Fluorescent Neutron Track Detectors for Boron-10 Microdistribution Measurement in BNCT: A Feasibility Study

Abstract

Boron Neutron-Capture Therapy (BNCT) is a form of radiation therapy that relies on the highly localized and enhanced biological effects of the ¹⁰B neutron capture (BNC) reaction products to selectively kill cancer cells. The efficacy of BNCT is, therefore, strongly dependent on the ¹⁰B spatial microdistribution at a subcellular level. Fluorescent Nuclear Track Detectors (FNTDs) could be a promising technology for measuring ¹⁰B microdistribution. They allow the measurement of the tracks of charged particles, and their biocompatibility allows cell samples to be deposited and grown on their surfaces. If a layer of borated cells is deposited and irradiated by a neutron field, the energy deposited by the BNC products and their trajectories can be measured by analyzing the corresponding tracks. This allows the reconstruction of the position where the measured particles were generated, hence the microdistribution of ¹⁰B. With respect to other techniques developed to measure ¹⁰B microdistribution, FNTDs would be a non-destructive, biocompatible, relatively easy-to-use, and accessible method, allowing the simultaneous measurement of the ¹⁰B microdistribution, the LET of particles, and the evolution of the related biological response on the very same cell sample. An FNTD was tested in three irradiation conditions to study the feasibility of FNTDs for BNCT applications. The FNTD allowed the successful measurement of the correct alpha particle range and mean penetration depth expected for all the radiation fields employed. This work proved the feasibility of FNTD in reconstructing the tracks of the alpha particles produced in typical BNCT conditions, thus the ¹⁰B microdistribution. Further experiments are planned at the University of Pavia’s LENA (Applied Nuclear Energy Laboratory) to test the final set-up coupling the FNTD with borated cell samples.

1. Introduction

The interest in Boron Neutron-Capture Therapy (BNCT) has been recently increasing thanks to the latest advancements in accelerator technology, which allow the generation of the required neutron field from accelerator facilities that could be hosted in hospitals or clinical centers. BNCT is a binary form of particle therapy used in the treatment of tumor pathologies. It consists of two distinct phases: (i) the intravenous administration of a compound enriched in ¹⁰B, which selectively accumulates in tumor cells, followed by (ii) the irradiation with a thermal or epithermal neutron beam, depending on the depth of the tumor target [1]. Apoptosis of the tumor cells is triggered by significant DNA damage resulting from ionizations caused by the charged products of the ¹⁰B neutron-capture reactions, specifically an $\alpha$ particle and a ⁷Li nucleus. These products have ranges of approximately 9 µm and 4 µm, respectively, making them capable of depositing their energy primarily within the cell. The selectivity and effectiveness of BNCT thus depend closely on the biodistribution of ¹⁰B within the intracellular environment. Specifically, the likelihood of inducing DNA damage and subsequently killing the tumor cell increases with the greater internalization of ¹⁰B into the nucleus or its adjacent regions.

Currently, the two main ¹⁰B carriers employed for BNCT treatments are disodium mercaptoundecahydrododecaborate (BSH, Na₂B₁₂H₁₂SH) and L-p-Boronophenylalanine (BPA, C₉H₁₂BNO₄). The former accumulates in the intercellular space, while BPA is able to penetrate the cell membrane and potentially reach the nucleus. However, research on improved and optimized boron carrier compounds to improve BNCT clinical outcomes is very active. A comprehensive review of the state of the art of boron agents for BNCT can be found in [2]. Many different boron carriers have been studied by following different design strategies, including cell membrane targeting, nuclear targeting, and tumor affinity. It is evident from the described scenario that the biodistribution of ¹⁰B depends on the type of carrier administered to the patient and how this, in turn, is internalized by the individual cell. It is, therefore, crucial to study the ¹⁰B microdistribution at a subcellular level, both inter- and intracellularly, as it has a significant impact on the efficacy of the treatment.

