First Attempt of Boron Neutron Capture Therapy (BNCT) for Hepatocellular Carcinoma

Effect of neutron beam properties on dose distributions in a water phantom for boron neutron capture therapy

From the viewpoints of the advantage depths (ADs), peak tumor dose and skin dose, we evaluated the effect on the dose distribution of neutron beam properties, namely the ratio between thermal and epithermal neutron fluxes (thermal/epithermal ratio), fast neutron component and γ-ray component. Several parameter surveys were conducted with respect to the beam properties of neutron sources for boron neutron capture therapy assuming boronophenylalanine as the boron agent using our dose calculation tool, called SiDE. The ADs decreased by 3% at a thermal/epithermal ratio of 20-30% compared with the current recommendation of 5%. The skin dose increased with the increasing thermal/epithermal ratio, reaching a restricted value of 14 Gyeq at a thermal/epithermal ratio of 48%. The fast neutron component was modified using two different models, namely the 'linear model', in which the fast neutron intensity decreases log-linearly with the increasing neutron energy, and the 'moderator thickness (MT) model', in which the fast neutron component is varied by adjusting the MT in a virtual beam shaping assembly. Although a higher fast neutron component indicated a higher skin dose, the increment was <10% at a fast neutron component of <1 × 10-12 Gy cm2 for both models. Furthermore, in the MT model, the epithermal neutron intensity at a fast neutron component of 6.8 × 10-13 Gy cm2 was 41% higher compared with that of 2 × 10-13 Gy cm2. The γ-ray component also caused no significant disadvantages up to several times larger compared with the current recommendation.

A clinician's perspective on boron neutron capture therapy: promising advances, ongoing trials, and future outlook

PURPOSE

This comprehensive review aims to provide a unique clinical perspective on the latest advances and ongoing boron neutron capture therapy (BNCT) trials for various cancers.

METHODS

We critically analyzed clinical data from BNCT trials for head and neck cancer, glioblastoma, melanoma, meningioma, breast cancer, and liver tumors. We investigated differences in tumor responses and normal tissue toxicities among trials and discussed potential contributing factors. We also identified the limitations of early BNCT trials and proposed strategies to optimize future trial design.

RESULTS

BNCT has shown promising results in treating head and neck cancer, with high response rates and improved survival in patients with recurrent disease. In glioblastoma, BNCT combined with surgery and chemotherapy has demonstrated survival benefits compared to standard treatments. BNCT has also been successfully used for recurrent high-grade meningiomas and shows potential for melanomas, extramammary Paget's disease, and liver tumors. However, differences in tumor responses and toxicities were observed among trials, potentially attributable to variations in treatment protocols, patient characteristics, and evaluation methods.

CONCLUSIONS

BNCT is a promising targeted radiotherapy for various cancers. Further optimization and well-designed randomized controlled trials are needed to establish its efficacy and safety. Future studies should focus on standardizing treatment protocols and addressing limitations to guide clinical decision-making and research priorities.

Nucleoside Scaffolds and Carborane Clusters for Boron Neutron Capture Therapy: Developments and Future Perspective

Nucleosides containing carboranes are one of the most important boron delivery agents for boron neutron capture therapy, BNCT, which are good substrates of hTK1. The development of several nucleosides containing carboranes at early stages led to the discovery of the first generation of 3CTAs by incorporating a hydrocarbon spacer between the thymidine scaffold and carborane cluster and attaching dihydroxylpropyl group on the second carbon (C2) atom of the carborane cluster (e.g., N5 and N5-2OH). Phosphorylation rate, tumor cellular uptake, and retention have been evaluated in parallel to change the length of the tether arm of spacers in these compounds. Many attempts were reported and discussed to overcome the disadvantage of the first generation of 3CTAs by a) incorporating modified spacers between thymidine and carborane clusters, such as ethyleneoxide, polyhydroxyl, triazole, and tetrazole units, b) attaching hydrophilic groups at C2 of the carborane cluster, c) transforming lipophilic closo-carboranes to hydrophilic nidocarborane. The previous modifications represented the second generation of 3CTAs to improve the hydrogen bond formation with the hTK1 active site. Moreover, amino acid prodrugs were developed to enhance biological and physicochemical properties. The structure-activity relationship (SAR) of carboranyl thymidine analogues led to the roadmap for the development of the 3rd generation of the 3CTAs for BNCT.

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.

