Radiobiological impact of gadolinium neutron capture from proton therapy and alternative neutron sources using TOPAS-nBio.

PURPOSE: A multi-scale investigation of the biological properties of gadolinium neutron capture (GdNC) therapy with applications in particle therapy is conducted using the TOPAS Monte Carlo (MC) simulation code. The simulation results are used to quantify the amount of gadolinium dose enhancement produced as a result of the secondary neutron production from proton therapy scaled by measured data. MATERIALS AND METHODS: MC modeling was performed using the radiobiology extension TOol for PArticle Simulation TOPAS-nBio MC simulation code to study the radiobiological effects produced from GdNC on a segment of DNA, a spherical cellular model, and from the modeling of previous experimental measurements. The average RBE values were calculated from two methods, microdosimetric kinematic (MK) and biological weighting r(y) within a 2 nm DNA segment for GdNC. The single-strand breaks (SSBs) and double-strand breaks (DSBs) were calculated from within the nucleus of a 20 µm diameter, spherical cell model. From a previous experimental proton therapy measurement using a spread-out Bragg peak (SOBP) of 4.5-9.5 cm and a delivered absorbed dose of 10.4 Gy, the amount of Gd neutron captures was calculated and used to quantify the amount of GdNC absolute dose from particle therapy. RESULTS: The average RBE from microdosimetric kinematic and biological weighting was 1.35, and 1.70 for a 10% cell survival on HSG cell-line and weighting function data from early intestinal tolerance of mice. From a central isotropic GdNC source, the energy deposition is found to decrease from roughly 2.7 eV per capture down to approximately 0.01 eV per capture, a drop of two orders of magnitude within 50 nm. This result suggests that Gd needs to be close to the DNA (within 10-20 nm) in order for neutron capture to induce a significant dose enhancement due to the short-range electrons emitted after Gd neutron capture. Within a spherical cell model, the SSBs, and DSBs were determined to be 39 and 1.5 per neutron capture, respectively. From the total neutron captures produced from an experimental proton therapy measurement on a 3000 PPM Gd solution, an insignificant absolute Gd dose enhancement was quantified to be 5.4 × 10-6  Gy per Gy of administered proton dose. CONCLUSION: From this study and literature review, the production of secondary thermal neutrons from proton therapy is determined to be a limiting factor and unlikely to produce a clinically useful dose enhancement for secondary neutron capture therapy. Moreover, alternative neutron sources, such as, a compact deuterium-tritium (D-T) neutron generator, a "high yield" deuterium-deuterium (D-D) generator, or an industrial strength (100 mg) 252 Cf source were investigated, with the 252 Cf source the most likely to be capable of producing enough neutrons for 1 Gy of localized GdNC absolute dose within a reasonable treatment time.

Samarium isotope compositions of uranium ore concentrates: A novel nuclear forensic signature.

Uranium ore is mined and milled to produce uranium ore concentrate (UOC), a regulated product of the nuclear fuel cycle. Diversion of UOC from the fuel cycle into possible weapons production is a key concern in global nonproliferation efforts. As such, the ability to trace the origin of seized nuclear materials is imperative to law enforcement efforts. Although isotopic signatures of UOCs have proven fruitful to pinpoint sample provenance, new isotopic signatures are needed because most existing isotopic signatures are not indicative of the original ore body from which the U is derived. In this work, we developed a new method to separate samarium (Sm) from a U-rich sample matrix and report the first Sm isotope compositions of 32 UOCs derived from a variety of worldwide uranium mines. Relative to terrestrial standards, approximately half the UOCs have resolved and anticorrelated 149Sm-150Sm isotope compositions, consistent with the capture of thermal neutrons by 149Sm in the ore body. The UOCs with anomalous Sm isotope compositions tend to derive from older (~>1.5Ga) and higher-grade ore bodies, although other factors, such as the presence of neutron moderators like water, also play a role. Nonetheless, the Sm isotope compositions of UOCs directly reflects the neutron fluence over the history of the original ore body and can be used to discern different geologic conditions associated with that ore body. As such, this work demonstrates the potential use of Sm isotopes as a novel nuclear forensics signature for origin assessment of UOCs.

Thermal neutron-induced γ-ray background in 124Sn.

The thermal neutron-induced gamma-ray background in 124Sn is investigated in connection with neutrinoless double beta decay (0νßß) studies in 124Sn. For this purpose, a 99.26% enriched 124Sn sample was irradiated with a thermal neutron fluence of 3×1015/cm2 in the Dhruva reactor at the Bhabha Atomic Research Centre, Mumbai. The gamma ray spectra of the irradiated sample were measured in a low background counting setup to study both long-lived and short-lived activities. The present data give an independent measurement of the half-life of 125Sn*(32+) and 125Sn(112-) as 10.01(8) min and 9.63(2) d, respectively. The impact of the observed high-energy gamma rays and the residual activity due to 125Sb, on the background in the region of interest around the Qßß value of 124Sn (~2.291 MeV) is discussed.

Practical aspects of 153Gd as a radioactive source for use in brachytherapy.

The goal of this study was to investigate the production, purification and immobilization techniques for a 153Gd brachytherapy source. We have investigated the maximum attainable specific activity of 153Gd through the irradiation of Gd2O3 enriched to 30.6% 152Gd at McMaster Nuclear Reactor. The advantage of producing 153Gd through this production pathway is the possibility to irradiate pre-sealed pellets of 152Gd enriched Gd2O3, thereby removing the need to perform chemical separation with large quantities of radio-impurities. However, small amounts of long-lived impurities are produced from the irradiation of enriched 152Gd targets due to traces of Eu in the sample. If the amount of impurities produced is deemed unacceptable, 153Gd can be isolated as an aqueous solution, chemically separated from impurities and loaded onto a sorbent with a high affinity for Gd before encapsulation.