Several studies are currently underway to develop techniques capable of measuring the microdistribution of ¹⁰B within cells using different methods [1,3]: secondary ion mass spectrometry (SIMS) [4,5,6,7,8,9] and Nano-SIMS [10,11], high-resolution alpha autoradiography [12,13,14,15], neutron autoradiography [16,17,18,19,20,21,22,23,24], electron energy loss spectroscopy (EELS) [25,26,27,28,29], laser secondary neutral mass spectrometry (SNMS) [6,30,31], laser-induced breakdown spectroscopy (LIBS) [32,33,34,35], stimulated Raman scattering (SRS) [36], nuclear magnetic resonance or magnetic resonance imaging (MRI) on protons [37,38,39] or on ¹⁰B [40] and immunohistochemistry [41]. Nano-SIMS, EELS, and SNMS are the techniques allowing for the best spatial resolution, as low as a few hundred $\mathrm{n}\mathrm{m}$ and even lower in the case of EELS. However, they all require very complex set-up, procedures and specific facilities to be carried out. SIMS, besides its limits on sensitivity and quantification, necessitates measurement and analysis under vacuum conditions using highly focused low-energy ion beams and mass spectrometers [3]. SNMS and EELS are both time-, labor- and cost-intensive and require complex procedures for the preparation and analysis of cell samples [3]. They require vacuum conditions, cryo-fixation and cryo-sectioning of very thin frozen samples and cryo-analysis. Techniques such as LIBS and MRI have more accessible requirements in terms of sample preparation and analysis. However, MRI has a much lower spatial resolution and sensitivity, while LIBS requires a calibration curve relying on standard samples, which are difficult to produce from biological materials. While still under development, neutron autoradiography was proven a promising technique that is able to achieve subcellular spatial resolution for assessing how ¹⁰B is internalized and whether it reaches recognizable cell structures. However, to obtain such a resolution, the neutron autoradiography has to be overlapped with the histological image of the biological sample [22,23]. This is a critical step of the analysis procedure as it affects the measurement accuracy.

This work aims to study the feasibility of an experimental method relying on a Fluorescent Nuclear Track Detector (FNTD) [42]. This device has several key advantages, including excellent energy and spatial resolution, biocompatible surface, and reusability. FNTDs have already been applied in hadron therapy, with the FNTD being the central component of the biosensor Cell-Fit-HD^4D [43,44]. This study represents a significant innovation in the field of BNCT, as it is the first time that FNTD detectors have been explored as potential tools for measuring the microdistribution of ¹⁰B. To this end, an FNTD readout from a confocal laser-scanning microscope was tested in three different conditions to prove its capabilities of accurately measuring the particle tracks of boron neutron-capture (BNC) products. Hence, the device was first irradiated by the alpha particles emitted by an Am-241 source (with energy of about $5.5$ $\mathrm{M}$$\mathrm{e}\mathrm{V}$). The same radiation field moderated by a 23 $\mu \mathrm{m}$ layer of Mylar was then used to obtain alpha particles with a lower energy of about $2.5$ $\mathrm{MeV}$, thus being closer to BNC products. Finally, the FNTD was tested at the University of Pavia’s LENA (Applied Nuclear Energy Laboratory) under a thermal neutron beam. A standard reference material (SRM) consisting of boron implanted on a silicon wafer was placed on the detector surface to reproduce a typical BNCT radiation field.

2. Materials and Methods

The characterization of FNTD detectors was carried out in radiation fields relevant to BNCT by combining the results of different experiments with reference simulations. The particle transport was simulated by means of SRIM-2013 software [45] to describe the particle path in the detector.

2.1. The Fluorescent Neutron Track Detector

Fluorescent Neutron Track Detectors (FNTDs) are a well-established technology based on a single crystal made of aluminum oxide doped with carbon and magnesium impurities: $A{l}_2{O}_3:C,Mg$. The crystal is grown following the Czochralski technique [42]. Carbon and magnesium doping is obtained, respectively, by dissolving carbon monoxide and by adding magnesium compounds (such as $MgO$ or $MgA{l}_2{O}_4$) in the raw material.

The single and double vacancies found in $A{l}_2{O}_3:C,Mg$ crystals result in different fluorescent centers: F-centers characterized by an absorption band at 205 nm and by an emission band at 420 nm with a 2 ns lifetime, $F^{+}$-centers absorbing at 230 and 255 nm and emitting at 330 nm with 2 ns lifetime, and $F_2^{2+}$-centers absorbing at 435 nm and emitting at 520 nm with about 9 ns lifetime. $F_2^{2+}$-centers can easily absorb free-electrons generated during irradiations, creating $F_2^{+}$-centers by photochromic and radiochromic transformations. $F_2^{+}$-centers are characterized by excitation bands at 335 and 620 nm and emission band at 750 nm with a lifetime of 75 ns. The very short emission lifetime of these crystals allows for a fast laser-scanning readout. To reduce background luminescence, FNTDs are thermally annealed with a 17 h long specific heating profile up to 650 °C and successively treated with optical bleaching. Annealing and optical bleaching can be repeated to erase fluorescent tracks, thus allowing the detector to be reused. FNTDs can measure particles with LET ranging from 0.5 to 1800 $\mathrm{keV} \mu \mathrm{m}$⁻¹, allowing the reaching of a fluence of $5\times {10}^7$ cm⁻² without saturation and to sustain high dose-rate up to $1\times {10}^8$ $\mathrm{Gy} \mathrm{s}$⁻¹ [44].

