Salvage Boron Neutron Capture Therapy for Malignant Brain Tumor Patients in Compliance with Emergency and Compassionate Use: Evaluation of 34 Cases in Taiwan

Boric Acid Alters the Expression of DNA Double Break Repair Genes in MCF-7-Derived Breast Cancer Stem Cells

Breast cancer pathology ranks second in mortality among women worldwide due to the resistance of cancer stem cells in tumor tissue to radiotherapy and chemotherapy and their effective DNA damage response system (DDR). Targeting the expression of DNA double-strand break (DSB) repair genes in breast cancer stem cells (BC-SCs) is essential for facilitating their elimination with conventional therapies. This study aims to investigate the effects of boric acid (BA) on the expression of DNA DSB repair genes in BC-SCs, which has not been studied in the literature before. BS-SCs were isolated by the MACS method and characterized by flow cytometry. The effects of BA on BC-SCs' DNA DSB repair genes were deciphered by cell viability assay, inverted microscopy, and RT-qPCR. While the expression of the BRCA1 and BRCA2 was upregulated, the expression of the ATM (p < 0.001), RAD51 (p < 0.001), and KU70 (p < 0.001) was downregulated in dose-treated BC-SCs (p < 0.001) to the qPCR results. Consequently, BA affects some of the DNA DSB repair genes of breast cancer stem cells. Findings from this study could provide new insights into the potential therapeutic application of BA in BC-SC elimination and cancer intervention.

The impact of ZTE-based MR attenuation correction compared to CT-AC in 18F-FBPA PET before boron neutron capture therapy

Tumor-to-normal ratio (T/N) measurement of 18F-FBPA is crucial for patient eligibility to receive boron neutron capture therapy. This study aims to compare the difference in standard uptake value ratios on brain tumors and normal brains using PET/MR ZTE and atlas-based attenuation correction with the current standard PET/CT attenuation correction. Regarding the normal brain uptake, the difference was not significant between PET/CT and PET/MR attenuation correction methods. The T/N ratio of PET/CT-AC, PET/MR ZTE-AC and PET/MR AB-AC were 2.34 ± 0.95, 2.29 ± 0.88, and 2.19 ± 0.80, respectively. The T/N ratio comparison showed no significance using PET/CT-AC and PET/MR ZTE-AC. As for the PET/MRI AB-AC, significantly lower T/N ratio was observed (- 5.18 ± 9.52%; p < 0.05). The T/N difference between ZTE-AC and AB-AC was also significant (4.71 ± 5.80%; p < 0.01). Our findings suggested PET/MRI imaging using ZTE-AC provided superior quantification on 18F-FBPA-PET compared to atlas-based AC. Using ZTE-AC on 18F-FBPA-PET /MRI might be crucial for BNCT pre-treatment planning.

Prognostic assessment of 18F-boronophenylalanine positron emission tomography (BPA-PET) in salvage boron neutron capture therapy for malignant brain tumors

Background

Boron neutron capture therapy (BNCT) stands out as a propitious anti-cancer modality. 18F-boronophenylalanine positron emission tomography (BPA-PET) holds the potential to ascertain the concentration of BPA within the tumor, enabling meticulous treatment planning and outcome evaluation. However, no studies have been conducted on comparing the outcomes of those treated with BNCT to those who did not undergo this therapy. This study endeavors to analyze the correlation between BPA-PET and BNCT in the context of malignant brain tumors, and assess the survival outcomes following BNCT.

Methods

A cohort study was performed on patients who underwent BPA-PET between February 2017 and April 2022 in our hospital. Patients were stratified into two groups: those subjected to BNCT (Group 1) and those not (Group 2). The tumor to normal tissue (T/N) ratio derived from BPA-PET was set at 2.5. The findings were scrutinized based on clinical follow-up. Student's t-test and Chi-squared test were employed to discern differences between the groups. A cumulative survival curve was constructed employing the Kaplan-Meier method. Differences were considered statistically significant at P<0.05.

Results

In total, 116 patients with T/N ratios obtained from BPA-PET were enrolled. BNCT was administered to 58 patients, while mortality was observed in 100 patients. The median overall survival (OS) for the two groups was 8.5 and 6.0 months, respectively. The cumulative OS exhibited no significant discrepancy between the two groups, nor in their T/N ratios. Within Group 1, 44 out of 58 (75.9%) patients exhibited T/N ratios exceeding 2.5. Excluding 3 patients who expired within 3 months, 55 out of 58 patients were evaluated for response after BNCT. The objective response rate (ORR) was 30.9%. Patients achieving ORR displayed substantially higher survival rates compared to those without (median OS 13.5 vs. 8.3 months, P=0.0021), particularly when T/N ratio exceeded 2.5 (median OS 14.8 vs. 9.0 months, P=0.0199).

Conclusions

BNCT does not appear indispensable for prolonging the survival of patients afflicted with malignant brain tumors. Nevertheless, it proves advantageous when ORR is attained, a condition closely linked to the values of T/N ratio derived from BPA-PET.

