# Co-Adjuvant Nanoparticles for Radiotherapy Treatments of Oncological Diseases
## 4 The Effect of the Energy Release vs. X-PDT Efficacy
in [161] have developed theoretical dosimetric investigation, by using a Monte Carlo code developed for neutrons, photons, and electrons transport (MCNPX) on an idealized phantom representing a human head bearing a brain tumor volume. Simulations of irradiation have been conducted for diverse beams: monochromatic X-ray beams from a synchrotron source (30-120 keV); polychromatic X-ray beams have been used to simulate irradiations from a conventional X-ray tube (80, 120, and 180 kVp), and from a linear accelerator (6 MV). The results on the SSRT dose enhancement have been compared for the diverse ionizing radiation energy ranges and in the presence of iodineelements. The enthusiastic data proves the major RT enhancement in presence of high Z agents for the medium energy beam by monochromatic source at 80 keV, despite the lower penetration of the keV X-rays with respect to the conventionally used higher energy X-rays. At keV energies, the maximum dose can be delivered to the tumor while preserving skull and healthy tissues from excessive radiations exposure. In particular, the simulation on X-ray penetration and dose deposition performed by [PERSON] and co-workers reveals that by using 6 MV irradiation, the doses delivered to the skull are similar to the value obtained for 80 keV from monochromatic source but, at the same time, the doses delivered to the healthy brain are hazardously higher if compared to low energy monochromatic and polychromatic beams. Moreover, the dose enhancement for 6 MV irradiation is almost negligible. This may be imputed to the different type of X-rays interaction with the biological medium, ruled by Compton and photoelectric effects working in the MeV and keV range, respectively. This work highlights the importance of computational modeling in order to find an optimized treatment in presence of radiosensitizers, able to significantly reduce the dose to the healthy tissue, hopefully avoiding the implication of megavoltage irradiation. In another work, [PERSON] et al. in [162] have quantified, by Monte Carlo GEANT4 program, the amount of energy deposited in diverse scintillators such as gadolinium oxides (Gd\({}_{2}\)O\({}_{3}\)) nanoparticles, by varying the concentration of the NS in an aqueous medium, the NS dimension, and the energy of the first high energy beam, as well as the position of the first interaction event. In this work, the first estimation of the energy transfer from the primary interaction of an ionizing radiation beam with a sphere of water representing a volume of tumor, loaded with nanocintillators, has been conducted by means of the mass energy absorption coefficient. The results reveal that the maximum energy transfer enhancement occurs for energies around 40-60 keV, confirming the importance of exploiting beams of ionizing radiation in the kiloelectronvolt energy range in presence of high-Z elements. However, [PERSON] and co-workers have highlighted that in such a composite compound, made by a large volume of water loaded with a small concentration of NS, the interaction between the first photon and the matter can occur either in the NS or in water. Especially, when the interaction occurs in the NS a fraction of the expected transferred energy can escape from the nanoparticle, typically due to the larger migration distance of the secondary particles generated after the first interaction with respect to the NS dimensions, and the deposited energy is shared between the water and the NS. In this context, the Monte Carlo GEANT4 calculation has helped to determine the spatial distribution of deposited energy in the volume surrounding the nanoparticle where the primary interaction occurs and to evaluate the fraction of energy loss during the energy relaxation and charge migration processes in the NS. The new parameter \(\eta\) has been introduced, which quantifies the percentage of energy that is deposited in the NS itself, as a function of both NS and external source parameters.