Fluorinated borono-phenylalanine for optimizing BNCT: Enhancing boron absorption against hydrogen scattering for thermal neutrons

BACKGROUND

Boron-containing compounds, such as 4-borono-phenylalanine (BPA) are used as drugs for cancer treatment in the framework of Boron Neutron Capture Therapy (BNCT). Neutron irradiation of boron-rich compounds delivered to cancer cells triggers nuclear reactions that destroy cancer cells.

PURPOSE

We provide a modeling of the thermal neutron cross section of BPA, a drug used in Boron Neutron Capture Therapy (BNCT), to quantify the competing contributions of boron absorption against hydrogen scattering, for optimizing BNCT by minimizing the latter.

METHODS

We perform the experimental determination of the total neutron scattering cross section of BPA at thermal and epithermal neutron energies using neutron transmission measurements. We isolate the contribution related to the incoherent scattering by hydrogen atoms as a function of the neutron energy by means of the Average Functional Group Approximation, and we calculate the probability for a neutron of being absorbed as a function of the neutron energy both for BPA and for its variants where either one or all four aromatic hydrogen atoms are substituted by 19 F, and both for the samples with natural occurrence or enriched concentration of 10 B.

RESULTS

While referring to the already available literature for in vivo use of fluorinated BPA, we show that fluorine-rich variants of BPA increase the probability of neutrons being captured by the molecule. As the higher absorption efficiency of fluorinated BPA does not depend on whether the molecule is used in vivo or not, our results are promising for the higher efficiency of the boron neutron capture treatment.

CONCLUSIONS

Our results suggest a new advantage using fluorinated compounds for BNCT, in their optimized interaction with neutrons, in addition to their already known capability to be used for monitoring and pharmacokinetics studies using 19 F-Nuclear Magnetic Resonance or in 18 F-Positron Emission Tomography.

Thermal neutron cross sections of amino acids from average contributions of functional groups

The experimental thermal neutron cross sections of the 20 proteinogenic amino acids have been measured over the incident-neutron energy range spanning from 1 meV to 10 keV and data have been interpreted using the multi-phonon expansion based on first-principles calculations. The scattering cross section, dominated by the incoherent inelastic contribution from the hydrogen atoms, can be rationalised in terms of the average contributions of different functional groups, thus neglecting their correlation. These results can be used for modelling the total neutron cross sections of complex organic systems like proteins, muscles, or human tissues from a limited number of starting input functions. This simplification is of crucial importance for fine-tuning of transport simulations used in medical applications, including boron neutron capture therapy as well as secondary neutrons-emission induced during proton therapy. Moreover, the parametrized neutron cross sections allow a better treatment of neutron scattering experiments, providing detailed sample self-attenuation corrections for a variety of biological and soft-matter systems.

The effective isotropy of the hydrogen local potential in biphenyl and other hydrocarbons

We present an experimental investigation of the hydrogen nuclear momentum distribution in biphenyl using deep inelastic neutron scattering. Our experimental results suggest that the local potential affecting hydrogen is both harmonic and isotropic within experimental uncertainties. This feature is interpreted as a consequence of the central limit theorem, whereby the three-dimensional momentum distribution is expected to become a purely Gaussian function as the number of independent vibrational modes in a system increases. We also performed ab initio phonon calculations on biphenyl and other saturated hydrocarbons, from methane to decane. From the results of the simulations, one can observe that the nuclear momentum distribution becomes more isotropic as the number of atoms and normal modes in the molecule increases. Moreover, the predicted theoretical anisotropy in biphenyl is clearly larger than in the experiment. The reason is that the total number of normal modes necessary to reproduce the experimental results is much larger than the number of normal modes encompassed by a single unit cell due to the presence of structural disorder and intermolecular interactions in the real crystal, as well as coupling of different normal modes. Finally, experimental data were collected, over a subset of detectors on the VESUVIO spectrometer at ISIS, with a novel setup to increase the count rate and signal-to-background ratio. We envision that such an optimized experimental setup can provide faster measurements and more stringent constraints for phonon calculations.

Optimization of detection strategies for epithermal neutron spectroscopy using photon-sensitive detectors

In this work, we discuss an improved detection procedure for the photon-sensitive yttrium-aluminum-perovskite detectors installed on the VESUVIO spectrometer at the ISIS pulsed neutron and muon source. By decreasing the low-level energy threshold of detected photons, we observe an increased count rate up to a factor ∼3, and a decrease of relative error bars and noise of ∼40% and 35%, respectively, for deep inelastic neutron scattering measurements. In addition, we demonstrate how the reported optimization may increase the accuracy in the line shape analysis of neutron Compton profiles, as well as in the application of the mean-force approach to detect the anisotropy and anharmonicity in the single-particle local potential. We envisage that such an upgrade of the detection procedure would have a substantial impact on the VESUVIO scientific programme based on deep inelastic neutron scattering investigations.