A novel method for simultaneously measuring boronophenylalanine uptake in brain tumor cells and number of cells using inductively coupled plasma atomic emission spectroscopy

Boron neutron capture therapy (BNCT) combines neutron irradiation with boron compounds that are selectively uptaken by tumor cells. Boronophenylalanine (BPA) is a boron compound used to treat malignant brain tumors. The determination of boron concentration in cells is of great relevance to the field of BNCT. This study was designed to develop a novel method for simultaneously measuring the uptake of BPA by U87 and U251 cells (two brain tumor cell lines) and number of cells using inductively coupled plasma atomic emission spectroscopy (ICP-AES). The results revealed a linear correlation between phosphorus intensity and the numbers of U87 and U251 cells, with correlation coefficients (R2) of 0.9995 and 0.9994, respectively. High accuracy and reliability of phosphorus concentration standard curve were also found. Using this new method, we found that BPA had no significant effect on phosphorus concentration in either U87 or U251 cells. However, BPA increased the boron concentration in U87 and U251 cells in a concentration-dependent manner, with the boron concentration in U87 cells being higher than that in U251 cells. In both U87 and U251 cells, boron was mainly distributed in the cytoplasm and nucleus, accounting for 85% and 13% of the total boron uptake by U87 cells and 86% and 11% of the total boron uptake by U251 cells, respectively. In the U87 and U251 cell-derived xenograft (CDX) animal model, tumor exhibited higher boron concentration values than blood, heart, liver, lung, and brain, with a tumor/blood ratio of 2.87 for U87 cells and 3.11 for U251 cells, respectively. These results suggest that the phosphorus concentration in U87 and U251 cells can represent the number of cells and BPA is easily uptaken by tumor cells as well as in tumor tissue.

The current status and novel advances of boron neutron capture therapy clinical trials

Boron neutron capture therapy (BNCT) is a treatment method that focuses on improving the cure rate of patients with cancer who are difficult to treat using traditional clinical methods. By utilizing the high neutron absorption cross-section of boron, material rich in boron inside tumor cells can absorb neutrons and release high-energy ions, thereby destroying tumor cells. Owing to the short range of alpha particles, this method can precisely target tumor cells while minimizing the inflicted damage to the surrounding normal tissues, making it a potentially advantageous method for treating tumors. Globally, institutions have progressed in registered clinical trials of BNCT for multiple body parts. This review summarized the current achievements in registered clinical trials, Investigator-initiated clinical trials, aimed to integrate the latest clinical research literature on BNCT and to shed light on future study directions.

Amino Acid-Based Boron Carriers in Boron Neutron Capture Therapy (BNCT)

Interest in the design of boronated amino acids has emerged, partly due to the utilization of boronophenylalanine (BPA), one of the two agents employed in clinical Boron Neutron Capture Therapy (BNCT). The boronated amino acids synthesized thus far for BNCT investigations can be classified into two categories based on the source of boron: boronic acids or carboranes. Amino acid-based boron carriers, employed in the context of BNCT treatment, demonstrate significant potential in the treatment of challenging tumors, such as those located in the brain. This review aims to shed light on the developmental journey and challenges encountered over the years in the field of amino acid-based boron delivery compound development. The primary focus centers on the utilization of the large amino acid transporter 1 (LAT1) as a target for boron carriers in BNCT. The development of efficient carriers remains a critical objective, addressing challenges related to tumor specificity, effective boron delivery, and rapid clearance from normal tissue and blood. LAT1 presents an intriguing and promising target for boron delivery, given its numerous characteristics that make it well suited for drug delivery into tumor tissues, particularly in the case of brain tumors.

Targeting M-MDSCs enhances the therapeutic effect of BNCT in the 4-NQO-induced murine head and neck squamous cell carcinoma model

Purpose

Malignant head and neck squamous cell carcinoma (HNSCC) is characterized by a poor prognosis and resistance to conventional radiotherapy. Infiltrating myeloid-derived suppressive cells (MDSCs) is prominent in HNSCC and is linked to immune suppression and tumor aggressiveness. This study aimed to investigate the impact of boron neutron capture therapy (BNCT) on the MDSCs in the tumor microenvironment and peripheral blood and to explore the potential for MDSCs depletion combined with BNCT to reactivate antitumor immunity.

Methods and materials

Carcinogen, 4-NQO, -induced oral tumors were irradiated with a total physical dose of 2 Gy BNCT in Tsing Hua Open Reactor (THOR). Flow cytometry and immunohistochemistry accessed the dynamics of peripheral MDSCs and infiltrated MDSCs within the tumor microenvironment. Mice were injected with an inhibitor of CSF-1 receptor (CSF-1R), PLX3397, to determine whether modulating M-MDSCs could affect mice survival after BNCT.

Results

Peripheral CD11b+Ly6ChighLy6G- monocytic-MDSCs (M-MDSCs), but not CD11b+Ly6CloLy6Ghigh polymorphonuclear-MDSCs (PMN-MDSCs), increased as tumor progression. After BNCT treatment, there were temporarily decreased and persistent increases of M-MDSCs thereafter, either in peripheral blood or in tumors. The administration of PLX-3397 hindered BNCT-caused M-MDSCs infiltration, prolonged mice survival, and activated tumor immunity by decreasing tumor-associated macrophages (TAMs) and increasing CD8+ T cells.

Conclusion

M-MDSCs were recruited into 4-NQO-induced tumors after BNCT, and their number was also increased in peripheral blood. Assessment of M-MDSCs levels in peripheral blood could be an index to determine the optimal intervention window. Their temporal alteration suggests an association with tumor recurrence after BNCT, making M-MDSCs a potential intervention target. Our preliminary results showed that PLX-3397 had strong M-MDSCs, TAMs, and TIL (tumor-infiltrating lymphocyte) modulating effects that could synergize tumor control when combined with BNCT.

