## 4 Discussion  
### Dependence of DNA Damage on the Volume of Gold or Iron  
Table 2 presents the number of SSBs and DSBs for different percentages of gold to iron at various photon beam energies. Across all photon beam energies, it is evident that the number of DNA strand breaks increases with the percentage or volume of gold. For a photon beam energy of 50 keV (Table 2a), decreasing the percentage of gold from 100% to 75% results in a 5.1-fold reduction in DNA damage (SSBs and DSBs). Further decreasing the percentage of gold from 75% to 20% and from 50% to 25% leads to reductions of 5.9 times and 3.4 times, respectively. This phenomenon can be attributed to the reduction in dose enhancement resulting from the introduction of iron in the iron-gold nanoparticles. It was observed that the dose enhancement of iron was significantly lower than that of gold when subjected to kilovoltage photon beams [17]. When the volume of gold is reduced to zero and the nanoparticle heterojunction becomes purely iron, the number of DNA strand breaks is only 0.1% of that observed in the gold nanoparticle (100% gold and 0% iron). Similar trends are observed in the 100 keV (Table 2b) and 150 keV (Table 2c) photon beam energies, with the number of SSBs or DSBs decreasing with increasing beam energy. The increase in DNA damage with a larger volume of gold can be attributed to the generation of secondary electrons at the heterojunction when irradiated by the photon beam. It is well-known that SSBs and DSBs are caused by the ionization of the DNA molecule strands due to charged particles such as secondary electrons [29]. These electrons can be generated from the heterojunction through the interaction between the photon beam and the nanoparticle. In the kV photon energy range, the photoelectric effect is dominant. The cross section or probability of photoelectric interaction is proportional to the atomic number (Z) raised to the power exponent n, where n typically ranges from 4 to 5. For a fixed wavelength, materials with higher atomic numbers, such as gold (Z = 79), have larger cross sections than iron (Z = 26) due to this relationship. It is important to note that the cross-section values can vary with different wavelengths, and the stated proportionality is valid only for a specific wavelength. This implies that a heterojunction containing a larger volume of gold can generate more secondary electrons compared to the iron volume. These findings align with the dose enhancement factors of gold and iron nanoparticles in nanoparticle-enhanced radiotherapy. The dose enhancement factor is defined as the ratio of the dose at the DNA with the addition of nanoparticles to the dose at the DNA without the addition of nanoparticles [30]. It is observed that a larger volume of gold in the heterojunction can enhance the dose at the DNA, resulting in increased DNA damage. Consequently, this enhances cancer cell kill and tumor control in patients. In MRI-guided radiotherapy, it is essential to consider both the unique properties of gold and iron nanoparticles. While gold nanoparticles are known to be excellent radiation dose enhancers, the presence of iron nanoparticles is crucial due to their specific magnetic properties that contribute to enhancing contrast in MRI images. Iron nanoparticles possess strong magnetic characteristics and can become magnetized when exposed to a magnetic field. This magnetization leads to local disturbances in the surrounding magnetic field, causing changes in the relaxation times of nearby water molecules. These changes can be detected by MRI scanners, enabling enhanced contrast in the resulting images [7]. Therefore, incorporating iron into the nanoparticle structure in the iron-gold heterojunction is necessary to achieve the desired contrast enhancement in MRI. On the other hand, gold nanoparticles offer various advantages in the heterojunction. They exhibit excellent biocompatibility, stability, and ease of functionalization, making them suitable for a wide range of biomedical applications [10].