From the $A{l}_2{O}_3:C,Mg$ crystal, a plate-shaped cut with typical dimensions $8\times 4\times 0.5$ $\mathrm{m}{\mathrm{m}}^3$ is usually obtained, as for the FNTD employed in this work.

2.2. Readout System

The FNTD readout was carried out by Confocal Laser-Scanning Microscopy (CLSM). CLSM allowed a non-destructive readout with diffraction-limited spatial resolution. A Zeiss LSM710 ConfoCor 3 inverted CLSM (Zeiss AG, Oberkochen, Germany) was employed for the experiments. Data acquisition was managed using the ZEN software (version 2009). The microscope consists of a 63× oil objective and an Avalanche Photo-Diodes (APD) in Geiger mode, a main beam splitter (BS488/561/633), and a single long-pass emission filter (LP655) placed in front of the APDs for signal separation, and a 633 $\mathrm{n}$$\mathrm{m}$ helium-neon laser excitation source. The pinhole diameter was set to 1 Air Unity (AU) to achieve an optimal trade-off between resolution, contrast, and brightness. The FNTD was mounted in a glass-bottom dish. Immersion oil was applied between the dish and the objective lens.

2.3. Data Processing

The images acquired were processed using Fiji (https://imagej.net/software/fiji/ (accessed on 15 January 2023)) [46], an image processing software package based on ImageJ (https://imagej.net/software/imagej/ (accessed on 15 January 2023)). The 3D image processing routine created a maximum intensity projection along the detector depth, allowing for the separation of each track, which was then processed individually. For each track, the signal of each depth-slice was fitted with a 2D Gaussian. This made it possible to obtain the fluorescence amplitude, the fluorescence background offset, the coordinates of the track centroid ($x_0$, $y_0$), the track maximum and minimum width ($W_y$ and $W_x$, respectively), and the azimuthal angle of the track propagation ($\phi$). The radial extension of the track is the effect of secondary electrons emitted by the interaction of the particle tracking with the detector material.

Figure 1 schematically represents the quantities obtained through data analysis. From the fluorescence signals, the linear energy transfer (LET) of the ion traversing the FNTD can be assessed. In a BNCT context, if the particle type and emission energy are known, the position where the particle detected originated, thus the ¹⁰B position in the analyzed sample, can be estimated. This is achieved by combining the track direction measured, the energy released into the detector, the particle LET into the sample material, and the emission energy. However, only the track length and direction were considered in this work to study the feasibility of FNTD detectors for BNCT applications.

2.4. The Experiments

Three experiments were carried out to characterize the FNTD readout signal in BNCT-relevant radiation fields, thus to assess its feasibility for BNCT applications.

Two preliminary tests were carried out employing an Am-241 radioactive source. The three most probable decay channels of Am-241 emit $\alpha$ particles at 5.486, 5.442, and 5.388 $\mathrm{M}\mathrm{e}\mathrm{V}$ in the 84.8%, 13.1% and 1.6% of emissions, respectively. The source, disk-shaped with 4 $\mathrm{m}$$\mathrm{m}$ diameter, had an activity of 2638 $\mathrm{Bq}$ to the irradiation day. The measurement was carried out by placing the detector at $1.16$ $\mathrm{c}$$\mathrm{m}$ from the source inside a vacuum chamber. A fluence of $4.466\times {10}^5$ cm⁻² was estimated.

Since the boron neutron capture produces $\alpha$ particles at $1.472$ $\mathrm{MeV}$ in the 93.7% of reactions or at $1.776$ $\mathrm{MeV}$ in the 6.3%, a second test was carried out to better reproduce such irradiation condition. With this purpose, a 23 $\mu$$\mathrm{m}$ layer of Mylar ($H_8{C}_{1}0O_4$) was interposed between the Am-241 source and the FNTD. The Mylar layer worked as a moderator, lowering the energy of the $\alpha$ particles emitted to about $2.5$ $\mathrm{MeV}$. In addition, a hole was made in the center of the Mylar layer to assess the capability of the FNTD to discriminate between the two resulting $\alpha$ energies (about 5.5 and 2.5 $\mathrm{MeV}$).