Nanostructured boron agents for boron neutron capture therapy: a review of recent patents

Boron neutron capture therapy (BNCT) is a potential radiation therapy modality for cancer, and tumor-targeted stable boron-10 (10B) delivery agents are an important component of BNCT. Currently, two low-molecular-weight boron-containing compounds, sodium mercaptoundecahydro-closo-dodecaborate (BSH) and boronophenylalanine (BPA), are mainly used in BNCT. Although both have suboptimal tumor selectivity, they have shown some therapeutic benefit in patients with high-grade glioma and several other tumors. To improve the efficacy of BNCT, great efforts have been devoted for the development of new boron delivery agents with better uptake and favorable pharmacokinetic profiles. This article reviews the application and research progress of boron nanomaterials as boron carriers in boron neutron capture therapy and hopes to stimulate people's interest in nanomaterial-based delivery agents by summarizing various kinds of boron nanomaterial patents disclosed in the past decade.

Correcting for the heterogeneous boron distribution in a tumor for BNCT dose calculation

Most treatment planning systems of boron neutron capture therapy perform dose calculations based on the assumption of a homogeneous boron distribution in tumors, which leads to dose distortion due to the difference between the tumor-to-normal tissue ratio (TNR) range measured in positron emission tomography images (PET) and the target delineation in computed tomography images of the treatment plan. The heterogeneous boron distribution in the target of the treatment plan can be obtained by image fusion. This study provides a way to quantify a heterogeneous boron distribution based on PET images. Theoretically, the same mean TNR for dose calculation by homogeneous or heterogeneous boron distribution should get almost the same mean dose. However, slightly different mean doses are found due to the partial volume effect for a small target volume. The wider the boron distribution is, the higher the impact on the dose-volume histogram distribution is. Dose distribution with homogeneous boron distribution may be overestimated in low boron uptake regions by wrong boron concentration and neutron flux depression. To accurately give the tumor prescription dose and achieve better tumor control, for low dose regions of the tumor should be considered more boron neutron capture therapy treatments or combined with other treatment modalities. The heterogeneous boron distribution must be taken into consideration to have an accurate dose estimation. Therefore, the way how medical physicists and clinicians process the TNR in gross tumor volume should be refined, and the method demonstrated in the work provides a good reference.

Optimizing Boron Neutron Capture Therapy (BNCT) to Treat Cancer: An Updated Review on the Latest Developments on Boron Compounds and Strategies

Boron neutron capture therapy (BNCT) is a tumor-selective particle radiotherapy. It combines preferential boron accumulation in tumors and neutron irradiation. The recent initiation of BNCT clinical trials employing hospital-based accelerators rather than nuclear reactors as the neutron source will conceivably pave the way for new and more numerous clinical trials, leading up to much-needed randomized trials. In this context, it would be interesting to consider the implementation of new boron compounds and strategies that will significantly optimize BNCT. With this aim in mind, we analyzed, in this review, those articles published between 2020 and 2023 reporting new boron compounds and strategies that were proved therapeutically useful in in vitro and/or in vivo radiobiological studies, a critical step for translation to a clinical setting. We also explored new pathologies that could potentially be treated with BNCT and newly developed theranostic boron agents. All these radiobiological advances intend to solve those limitations and questions that arise during patient treatment in the clinical field, with BNCT and other therapies. In this sense, active communication between clinicians, radiobiologists, and all disciplines will improve BNCT for cancer patients, in a cost- and time-effective way.

Enhanced Resolution of Neutron Autoradiography with UV-C Sensitization to Study Boron Microdistribution in Animal Models

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.

Measurement of an evaporation coefficient in tissue sections as a correction factor for 10B determination

Boron neutron capture therapy (BNCT) is a cancer treatment option that combines preferential uptake of a boron compound in tumors and irradiation with thermal neutrons. For treatment planning, the boron concentration in different tissues must be considered. Neutron autoradiography using nuclear track detectors (NTD) can be applied to study both the concentration and microdistribution of boron in tissue samples. Histological sections are obtained from frozen tissue by cryosectioning. When the samples reach room temperature, they undergo an evaporation process, which leads to an increase in the boron concentration. To take this effect into account, certain correction factors (evaporation coefficients, CEv) must be applied. With this aim, a protocol was established to register and analyze mass variation of tissue sections, measured with a semimicro scale. Values of ambient temperature, pressure, and humidity were simultaneously recorded. Reproducible results of evaporation curves and CEv values were obtained for different tissue samples, which allowed the systematization of the procedure. This study could contribute to a more precise determination of boron concentration in tissue samples through the neutron autoradiography technique, which is of great relevance to make dosimetric calculations in BNCT.

Boron uptake of boronophenylalanine and the effect of boron neutron capture therapy in cervical cancer cells

There are few studies about boron neutron capture therapy (BNCT) for cervical cancer. The present study evaluated the biodistribution of boronophenylalanine (BPA) and the effect of BNCT on cervical cancer cell lines. BPA exposure and neutron irradiation of cervical cancer cell lines resulted in decreased survival fraction compared to irradiation only. In vivo cervical cancer tumor boron concentration was highest at 2.5 h after BPA intraperitoneal administration, and higher than in the other organs. BNCT may be effective against cervical carcinoma.

