Objective. Boron Neutron Capture Therapy is a cancer radiotherapy that uses the selective uptake of boron compounds by tumor cells, followed by neutron irradiation. Conventional dosimetry generally assumes a homogeneous boron distribution within tissues, yet evidence indicates intracellular heterogeneity. This work aims to improve the Photon Isoeffective Dose model (PID) for Glioblastoma Multiforme (GBM) by incorporating subcellular-scale effects: (i) a correction factor for the stochastic nature of energy deposition due to intracellular boron localization, and (ii) the treatment of the nucleus-to-cytoplasm boron concentration ratio as a stochastic variable. Approach. The boron-10 microdistribution in U-87 glioblastoma cells was quantified for the first time through neutron autoradiography, revealing preferential accumulation in the nucleus. Following these experimental data, the nucleus-to-cytoplasm boron concentration ratio was described by a lognormal random variable, consistent with biological uptake processes. The correction factor was applied to the dosimetry of U-87 radiobiological data. Then, updated radiobiological parameters and subcellular-scale effects were integrated into the PID formalism and applied to a clinical case of GBM. Main results. The outcome was a Microdosimetric Photon Isoeffective Dose Model, which extends conventional PID by explicitly including intracellular boron heterogeneity. Applied to U-87 data, proposed corrections revealed a 47% reduction in the Compound Biological Effectiveness factor compared to conventional calculations, showing that neglecting subcellular distribution substantially overestimates the boron dose. For the clinical case, the total dose and 1-year Progression-Free Survival (PFS) differed only by 4% and 3%, respectively, compared to conventional dosimetry. However, perturbation analyses indicated that under higher intracellular heterogeneity, plausible in vivo, the deviations could become substantial (up to 22% in dose and 68% in PFS). Significance. These findings highlight the relevance of subcellular-scale modeling. The proposed Microdosimetric Model, grounded on experimentally derived microdosimetric corrections, provides a robust framework to improve both the accuracy and the personalization of BNCT treatment planning.

The assessment of boron microdistribution is essential to evaluate the suitability of boron neutron capture therapy (BNCT) in different biological models. In our laboratory, we have reported a methodology to produce cell imprints on polycarbonate through UV-C sensitization. The aim of this work is to extend the technique to tissue samples in order to enhance spatial resolution. As tissue structure largely differs from cultured cells, several aspects must be considered. We studied the influence of the parameters involved in the imprint and nuclear track formation, such as neutron fluence, different NTDs, etching and UV-C exposure times, tissue absorbance, thickness, and staining, among others. Samples from different biological models of interest for BNCT were used, exhibiting homogeneous and heterogeneous histology and boron microdistribution. The optimal conditions will depend on the animal model under study and the resolution requirements. Both the imprint sharpness and the fading effect depend on tissue thickness. While 6 h of UV-C was necessary to yield an imprint in CR-39, only 5 min was enough to observe clear imprints on Lexan. The information related to microdistribution of boron obtained with neutron autoradiography is of great relevance when assessing new boron compounds and administration protocols and also contributes to the study of the radiobiology of BNCT.

Background: The neutron autoradiography technique using nuclear track detectors (NTDs) is widely applied to study boron microdistribution in tissue samples previously infused with a boron compound. For this purpose, tissue sections are mounted on an NTD, and by irradiating the assembly with thermal neutrons, the ¹⁰B (n,α) ⁷Li reaction takes place and the resulting particles entering the detector cause permanent damage along their paths. This latent damage can be amplified at light microscopy level by a chemical etching, enabling their observation and quantification. Thus, mapping the nuclear tracks allows the assessment of the spatial distribution of ¹⁰B in the biological sample. For this technique, tissue sections are obtained by freeze slicing using a cryostatic microtome. In the transition to room temperature, they undergo an evaporation process that leads to a thickness reduction. Consequently, a natural amplification of the number of tracks in the detector occurs. Since evaporation implies a loss of weight in the sample, the ¹⁰B concentration can be corrected by an evaporation coefficient (CEv) defined as: CEv = m_s/m_h, where m_h corresponds to the “wet” section mass, measured immediately after the tissue is sectioned in the cryostat, and m_s is the “dry” section mass, measured when the evaporation process is finished. This study aimed to establish reproducible CEv reference values for healthy and tumour tissues across multiple tissues and species. Methods: A protocol previously established at our laboratory to determine CEv was applied to obtain reproducible results of evaporation curves and reference values of CEv for different tissues of interest. Measurements were carried out in normal liver, lung, kidney, and metastatic lung for different species: BDIX rats, nude mice, hamsters, and sheep. Moreover, different types of tissue coming from a hamster’s cheek pouch oral cancer model (tumor, normal pouch tissue, and precancerous tissue) were also analyzed. Results: Distributions of the CEv values for each tissue were studied and consistent results were obtained, allowing the establishment of reference values. Furthermore, it was observed that rat tissue CEvs correlate with the characteristic water content measured by desiccation in lung, liver, and kidney reported in the literature. Conclusions: These results will allow the correct quantification of boron in tissue sections through neutron autoradiography technique.

Background: The lung is the most frequent site of metastasis for many types of tumors, and for some types of cancer, it is the only site of metastatic lesions. In Argentina, a multi-institutional project has been established to assess the feasibility of applying boron neutron capture therapy (BNCT) ex-situ to the treatment of patients with multiple lung metastases, seeking to deliver a therapeutic dose to tumor tissue without exceeding the radiotolerance of the normal lung. The procedure consists of boron infusion and irradiation of the explanted organ followed by surgical reimplant, which requires the perfusion of the organ with a preservation solution. Within this context, the validity of the ovine model as an adequate human surrogate has been established. The objective of this work was to characterize the microdistribution of boron between broncho-vascular and alveolar regions in an ovine lung model and to determine how this heterogeneity affects photon-equivalent dose estimates for an ex-situ BNCT protocol. Methods: During a new boron biodistribution study of a normal sheep where boronophenylalanine (BPA)-fructose complex (350 mg/kg) was infused, lung samples were extracted for boron determination. Inductively coupled plasma optical emission spectroscopy (ICP-OES) measurements were carried out, and sections of normal lung were processed to obtain autoradiographic images using polycarbonate as nuclear track detector (NTD). The sample-detector assemblies were irradiated with a high fluence (10¹³ n/cm²) and etched for 4 min. This allowed for the analysis of the collective optical effect of nuclear tracks using optical density (OD) as the end point. Results: The analysis revealed a preferential uptake of BPA in the broncho-vascular tree at the microscopic level. OD measurements were performed in order to quantify this trend, as it could explain potential vascular damage in normal lung after BNCT. Regions of interest were delimited in the histological images, and the OD in each of them was calculated in the corresponding autoradiographies. These results showed that the broncho-vascular tree captures almost twice as much boron as the alveolar region. The same relation was observed in both pre- and post-perfusion lung samples, indicating that the uptake differences are not a consequence of the conservation process. The impact of these findings on the dosimetry of a potential BNCT ex-situ clinical setting was assessed. Conclusions: Photon equivalent dose in each tissue structure was determined by initially considering the boron concentration of the alveolar region and then using the boron concentration relative to the broncho-vascular tree. This analysis revealed that the percentage variances in the estimated dose in the organ voxels could reach up to 60%, with an average of 30%.