## 4 Discussion
### Dependence of DNA Damage on the Photon Beam Energy  
Figure 3 depicts the relationship between DNA damage and photon beam energy for different percentages of gold and iron in the nanoparticle heterojunction. The results reveal that higher photon beam energies result in lower levels of DNA damage caused by the irradiated nanoparticle heterojunction. Specifically, when the percentage volume of gold is 75%, decreasing the photon beam energy from 100 keV to 50 keV leads to a 1.8-fold increase in strand breaks. Further decreasing the photon beam energy from 150 keV to 50 keV results in a larger increase, with strand breaks rising 2.4 times. Similar trends can be observed when the percentage volume of gold is reduced to 50%, where strand breaks increase 1.5 times and 2.8 times when the photon beam energy is decreased from 100 keV and 150 keV to 50 keV, respectively. However, when the percentage volume of gold is equal to or less than 25%, the decrease in strand breaks with an increase in photon beam energy is not significant. This highlights the significant role played by the volume of gold in DNA damage in relation to changes in photon beam energy.  
The increase in the number of DNA strand breaks with an increase in the volume of gold in the nanoparticle heterojunction can be attributed to an augmented secondary electron yield from the nanoparticle. This, in turn, enhances the radiation dose at the DNA. The observed effect is primarily due to the dominance of the photoelectric effect in the kV photon beam energy range. The cross section of the photoelectric effect is inversely proportional to the photon energy (\(1/\mathrm{E}^{3.5}\)), where E represents the energy of the incident photon. Consequently, lower photon beam energies result in larger photoelectric cross section and the generation of a greater number of photoelectric or secondary electrons. However, this effect is particularly evident when the percentage of gold in the nanoparticle heterojunction is substantial, as the photoelectric effect is also influenced by the atomic number of the nanoparticle material.  
In radiotherapy, tissue penetration is a crucial consideration alongside dose enhancement resulting from photon beam energy. In nanoparticle-enhanced radiotherapy, the objective is to selectively enhance the radiation dose within the tumor while minimizing damage to surrounding healthy tissues. The use of kV photon beams is advantageous in terms of tissue penetration and the controlled delivery of radiation dose, enabling precise targeting of the tumor region. This makes kV photon beams suitable for intraoperative radiotherapy (IORT), a specialized form of radiotherapy conducted during surgery [31]. Unlike conventional external beam radiotherapy, which originates from outside the body, IMRT involves placing a radiation source directly inside the patient's body, usually in close proximity to the tumor. IORT finds application in the treatment of various cancers, including breast cancer, colorectal cancer, pancreatic cancer, and sarcomas [32; 33]. It is typically integrated into a comprehensive treatment plan that may also involve surgery, chemotherapy, and/or external beam radiation therapy.