###### Abstract  
This study aims to evaluate the dependence of DNA damage on the proportion of iron and gold in iron-gold nanoparticle heterojunctions using Monte Carlo simulations. The simulation setup included a spherical nanoparticle with varying percentages of iron and gold, irradiated by photon beams of different energies (50-150 keV). The Geant4-DNA Monte Carlo code was utilized for the accurate tracking of radiation transport. The results reveal that DNA damage increases with a higher percentage of gold volume in the heterojunction, primarily due to photoelectric enhancement. Furthermore, a lower photon beam energy of 50 keV induces greater DNA damage compared to energies of 100 keV and 150 keV. The findings suggest that for effective cancer cell eradication through DNA damage, the gold volume should be equal to or greater than 50% in the iron-gold nanoparticle heterojunction. In conclusion, the findings from this study will shed light on the potential of iron-gold nanoparticle heterojunctions in enhancing radiotherapy outcomes. The investigation of DNA damage resulting from the combination of contrast agents and radiosensitizers is crucial for advancing cancer research and treatment. The knowledge gained from this research will aid in the development of personalized and effective radiotherapy approaches, ultimately improving patient outcomes in cancer treatment. D 2023 13 8942  
advancements in healthcare and treatment, it is projected that the number of survivors will increase by 5.3 million by the year 2030 [2]. Despite this positive trend, there is an expected rise in the number of cancer diagnoses and deaths as well [3]. Radiotherapy remains a crucial component of treatment planning in hospitals, playing a role in the treatment of approximately 50 percent of cancer patients. Furthermore, radiotherapy has contributed to the cure of around 40 percent of these patients [4]. Given the significance of radiotherapy in treatment planning, it is imperative to enhance techniques and equipment. At its core, radiotherapy utilizes radiation to eradicate cancer cells. Ionizing radiation generates ions or charged particles that release energy as they pass through tissue cells. The amount of energy deposited per unit mass is referred to as the dose delivered to the cells. This dose prompts cellular apoptosis or genetic mutations that can result in cell death [5]. The administration of the dose is carried out using a medical linear accelerator (LINAC). These devices are employed to externally deliver photon and electron beams, a method known as external beam radiotherapy. LINACs are highly efficient as they feature a rotating head capable of delivering radiation from various angles depending on the patient's case and the organs at risk. This approach minimizes unnecessary exposure to specific areas, granting medical physicists the flexibility to precisely target the tumor based on the treatment plan. In conventional radiotherapy, photons are utilized as the primary form of ionizing radiation. Photons, including X-rays and gamma rays, have the ability to interact with electrons, whether they are bound to an atom or free electrons, as well as with the electric field of the nucleus. When photons interact with tightly bound electrons, it leads to the photoelectric effect or Rayleigh scattering. The photoelectric effect results in the release of a photoelectron, while Rayleigh scattering does not transfer energy and therefore does not contribute to the dose [6]. On the other hand, if a photon interacts with a free electron, it undergoes Compton scattering, resulting in the production of another photon and a recoil electron. Compton scattering involving X-rays is predominantly associated with free electrons due to their low binding energy compared to the incoming photon energy. However, in practice, Compton scattering can also occur for bound electrons, as long as the incoming photon possesses very high energies.