Following the successful results of the two preliminary tests, the FNTD was eventually used to measure the radiation field produced by the boron neutron-capture reaction.

A sample of the standard reference material (SRM) SRM-2137 [47], certificated by the National Institute of Standards and Technology (NIST), was irradiated by a thermal neutron field in the thermal column of the TRIGA Mark II research reactor of University of Pavia’s LENA (Applied Nuclear Energy Laboratory). The SRM-2137 consists of a single-crystal silicon substrate with a $1.018·{10}^{15}\pm 0.035·{10}^{15}$ $\mathrm{atoms}/\mathrm{c}{\mathrm{m}}^2$ superficial concentration of ¹⁰B obtained by ion implantation. As shown in Figure 2, the FNTD was placed in contact with the SRM sample inside a Teflon structure, which held them in position. The low neutron activation of Teflon allowed for the minimization of the background radiation generated by the holder structure. A portion of FNTD was kept outside the SRM sample to observe and evaluate the background resulting from neutron irradiation. The irradiation was carried out with a reactor nominal power of 10 $\mathrm{k}$$\mathrm{W}$. The irradiation time was evaluated in order to allow the measurement of an $\alpha$ fluence of about $5\times 10$⁵ cm⁻².

3. Results

3.1. Preliminary Tests with Am-241

The results of SRIM simulations are shown in Figure 3 and Figure 4 for the measurement without and with the Mylar layer, respectively.

The simulations were carried out considering an isotropic surface source of $\alpha$ particles, reproducing the Am-241 emission. The thickness of the Am-241 source was not considered in the simulations. Figure 3b and Figure 4b represent the distribution of $\alpha$ particles as a function of their penetration depth inside the FNTD. In the case of isotropically emitted $\alpha$ without a Mylar layer, their penetration depth was evenly distributed between 0 and about 15 $\mu \mathrm{m}$. 15 $\mu$$\mathrm{m}$ corresponds to the particle range in the FNTD material (i.e., the penetration depth of those particles emitted in the detector-axis direction). The mean penetration depth was $7.43$ $\mu$$\mathrm{m}$. When the 23 $\mu$$\mathrm{m}$ layer of Mylar is interposed between the Am-241 source and the detector, as clearly shown in Figure 4b, a significant number of particles was not able to reach the FNTD detector. According to the results of SRIM simulations, the $\alpha$ particles that were capable of reaching the detector had a mean energy of about $2.25$ $\mathrm{MeV}$ and could penetrate up to 5 $\mu$$\mathrm{m}$ into the FNTD detector.

The results of the experiments well agreed with what was expected from the simulations. The preliminary experiments also proved the capability of the FNTD detector read by the CLSM to measure the tracks of the $\alpha$ particle produced by Am-241, both with and without the 23 $\mu$$\mathrm{m}$ Mylar layer moderation. Figure 5 shows an example of the $\alpha$ tracks measured in a subregion of the FNTD at different depths in the case of no moderator.

Dirt on the FNTD surface, such as the two brightest features at the bottom-right of the FNTD region shown in Figure 5, was excluded from data analysis by properly setting a threshold system. After reaching a maximum brightness at about 4 $\mu$$\mathrm{m}$, the majority of the tracks measured vanished at a depth of around 7 $\mu$$\mathrm{m}$ into the FNTD. This was in agreement with the mean penetration depth predicted by the simulations. The tracks had an oval shape as their direction was predominantly different from the detector axis due to the isotropic emission. The average track area resulted $0.329$ $\mu$$\mathrm{m}$², corresponding to an equivalent diameter of approximately $0.5$ $\mu$$\mathrm{m}$. The result obtained well aligned with previous studies in literature [48]. The fluence resulting from the measurement was $6.1\times 10$⁵ cm⁻², which agreed within uncertainty to the nominal fluence expected of $4.466\times 10$⁵ cm⁻².

The measurement of the Am-241 source moderated by a 23 $\mu$$\mathrm{m}$ thick Mylar layer is reported in Figure 6 and Figure 7. They show, respectively, a subregion of the FNTD beneath the Mylar layer and a subregion beneath the central hole.