NeuTHOR Station—A Novel Integrated Platform for Monitoring BNCT Clinical Treatment, Animal and Cell Irradiation Study at THOR

(1) Background: A well-established Boron Neutron Capture Therapy (BNCT) facility includes many essential systems, which are the epithermal neutron beam system, on-line monitoring system (OMS), QA/QC (quality assurance or quality control) system, boron concentration (BC) measurement system, and treatment planning system (TPS). Accurate data transmission, monitoring, and deposition among these systems are of vital importance before, during, and after clinical, animal, and cell BNCT irradiation. This work developed a novel integrated platform NeuTHOR Station (NeuTHORS) for BNCT at Tsing Hua Open-pool Reactor (THOR). Apart from the data of the OMS and QA/QC system, the data of BC and TPS can be loaded on NeuTHORS before BNCT clinical, animal, and cell irradiation. (2) Methods: A multi-paradigm computer programming language c# (c sharp) was used to develop the integrated platform NeuTHORS. The design of NeuTHORS is based on the standard procedures of BNCT treatment or experiment at THOR. Moreover, parallel testing with OMS-BNCT (the former OMS) and QA/QC of THOR was also performed for more than 70 times to verify the validation of NeuTHORS. (3) Results: According to the comparisons of the output, NeuTHORS and OMS-BNCT and QA/QC of THOR show very good consistency. NeuTHORS is now installed on an industrial PC (IPC) and successfully performs the monitoring of BNCT Treatment at THOR. Patients’ f BC and TPS data are also input into NeuTHORS and stored on IPC through an internal network from BC measurement room and TPS physicist. Therefore, the treatment data of each patient can be instantaneously established after each BNCT treatment for further study on BNCT. NeuTHORS can also be applied on data acquisition for a BNCT-related study, especially for animal or cell irradiation experiments. (4) Conclusions: A novel integrated platform NeuTHOR Station for monitoring BNCT clinical treatment and animal and cell irradiation study has been successfully established at THOR. With this platform, BNCT radiobiology investigations will be efficiently performed and a thorough data storage and analysis system of BNCT treatments or experiments can thus be systematically built up for the further investigation of BNCT at THOR.

Advances in Boron Neutron Capture Therapy (BNCT) for Recurrent Intracranial Meningioma

Meningiomas are the most frequently diagnosed primary intracranial tumors in adults. Surgical resection is preferred if the meningioma is accessible; for those that are not suitable for surgical resection, radiotherapy should be considered to improve local tumor control. However, recurrent meningiomas are challenging to treat, as the recurrent tumor might be located in the previously irradiated area. Boron Neutron Capture Therapy (BNCT) is a highly selective radiotherapy modality in which the cytotoxic effect focuses mainly on cells with increased uptake of boron-containing drugs. In this article, we describe four patients with recurrent meningiomas treated with BNCT in Taiwan. The mean boron-containing drug tumor-to-normal tissue uptake ratio was 4.125, and the tumor mean dose was 29.414 GyE, received via BNCT. The treatment response showed two stable diseases, one partial response, and one complete response. We also introduce and support the effectiveness and safety of BNCT as an alternative salvage treatment for recurrent meningiomas.

Salvage boron neutron capture therapy for pediatric patients with recurrent diffuse midline glioma

PURPOSE

Pediatric diffuse malignant glioma located in the brainstem was officially named "diffuse midline glioma" (DMG) by the World Health Organization in 2016. For this disease, radical surgery is not beneficial, and the only major treatment strategy is radiotherapy. However, the dose limitations to brainstem tissue mean that treatment by radiotherapy can only control and not eradicate the tumors, and there is no effective treatment for recurrence, resulting in short overall survival of 6-12 months. This paper reports our experience with boron neutron capture therapy (BNCT), a new treatment process, and its efficacy in treating children with recurrent DMG.

METHODS

From September 2019 to July 2022, we treated 6 children affected by recurrent DMG. With the collaboration of Taipei Veteran General Hospital (TVGH) and National Tsing-Hua University (NTHU), each patient received two sessions of BNCT within 1 month.

RESULTS

Among the six patients, three showed partial response and the rest had stable disease after the treatment. The overall survival and recurrence-free survival duration after treatment were 6.39 and 4.35 months, respectively. None of the patients developed severe side effects, and only one patient developed brain necrosis, which was most likely resulted from previous hypofractionated radiotherapy received.

CONCLUSION

BNCT elicited sufficient tumor response with low normal tissue toxicity; it may benefit vulnerable pediatric patients with DMG.

Therapeutic Efficacy and Radiobiological Effects of Boric-Acid-Mediated BNCT in an Osteosarcoma-Bearing SD Rat Model

BACKGROUND

Osteosarcoma (OS) is the most common primary malignancy of the bone and is notoriously resistant to radiation therapy. High-dose cytotoxic chemotherapy and surgical resection have improved the survival rate and prognosis of patients with OS. Nonetheless, treatment challenges remain when the tumor cannot be removed by surgery. Boron neutron capture therapy (BNCT) provides high linear energy transfer (LET) radiation, and its internal targeted characteristics make BNCT a novel therapy for removing OS and reducing radiation damage to adjacent healthy tissues.

