these results are significant in radiation planning for patients with metal implants. It is essential to accurately model the effect of metal on dose distribution to avoid underdosing tumor tissue or over-irradiating surrounding healthy tissue. For proton therapy, the Bragg peak shift should be considered, and the treatment plan should be adjusted based on the type of implant. In the case of electron therapy, consideration could be given to using a different modality in areas with significant dose attenuation behind the metal. The ideal case is to avoid irradiation through the metal implant.

The advantage of the new planning systems is that they can account for heterogeneities in the patient’s body. They often also consider the characteristics of the implants, which contributes to accurate and efficient dose delivery.

The findings directly relate to radiation treatment planning in oncology patients with metallic implants. High-density metals such as Co-Cr-Mo cause significant dose perturbations—both dose build-up in the proximal region and attenuation beyond the implant—potentially resulting in underdose of the target volume and overdose (hot spots) or underdose (cold spots) in surrounding tissues due to dose scattering and backscatter effects.

Additionally, in the high-gradient dose regions around implants, the interaction of radiation with metallic structures may lead not only to complex spatial redistribution of the dose but also to structural changes in the implant material itself, especially under repeated exposure or with certain beam types (e.g., high-energy electrons or protons). While this effect may be less pronounced with clinical photon beams, it remains an important consideration for future studies and quality assurance protocols.

To mitigate these effects, we strongly recommend using avoidance sectors during VMAT plan optimization, preventing beam paths from crossing the implant. Similarly, for 3DCRT, IMRT, or FIMRT techniques, it is crucial to carefully select beam angles to avoid traversing metallic implants, regardless of their composition.

These findings are summarized in

Table 1. Dose attenuation and proton range shifts caused by metallic implants across radiation modalities, including clinical recommendations.

Beam	Material	Dose Attenuation at 15 cm Depth	Bragg Peak Shift	Clinical Recommendation
VHEE	Titanium alloy	−28%	-	Mild impact: acceptable if there is no alternative. Prefer avoidance if multiple arcs are available.
	Stainless steel	−35%	-	Use avoidance sectors; avoid direct beam path through implant.
	Co-Cr-Mo alloy	−39%	-	Strong attenuation; always use avoidance sectors; re-optimize plan geometry.
Photons	Titanium alloy	−3%	-	Minimal impact; beam path acceptable in specific clinical contexts.
	Stainless steel	−6%	-	Moderate attenuation; avoid beam path if possible.
	Co-Cr-Mo alloy	−7%	-	Avoid beam path; use avoidance sectors in VMAT optimization.
Electrons	Titanium alloy	Dose at 5 cm: −39%	-	Strong attenuation; avoid central beam axis through implant.
	Stainless steel	−39%	-	Similar to titanium; avoid implant regions when possible.
	Co-Cr-Mo alloy	−39%	-	Similar to titanium; do not place fields through implant.
Protons	Titanium alloy	-	−4 cm	Recalculate range; add safety margins; avoid direct targeting behind implant.
	Stainless steel	-	−7 cm	Strong shift; adjust beam range; avoid implant shadow region.
	Co-Cr-Mo alloy	-	−9 cm	Severe shift; avoid implant in beam path; significant replanning required.

, which serves as a practical reference for clinicians involved in treatment planning. Integrating such strategies alongside advanced calculation algorithms (e.g., Monte Carlo) and photon counting CT can improve dose accuracy and patient safety. This table summarizes the dose attenuation at a depth of 15 cm for VHEE and photon beams and the Bragg peak shift for proton beams across different metallic implant materials. Dose values were extracted using the following methods:

• For VHEE, electrons and photons: $\left({\displaystyle \frac{D_{metal}-D_{water}}{D_{water}}}\right)$ at 15 cm depth.
• For protons, D_max range shift, defined as the difference in Bragg peak position compared to water D_max.

These metrics quantify the relative impact of each material on the dose distribution. The table also includes clinical recommendations for treatment planning, such as using avoidance sectors in VMAT or beam angle restrictions in 3DCRT/IMRT techniques.

This study was performed using simplified geometries and homogeneous water phantoms, which do not fully represent the complexity of human anatomy. Although Monte Carlo simulations inherently account for both primary and scattered radiation, including contributions from surrounding materials and structures, the use of simplified phantoms may limit the realism of anatomical scatter conditions. Future work will address these limitations by validating the results in anthropomorphic phantoms and incorporating anatomical heterogeneities such as bone and soft tissue into the simulations. Additionally, further research will explore potential structural changes in metallic implants due to irradiation, including microcrack formation, material density changes, and early signs of corrosion. A 3D pelvic phantom representing the male anatomy will also be developed, allowing the replacement of metallic implants with bone to assess their impact on treatment planning. This phantom will be used to compare VMAT (with a single arc) and conventional 3D-CRT box techniques in detail. The study will include experimental verification through physical measurements conducted at the Department of Clinical and Radiation Oncology of the Faculty Hospital Žilina, providing real-world insight into dose distribution effects. The ultimate aim is to offer precise and clinically applicable recommendations for medical physicists involved in radiotherapy planning for patients with metallic implants.

5. Conclusions

Monte Carlo simulations demonstrated how metals influence distribution as they traverse the metal implant. Our findings indicate that radiation therapy beams should avoid metal implants whenever possible due to the production of low- and high-dose regions before and after the implant, respectively. Owing to the potential for hot spots and cold spots in dose distribution. When treating these patients, it is essential to consider the position of individual metal components throughout their bodies to prevent them from reaching the irradiation area. This entails adhering to the principles of IGRT while ensuring adequate avoidance zones around metallic implants. It is essential to prevent superfluous bremsstrahlung and backscatter effects in the irradiated region. In the context of VHEE, evaluating the consequences induced by metallic implants within the human body is essential.

Our results offer a critical understanding of the significant influence of metallic implants on dose distributions, particularly for high-density materials like Co-Cr-Mo. High-density materials like Co-Cr-Mo exhibit significant dose attenuation. For example, at 15 cm depth, dose attenuation for 6 MV photons was −15.16% with Co-Cr-Mo, −8.11% with titanium, and −13.23% with steel. For 150 MeV VHEE, the impact was even more pronounced, with dose attenuation of −19.26% for Co-Cr-Mo, −3.46% for titanium, and −11.8% for steel. This trend continues with 100 MeV VHEE, demonstrating significant sensitivity to high-density materials. Furthermore, for electron therapy at 15 MeV, dose attenuation at 3 cm depth was −55.52% for Co-Cr-Mo, −54.79% for titanium, and −52.87% for steel, highlighting substantial attenuation even at lower energies and shallower depths. The Bragg peak shift in proton therapy, with a displacement of −9.5 cm for Co-Cr-Mo, −4.4 cm for titanium, and −7 cm for steel at 170 MeV, illustrates the challenges of dose accuracy around metallic implants for proton therapy.

The findings of this work demonstrate that all treatment beams, including novel VHEE beams, must carefully consider patient implants to produce optimal dose distributions. The pronounced impact of titanium, stainless steel, and Co-Cr-Mo alloy on dose attenuation indicates that meticulous material selection and beam angle modification are essential when treating patients with metallic implants. Our research establishes a basis for enhancing radiotherapy planning systems, promoting comprehensive simulations between metal and ionizing radiation. The research substantiates the imperative to customize radiation treatments according to each patient’s anatomical and material characteristics, providing a pathway to safer and more successful treatment for individuals with metal implants.