In both cases, the resulting track size was consistent with that measured in the case without a moderator. In the subregion beneath the Mylar, most of the tracks could penetrate to 4–5 $\mu \mathrm{m}$, and almost none could reach 6 $\mu$$\mathrm{m}$, as predicted by the simulations. In correspondence with the central hole, as expected, the tracks measured were all able to penetrate down to 14 $\mu$$\mathrm{m}$ into the FNTD. The number of $\alpha$ that were detected at 15 $\mu$$\mathrm{m}$ significantly dropped and almost zeroed at 16 $\mu$$\mathrm{m}$. The Mylar layer, indeed, basically worked as a collimator in the central hole region. Considering the small diameter of the hole with respect to the Am-241 source, only those non-moderated $\alpha$ particles with a direction similar to the detector axis (perpendicular to the detector surface) were able to reach the detector in the hole region. As already discussed, the penetration depth of such $\alpha$ particles could be reasonably approximated with their range, 15 $\mu$$\mathrm{m}$.

The results obtained also allowed the reconstruction of the 3D particle track, as shown by Figure 8.

3.2. Measurement of a Reference Boron Neutron-Capture Therapy Radiation Field

Boron neutron capture has two reaction channels. The first occurs with the 93.7% of probability and produces a $1.472$ $\mathrm{MeV}$ $\alpha$ particle, a $0.84$ $\mathrm{MeV}$ lithium ion and a 478 $\mathrm{keV}$ $\gamma$-ray. The second reaction channel occurs with the 6.3% of probability and produces a $1.776$ $\mathrm{MeV}$ $\alpha$ particle and a $1.015$ $\mathrm{MeV}$ lithium ion. While the $\alpha$ particles are characterized by a range in the FNTD material of $2.9$ $\mu$$\mathrm{m}$ (at $1.472$ $\mathrm{MeV}$) and $3.54$ $\mu$$\mathrm{m}$ (at $1.776$ $\mathrm{MeV}$), the lithium ions have a range of just $1.52$ $\mu$$\mathrm{m}$ (at $0.84$ $\mathrm{MeV}$) and $1.88$ $\mu$$\mathrm{m}$ (at $1.015$ $\mathrm{MeV}$). Since the $\alpha$ and the lithium produced by the neutron reaction are emitted isotropically in opposite directions, the FNTD detects a comparable amount of the two particles. The readout and data analysis were significantly more complicated than in the experiment with the Am-241, especially within the first 2 $\mu$$\mathrm{m}$ of detector-sensitive volume due to the radiation field heterogeneity. However, since lithium has a higher atomic number than helium, its corresponding track spot should be of greater diameter and of higher intensities (because of the higher ionization density and stopping power). Hence, a meticulous analysis might allow the discrimination between the two particles, especially when their impinging angle is perpendicular to the detector surface, and their track shape is circular. When considering an isotropic emission of $\alpha$ and lithium with boron neutron-capture reaction energies and probabilities, the mean penetration depth resulting from SRIM simulations was $1.46$ $\mu$$\mathrm{m}$ for $\alpha$ particles and $0.81$ $\mu$$\mathrm{m}$ for lithium ions.

The data processing was further complicated by the high background created by the neutron field and by the photons produced in the reactor. Nevertheless, thanks to the background data collected, allowing for an accurate setting of thresholds and for clean CLSM images, the tracks of boron neutron-capture reaction products were successfully discriminated from the background. Figure 9 shows the particle tracks measured by a 67.5 × 67.5 $\mu {\mathrm{m}}^2$ subregion of the FNTD. The results of the experiment were in agreement with what was predicted by the simulations. The higher density of tracks was observed within the first $\mu \mathrm{m}$ of the FNTD, where the radiation field was composed of both reaction products. The number of tracks measured deeper than 2 $\mu$$\mathrm{m}$ significantly dropped, reasonably in line with the $\alpha$ mean penetration depth simulated. Eventually, only very few tracks were found at a depth of $3.5$ $\mu$$\mathrm{m}$, all characterized by a circular shape. These were the few $1.776$ $\mathrm{MeV}$ $\alpha$ particles which entered the FNTD parallel to the detector axis and whose penetration depth thus corresponded to their range in the FNTD material.

4. Discussion

While a clear 3D reconstruction of the $\alpha$ tracks was obtained for the experiments with the Am-241 source, the higher background generated in the LENA’s reactor significantly complicated the 3D track reconstruction for the experiment with the SRM. Additional efforts should be put into the improvement of the radiation background to allow a clearer reconstruction of 3D tracks even in this experimental condition. However, this limitation did not hinder the successful visualization and measurement of particle tracks on 2D planes at different depths in the FNTD. This allowed the measurement of the track shape, size, length, and direction. Hence, despite the challenges posed by the high radiation background, the quantities required to estimate the emission point of the reaction products could still be measured by the FNTD.