METHODS

In this study, a UMR-106-grafted OS rat model was developed, and boric acid (BA) was used as the boron drug for BNCT. The pharmacokinetics of BA, following intravenous injection, were evaluated to determine the optimal time window for neutron irradiation. OS-bearing rats were irradiated by an epithermal neutron beam at Tsing Hua Open-Pool Reactor (THOR). The therapeutic efficacy of and tissue response after BNCT were evaluated by radiographic and histopathological observations.

RESULTS

OS-bearing rats were irradiated by neutrons in the first hour following the intravenous injection of BA. The prescription-absorbed doses in the tumor regions were 5.8 and 11.0 Gy. BNCT reduced the body weight of the tumor-bearing rats, but they recovered after a few days. The BA-mediated BNCT effectively controlled the orthotopic OS tumor, reduced osteolysis, and induced bone healing. Autoradiography and histological analysis confirmed that the BA retention region is consistent with the calcification region in OS tissue.

CONCLUSION

BA is specifically retained in OS, and the BA-mediated BNCT can significantly reduce the tumor burden and osteolysis in OS-bearing rats.

Multi-Targeted Neutron Capture Therapy Combined with an 18 kDa Translocator Protein-Targeted Boron Compound Is an Effective Strategy in a Rat Brain Tumor Model

BACKGROUND

Boron neutron capture therapy (BNCT) has been adapted to high-grade gliomas (HG); however, some gliomas are refractory to BNCT using boronophenylalanine (BPA). In this study, the feasibility of BNCT targeting the 18 kDa translocator protein (TSPO) expressed in glioblastoma and surrounding environmental cells was investigated.

METHODS

Three rat glioma cell lines, an F98 rat glioma bearing brain tumor model, DPA-BSTPG which is a boron-10 compound targeting TSPO, BPA, and sodium borocaptate (BSH) were used. TSPO expression was evaluated in the F98 rat glioma model. Boron uptake was assessed in three rat glioma cell lines and in the F98 rat glioma model. In vitro and in vivo neutron irradiation experiments were performed.

RESULTS

DPA-BSTPG was efficiently taken up in vitro. The brain tumor has 16-fold higher TSPO expressions than its brain tissue. The compound biological effectiveness value of DPA-BSTPG was 8.43 to F98 rat glioma cells. The boron concentration in the tumor using DPA-BSTPG convection-enhanced delivery (CED) administration was approximately twice as high as using BPA intravenous administration. BNCT using DPA-BSTPG has significant efficacy over the untreated group. BNCT using a combination of BPA and DPA-BSTPG gained significantly longer survival times than using BPA alone.

CONCLUSION

DPA-BSTPG in combination with BPA may provide the multi-targeted neutron capture therapy against HG.

Failure modes and effects analysis study for accelerator-based Boron Neutron Capture Therapy

BACKGROUND

Boron Neutron Capture Therapy (BNCT) has recently been used in clinical oncology thanks to recent developments of accelerator-based BNCT systems. Although there are some specific processes for BNCT, they have not yet been discussed in detail.

PURPOSE

The aim of this study is to provide comprehensive data on the risk of accelerator-based BNCT system to institutions planning to implement an accelerator-based BNCT system.

METHODS

In this study, failure mode and effects analysis (FMEA) was performed based on a treatment process map prepared for the accelerator-based BNCT system. A multidisciplinary team consisting of a medical doctor (MD), a registered nurse (RN), two medical physicists (MP), and three radiologic technologists (RT) identified the failure modes (FMs). Occurrence (O), severity (S), and detectability (D) were scored on a scale of 10, respectively. For each failure mode (FM), risk priority number (RPN) was calculated by multiplying the values of O, S, and D, and it was then categorized as high risk, very high risk, and other. Additionally, FMs were statistically compared in terms of countermeasures, associated occupations, and whether or not they were the patient-derived.

RESULTS

The identified FMs for BNCT were 165 in which 30 and 17 FMs were classified as high risk and very high risk, respectively. Additionally, 71 FMs were accelerator-based BNCT-specific FMs in which 18 and 5 FMs were classified as high risk and very high risk, respectively. The FMs for which countermeasures were "Education" or "Confirmation" were statistically significantly higher for S than the others (p = 0.019). As the number of BNCT facilities is expected to increase, staff education is even more important. Comparing patient-derived and other FMs, O tended to be higher in patient-derived FMs. This could be because the non-patient-derived FMs included events that could be controlled by software, whereas the patient-derived FMs were impossible to prevent and might also depend on the patient's condition. Alternatively, there were non-patient-derived FMs with higher D, which were difficult to detect mechanically and were classified as more than high risk. In O, significantly higher values (p = 0.096) were found for FMs from MD and RN associated with much patient intervention compared to FMs from MP and RT less patient intervention. Comparing conventional radiotherapy and accelerator-based BNCT, although there were events with comparable risk in same FMs, there were also events with different risk in same FMs. They could be related to differences in the physical characteristics of the two modalities.

CONCLUSIONS

This study is the first report for conducting a risk analysis for BNCT using FMEA. Thus, this study provides comprehensive data needed for quality assurance/quality control (QA/QC) in the treatment process for facilities considering the implementation of accelerator-based BNCT in the future. Because many BNCT-specific risks were discussed, it is important to understand the characteristics of BNCT and to take adequate measures in advance. If the effects of all FMs and countermeasures are discussed by multidisciplinary team, it will be possible to take countermeasures against individual FMs from many perspectives and provide BNCT more safely and effectively.

