Enhancing and verification of dose in external radiation therapy using Gd nanoparticles as a theranostic agent: A Monte Carlo simulation study

Introduction: Theranostics, in particular, the use of radionuclides with the capability of simultaneous imaging and treatment has opened new horizons in personalized treatment planning of targeted radiation therapy. In this approach, positive beta or gamma emitters are required for imaging and alpha, beta and Auger electrons for treatment purpose. On the other hand, studies have shown that due to high biological effectiveness the combination of external radiation modalities such as neutrons and protons with internal radiation modalities, can have a potential impact on cancer cell killing. Therefore, given the advances made in nanoparticles and radiopharmaceuticals, the necessity of designing combined therapy modalities and radiopharmaceuticals with a diagnostic-therapeutic capability is of particular importance, and Monte Carlo computational methods are a powerful tool in this regard.     Materials and Methods: In this study, by using the Geant4 and MCNPX Monte Carlo Codes, the absorbed dose of the Auger electrons, KERMA and the flux of the prompt gamma photons was calculated. the distribution of gadolinium nanoparticles with different concentrations and also without them, in two geometries of uniform PMMA environments surrounded by water and the cell model, was subjected to neutron irradiation and the dose was measured in macroscopic and microscopic conditions. Regarding the higher energy range compared to the diagnostic items in nuclear medicine, a gamma camera was optimized for counting the prompt gamma photons by using the F4 and F2 flux calculations as well as count calculation F1 MCNP tallies   Results: Dose enhancement was observed in micro and macro dosimetry conditions. For example, the amount of dose in the cellular model assigned to the nucleus was 5.1 E-5 Gy, 3.01E-03 Gy, and 4.05E-7 Gy for the distribution of Gd in the core, total nucleus, and cytoplasm for particle history, respectively. There was also a direct correlation between the number of prompt gamma rays and the nanoparticle concentration   Conclusion: Labeling of antibodies or peptides with gadolinium nanoparticles in the presence of external radiation beam can be used as a marker for drug distribution, as well as targeted radiation therapy theranostic agent because of enhancement of doses due to auger electrons and prompt gamma photons with imaging capability that showing the synergy impact of diagnostic imaging and therapeutics.