## 4 Discussion
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 3 DCRT, 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, 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: \(\left(\frac{D_{metal}-D_{matter}}{D_{matter}}\right)\) at 15 cm depth. * For protons, D\({}_{\text{max}}\) range shift, defined as the difference in Bragg peak position compared to water D\({}_{\text{max}}\). 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 3 DCRT/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.