Strategies for Preclinical Studies Evaluating the Biological Effects of an Accelerator-Based Boron Neutron Capture Therapy System

This review discusses the strategies of preclinical studies intended for accelerator-based (AB)-boron neutron capture therapy (BNCT) clinical trials, which were presented at the National Cancer Institute (NCI) Workshop on Neutron Capture Therapy held from April 20 to 22, 2022. Clinical studies of BNCT have been conducted worldwide using reactor neutron sources, with most targeting malignant brain tumors, melanoma, or head and neck cancer. Recently, small accelerator-based neutron sources that can be installed in hospitals have been developed. AB-BNCT clinical trials for recurrent malignant glioma, head and neck cancers, high-grade meningioma, melanoma, and angiosarcoma have all been conducted in Japan. The necessary methods, equipment, and facilities for preclinical studies to evaluate the biological effects of AB-BNCT systems in terms of safety and efficacy are described, with reference to two examples from Japan. The first is the National Cancer Center, which is equipped with a vertical downward neutron beam, and the other is the University of Tsukuba, which has a horizontal neutron beam. The preclinical studies discussed include cell-based assays to evaluate cytotoxicity and genotoxicity, in vivo cytotoxicity and efficacy of BNCT, and radioactivation measurements.

Animal Tumor Models for Boron Neutron Capture Therapy Studies (Excluding Central Nervous System Solid Tumors)

Translational research in adequate experimental models is necessary to optimize boron neutron capture therapy (BNCT) for different pathologies. Multiple radiobiological in vivo studies have been performed in a wide variety of animal models, studying multiple boron compounds, routes of compound administration, and a range of administration strategies. Animal models are useful for the study of the stability and potential toxicity of new boron compounds or delivery systems, BNCT theranostic strategies, the evaluation of biomarkers to monitor BNCT therapeutic and adverse effects, and to study the BNCT immune response by the host against tumor cells. This article will mention examples of these studies, highlighting the importance of experimental animal models for the advancement of BNCT. Animal models are essential to design novel, safe, and effective clinical BNCT protocols for existing or new targets for BNCT.

Evaluation of the Key Advantages between Two Modalities of Boronophenylalanine Administration for Clinical Boron Neutron Capture Therapy Using an Animal Model

In clinical boron neutron capture therapy (BNCT), boronophenylalanine (BPA) administrations through one-step infusion (OSI) and two-step infusion (TSI) are the most widely used. This study compared the advantages of OSI and TSI using a human oral squamous cell carcinoma-bearing animal model. OSI was administered at a high-dose rate of 20 mg/kg/min for 20 min (total dose: 400 mg/kg) as the first step infusion. TSI was a prolonged infusion at a low-dose rate of 1.67 mg/kg/min for 15, 30, 45, and 60 min (total dose: 25, 50, 75, and 100 mg/kg) following the first step infusion. The sigmoid Emax model was used to evaluate the boron accumulation effect in the tumor. The advantages of TSI were observed to be greater than those of OSI. The observed advantages of TSI were as follows: a stable level of boron concentration in blood; tumor to blood boron ratio (T/B); tumor to muscle boron ratio (T/M); and skin to blood boron ratio (S/B). The boron accumulation effect in tumors increased to 68.98%. Thus, effective boron concentration in these tumor cells was achieved to enhance the lethal damage in BNCT treatment. Boron concentration in the blood was equal to that in the skin. Therefore, the equivalent dose was accurately estimated for the skin.

The Anti-Tumor Effect of Boron Neutron Capture Therapy in Glioblastoma Subcutaneous Xenograft Model Using the Proton Linear Accelerator-Based BNCT System in Korea

Boron neutron capture therapy (BNCT) is a radiation therapy that selectively kills cancer cells and is being actively researched and developed around the world. In Korea, development of the proton linear accelerator-based BNCT system has completed development, and its anti-cancer effect in the U-87 MG subcutaneous xenograft model has been evaluated. To evaluate the efficacy of BNCT, we measured ¹⁰B-enriched boronophenylalanine (BPA) uptake in U-87 MG, FaDu, and SAS cells and evaluated cell viability by clonogenic assays. In addition, the boron concentration in the tumor, blood, and skin on the U-87 MG xenograft model was measured, and the tumor volume was measured for 4 weeks after BNCT. In vitro, the intracellular boron concentration was highest in the order of SAS, FaDu, and U-87 MG, and cell survival fractions decreased depending on the BPA treatment concentration and neutron irradiation dose. In vivo, the tumor volume was significantly decreased in the BNCT group compared to the control group. This study confirmed the anti-cancer effect of BNCT in the U-87 MG subcutaneous xenograft model. It is expected that the proton linear accelerator-based BNCT system developed in Korea will be a new option for radiation therapy for cancer treatment.

Exploring the Biological and Physical Basis of Boron Neutron Capture Therapy (BNCT) as a Promising Treatment Frontier in Breast Cancer

Simple Summary

BNCT is a biologically targeted, densely ionizing form of radiation therapy that allows for increased tumor cell kill, while reducing toxicity to surrounding normal tissues. Although BNCT has been investigated in the treatment of head and neck cancers and recurrent brain tumors, its applicability to breast cancer has not been previoulsy investigated. In this review we discuss the physical and biological properties of various boronated compounds, and advantages and challenges associated with the potential use of BNCT in the treatment of breast cancer.