A spatial resolution of a few $\mu \mathrm{m}$ was obtained for the particle track measured. This will allow, once the FNTD detector is coupled with borated cell samples, for the measurement of the ¹⁰B microdistribution with a subcellular spatial resolution.

Table 1. Summary of the qualitative spatial resolution of different techniques for the ¹⁰B microdistribution measurement. Most of the estimates are derived from the discussions in [1,3].

Technique	Qualitative Spatial Resolution
FNTD	Subcellular	$\mu \mathrm{m}$
EELS	Subcellular	tens of $\mathrm{n}\mathrm{m}$
SNMS	Subcellular	hundreds of $\mathrm{n}\mathrm{m}$
SIMS	Subcellular	a few $\mu \mathrm{m}$
Nano-SIMS	Subcellular	hundreds of $\mathrm{n}\mathrm{m}$
Autoradiography	Subcellular	$\mu \mathrm{m}$
LIBS	Cellular/Subcellular	10 $\mu$$\mathrm{m}$
MRI	Organs

summarizes the qualitative spatial resolution achieved by the different ¹⁰B microdistribution measurement techniques available in the literature. The resolution of FNTDs could be increased to approximately 80 $\mathrm{n}$$\mathrm{m}$ if optical nanoscopy is used for the readout [49]. The final stage of FNTD systems for ¹⁰B microdistribution measurement will be their use with cell samples and the creation of the biosensor ${\mathrm{Cell}-\mathrm{Fit}-\mathrm{HD}}^{4D}$ by coupling the FNTD with time-lapse microscopy. Besides its promising spatial resolution, the main advantages of using FNTD detectors are their capability to simultaneously measure the ¹⁰B microdistribution, the particle track, and energy deposition and to follow the evolution of biological effects. This would be a unique feature of FNTD systems, which would allow the study, on the very same sample of cells, of the ¹⁰B microdistribution and the evolution of the biological effects induced by the BNC reactions, directly linking them with the LET of the particles causing them. In addition, unlike other techniques, FNTDs allow a non-destructive analysis and are relatively easy to use and easily accessible, as they rely on equipment that is usually available in radiation laboratories.

5. Conclusions

Fluorescent Neutron Track Detectors (FNTDs) can have a pivotal role in Boron Neutron-Capture Therapy (BNCT) as they allow the measurement of the ¹⁰B microdistribution in tumor cells. This work presented a feasibility study and the very first characterization of an FNTD for BNCT. The detector was read by Confocal Laser-Scanning Microscopy (CLSM). To reproduce a typical BNCT radiation field, the FNTD was employed in three experimental conditions: using an Am-241 source, using the same Am-241 source moderated by a 23 $\mu$$\mathrm{m}$ thick Mylar layer, and using the NIST Standard Reference Material (SRM) 2137 irradiated by a thermal neutron field. The third experiment was carried out at the University of Pavia’s LENA (Applied Nuclear Energy Laboratory), where the system made of the FNTD and the SRM-2137 sample was irradiated in the research reactor thermal column. The FNTD successfully measured the range and mean penetration depth of the $\alpha$ particles in all the experimental conditions tested. Hence, the results of this work proved the feasibility of FNTDs to reconstruct the tracks of the reaction products created in typical BNCT conditions.

By proving the track reconstruction capability of FNTDs for boron neutron-capture products, this work proved the feasibility of this type of detector to measure the ¹⁰B microdistribution in a material sample placed on the detector surface. Once the particle track is reconstructed, indeed, the particle emission point can be estimated by employing existing accredited software. The emission point coincides with the ¹⁰B location, and its microdistribution could finally be estimated.

To characterize the ¹⁰B microdistribution at the cell level, future experiments will be carried out, depositing a layer of ¹⁰B enriched cells on the FNTD. This hybrid detector, known as Cell-Fit-HD, can be coupled with time-laps microscopy to create the biosensor Cell-Fit-HD^4D. Tracking specific proteins responsible for DNA repair, Cell-Fit-HD^4D will allow the study of the correlation between DNA damage dynamics and the stochastic energy deposition at a subcellular level. The development of such a technique will be pivotal in BNCT research, as it will allow the experimental study of the double stochastic nature of radiation interaction involved in BNCT: the ¹⁰B position with respect to the cell nucleus and the stochastic nature of energy deposition.