###### Abstract
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 [PERSON] [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 \(\geq\) 40 Gy/s) and have called this the FLASH effect [13]. [PERSON] 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. [PERSON] 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 [PERSON] 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 [PERSON] et al. [17], can be performed using point scintillation detectors. [PERSON] 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.