Abstract

BNCT is a high LET radiation therapy modality that allows for biologically targeted radiation delivery to tumors while reducing normal tissue impacts. Although the clinical use of BNCT has largely been limited to phase I/II trials and has primarily focused on difficult-to-treat malignancies such as recurrent head and neck cancer and recurrent gliomas, recently there has been a renewed interest in expanding the use of BNCT to other disease sites, including breast cancer. Given its high LET characteristics, its biologically targeted and tumor specific nature, as well as its potential for use in complex treatment settings including reirradiation and widespread metastatic disease, BNCT offers several unique advantages over traditional external beam radiation therapy. The two main boron compounds investigated to date in BNCT clinical trials are BSH and BPA. Of these, BPA in particular shows promise in breast cancer given that is taken up by the LAT-1 amino acid transporter that is highly overexpressed in breast cancer cells. As the efficacy of BNCT is directly dependent on the extent of boron accumulation in tumors, extensive preclinical efforts to develop novel boron delivery agents have been undertaken in recent years. Preclinical studies have shown promise in antibody linked boron compounds targeting ER/HER2 receptors, boron encapsulating liposomes, and nanoparticle-based boron delivery systems. This review aims to summarize the physical and biological basis of BNCT, the preclinical and limited clinical data available to date, and discuss its potential to be utilized for the successful treatment of various breast cancer disease states.

Clinical Viability of Boron Neutron Capture Therapy for Personalized Radiation Treatment

Simple Summary

Usually, for dose planning in radiotherapy, the tumor is delimited as a volume on the image of the patient together with other clinical considerations based on populational evidence. However, the same prescription dose can provide different results, depending on the patient. Unfortunately, the biological aspects of the tumor are hardly considered in dose planning. Boron Neutron Capture Radiotherapy enables targeted treatment by incorporating boron-10 at the cellular level and irradiating with neutrons of a certain energy so that they produce nuclear reactions locally and almost exclusively damage the tumor cell. This technique is not new, but modern neutron generators and more efficient boron carriers have reactivated the clinical interest of this technique in the pursuit of more precise treatments. In this work, we review the latest technological facilities and future possibilities for the clinical implementation of BNCT and for turning it into a personalized therapy.

Abstract

Boron Neutron Capture Therapy (BNCT) is a promising binary disease-targeted therapy, as neutrons preferentially kill cells labeled with boron (¹⁰B), which makes it a precision medicine treatment modality that provides a therapeutic effect exclusively on patient-specific tumor spread. Contrary to what is usual in radiotherapy, BNCT proposes cell-tailored treatment planning rather than to the tumor mass. The success of BNCT depends mainly on the sufficient spatial biodistribution of ¹⁰B located around or within neoplastic cells to produce a high-dose gradient between the tumor and healthy tissue. However, it is not yet possible to precisely determine the concentration of ¹⁰B in a specific tissue in real-time using non-invasive methods. Critical issues remain to be resolved if BNCT is to become a valuable, minimally invasive, and efficient treatment. In addition, functional imaging technologies, such as PET, can be applied to determine biological information that can be used for the combined-modality radiotherapy protocol for each specific patient. Regardless, not only imaging methods but also proteomics and gene expression methods will facilitate BNCT becoming a modality of personalized medicine. This work provides an overview of the fundamental principles, recent advances, and future directions of BNCT as cell-targeted cancer therapy for personalized radiation treatment.

Compassionate Treatment of Brainstem Tumors with Boron Neutron Capture Therapy: A Case Series

Brainstem tumors are heterogenous and cancerous glioma tumors arising from the midbrain, pons, and the medulla that are relatively common in children, accounting for 10% to 20% of all pediatric brain tumors. However, the prognosis of aggressive brainstem gliomas remains extremely poor despite aggressive treatment with chemotherapy and radiotherapy. That means there are many life-threatening patients who have exhausted all available treatment options and are beginning to face end-of-life stage. Therefore, the unique properties of highly selective heavy particle irradiation with boron neutron capture therapy (BNCT) may be well suited to prolong the lives of patients with end-stage brainstem gliomas. Herein, we report a case series of life-threatening patients with end-stage brainstem glioma who eligible for Emergency and Compassionate Use, in whom we performed a scheduled two fractions of salvage BNCT strategy with low treatment dosage each time. No patients experienced acute or late adverse events related to BNCT. There were 3 patients who relapsed after two fractionated BNCT treatment, characterized by younger age, lower T/N ratio, and receiving lower treatment dose. Therefore, two fractionated low-dose BNCT may be a promising treatment for end-stage brainstem tumors. For younger patients with low T/N ratios, more fractionated low-dose BNCT should be considered.

Importance of radiobiological studies for the advancement of boron neutron capture therapy (BNCT)

Boron neutron capture therapy (BNCT) is a tumour selective particle radiotherapy, based on the administration of boron carriers incorporated preferentially by tumour cells, followed by irradiation with a thermal or epithermal neutron beam. BNCT clinical results to date show therapeutic efficacy, associated with an improvement in patient quality of life and prolonged survival. Translational research in adequate experimental models is necessary to optimise BNCT for different pathologies. This review recapitulates some examples of BNCT radiobiological studies for different pathologies and clinical scenarios, strategies to optimise boron targeting, enhance BNCT therapeutic effect and minimise radiotoxicity. It also describes the radiobiological mechanisms induced by BNCT, and the importance of the detection of biomarkers to monitor and predict the therapeutic efficacy and toxicity of BNCT alone or combined with other strategies. Besides, there is a brief comment on the introduction of accelerator-based neutron sources in BNCT. These sources would expand the clinical BNCT services to more patients, and would help to make BNCT a standard treatment modality for various types of cancer. Radiobiological BNCT studies have been of utmost importance to make progress in BNCT, being essential to design novel, safe and effective clinical BNCT protocols.

Boron neutron capture therapy for urological cancers

Boron neutron capture therapy is based on a nuclear reaction between the nonradioactive isotope boron-10 and either low-energy thermal neutrons or high-energy epithermal neutrons, which generate high linear energy transfer α particles and a recoiled lithium nucleus (⁷ Li) that selectively destroys the DNA helix in tumor cells. Boron neutron capture therapy is an emerging procedure aimed at improving the therapeutic ratio for the traditional treatment of various malignancies, which has been studied clinically in a variety of diseases, including glioblastoma, head and neck cancer, cutaneous melanoma, hepatocellular carcinoma, lung cancer, and extramammary Paget's disease. However, boron neutron capture therapy has not been clinically performed for urological cancers, excluding genital extramammary Paget's disease that appeared at the scrotum to penis area. In this review, we aimed to provide an updated summary of the current clinical literature of patients treated with boron neutron capture therapy and to focus on the future prospects of boron neutron capture therapy for urological cancers.

Boron neutron capture therapy and add-on bevacizumab in patients with recurrent malignant glioma

BACKGROUND

Although boron neutron capture therapy has shown excellent survival data, previous studies have shown an increase in radiation necrosis against recurrent malignant glioma. Herein, we proposed that bevacizumab may reduce radiation injury from boron neutron capture therapy by re-irradiation. We evaluated the efficacy and safety of a boron neutron capture therapy and add-on bevacizumab combination therapy in patients with recurrent malignant glioma.

METHODS

Patients with recurrent malignant glioma were treated with reactor-based boron neutron capture therapy. Treatment with bevacizumab (10 mg/kg) was initiated 1-4 weeks after boron neutron capture therapy and was administered every 2-3 weeks until disease progression. Initially diagnosed glioblastomas were categorized as primary glioblastoma, whereas other forms of malignant glioma were categorized as non-primary glioblastoma.

RESULTS

Twenty-five patients (14 with primary glioblastoma and 11 with non-primary glioblastoma) were treated with boron neutron capture therapy and add-on bevacizumab. The 1-year survival rate for primary glioblastoma and non-primary glioblastoma was 63.5% (95% confidence interval: 33.1-83.0) and 81.8% (95% confidence interval: 44.7-95.1), respectively. The median overall survival was 21.4 months (95% confidence interval: 7.0-36.7) and 73.6 months (95% confidence interval: 11.4-77.2) for primary glioblastoma and non-primary glioblastoma, respectively. The median progression-free survival was 8.3 months (95% confidence interval: 4.2-12.1) and 15.6 months (95% confidence interval: 3.1-29.8) for primary glioblastoma and non-primary glioblastoma, respectively. Neither pseudoprogression nor radiation necrosis were identified during bevacizumab treatment. Alopecia occurred in all patients. Six patients experienced adverse events ≥grade 3.

CONCLUSIONS

Boron neutron capture therapy and add-on bevacizumab provided a long overall survival and a long progression-free survival in recurrent malignant glioma compared with previous studies on boron neutron capture therapy alone. The add-on bevacizumab may reduce the detrimental effects of boron neutron capture therapy, including pseudoprogression and radiation necrosis. Further studies of the combination therapy with a larger sample size and a randomized controlled design are warranted.

Development of an Imaging Technique for Boron Neutron Capture Therapy

The development of 4-¹⁰B-borono-2-¹⁸F-fluoro-L-phenylalanine (¹⁸FBPA) for use in positron emission tomography (PET) has contributed to the progress of boron neutron capture therapy (BNCT). ¹⁸FBPA has shown similar pharmacokinetics and distribution to 4-¹⁰B-borono-L-phenylalanine (BPA) under various conditions in many animal studies. ¹⁸FBPA PET is useful for treatment indication. A higher ¹⁸FBPA accumulation ratio of the tumor to the surrounding normal tissue (T/N ratio) indicates that a superior treatment effect is expected. In clinical settings, a T/N ratio of higher than 2.5 or 3 is often used for patient selection. Moreover, ¹⁸FBPA PET is useful for predicting the ¹⁰B concentration delivered to the tumor and surrounding normal tissues, enabling high-precision treatment planning. Precise dose prediction using ¹⁸FBPA PET data has greatly improved the treatment accuracy of BNCT. However, the methodology used for the data analysis of ¹⁸FBPA PET findings varies; thus, data should be evaluated using a consistent methodology so as to be more reliable. In addition to PET applications, the development of ¹⁸FBPA as a contrast agent for magnetic resonance imaging that combines gadolinium and ¹⁰B is also in progress.