Experimentally determined relative biological effectiveness of cyclotron-based epithermal neutrons designed for clinical BNCT: in vitro study

A neutron beam for boron neutron capture therapy (BNCT) of deep-seated tumours is designed to maintain a high flux of epithermal neutrons, while keeping the thermal and fast neutron component as low as possible. These neutrons (thermal and fast) have a high relative biological effectiveness in comparison with high energy photon beams used for conventional X-ray radiotherapy. In the past, neutrons for the purpose of BNCT were generated using nuclear reactors. However, there are various challenges that arise when installing a reactor in a hospital environment. From 2006, the Kyoto University Research Reactor Institute, in collaboration with Sumitomo Heavy Industries, began the development of an accelerator-based neutron source for clinical BNCT in a bid to overcome the shortcomings of a nuclear reactor-based neutron source. Following installation and beam performance testing, in vitro studies were performed to assess the biological effect of the neutron beam. Four different cell lines were prepared and irradiated using the accelerator-based neutron source. Following neutron and gamma ray irradiation, the survival curve for each cell line was calculated. The biological end point to determine the relative biological effectiveness (RBE) was set to 10% cell survival, and the D10 for each cell line was determined. The RBE of the accelerator-based neutron beam was evaluated to be 2.62.

Influence of lung physical density on dose calculation in boron neutron capture therapy for malignant pleural mesothelioma

The boron neutron capture therapy treatment planning systems such as SERA and TSUKUBA Plan, which are mainly based on the Monte Carlo method, require the lung physical density and composition of the tissue for the dose calculation. However, the physical density and composition of lungs may change because of diseases such as pneumonia and emphysema. We investigated the effect of the lung physical density on the neutron flux distribution and dose for the lung and tumor.

Boron Neutron Capture Therapy Eliminates Radioresistant Liver Cancer Cells by Targeting DNA Damage and Repair Responses

Introduction

For advanced hepatocellular carcinoma (HCC), resistance to conservative treatments remains a challenge. In previous studies, the therapeutic effectiveness and DNA damage responses of boric acid-mediated boron neutron capture therapy (BA-BNCT) in HCC have been demonstrated in animal models and HCC cell line. On the other hand, numerous studies have shown that high linear energy transfer (LET) radiation can overcome tumor resistance. Since BNCT yields a mixture of high and low LET radiation, we aimed to explore whether and how BA-BNCT could eliminate radioresistant HCC cells.

Methods

Radioresistant human HCC (HepG2-R) cells were established from HepG2 cells via intermittent irradiation. HepG2 and HepG2-R cells were then irradiated with either γ-ray or neutron radiation of BA-BNCT. Colony formation assays were used to assess cell survival and the relative biological effectiveness (RBE). The expression of phosphorylated H2AX (γH2AX) was also examined by immunocytochemistry and Western blot assays to evaluate the extent of DNA double-strand breaks (DSBs). Finally, the expression levels of DNA damage response-associated proteins were determined, followed by cell cycle analysis and caspase-3 activity analysis.

Results

Our data demonstrated that under the same dose by γ-ray, BNCT effectively eliminated radioresistant HCC by increasing the number of DNA DSBs (p < 0.05) and impeding their repair (p < 0.05), which verified the high RBE of BNCT. We also found that BNCT resulted in delayed homologous recombination (HR) and inhibited the nonhomologous end-joining (NHEJ) pathway during DNA repair. Markedly, BNCT increased cell arrest (p < 0.05) in the G₂/M phase by altering G₂ checkpoint signaling and increased PUMA-mediated apoptosis (p < 0.05).

Conclusion

Our data suggest that DNA damage and repair responses could affect the anticancer efficiency of BNCT in radioresistant HepG2-R cells, which highlights the potential of BNCT as a viable treatment option for recurrent HCC.

Design of a filtration system to improve the dose distribution of an accelerator-based neutron capture therapy system

PURPOSE

The aim of this study is to design and evaluate a neutron filtration system to improve the dose distribution of an accelerator-based neutron capture therapy system.

METHODS

An LiF-sintered plate composed of 99%-enriched ⁶ Li was utilized to filter out low-energy neutrons to increase the average neutron energy at the beam exit. A 5-mm thick filter to fit inside a 12-cm diameter circular collimator was manufactured, and experimental measurements were performed to measure the thermal neutron flux and gamma-ray dose rate inside a water phantom. The experimental measurements were compared with the Monte Carlo simulation, particle, and heavy ion transport code system. Following the experimental verification, three filter designs were modeled, and the thermal neutron flux and the biologically weighted dose distribution inside a phantom were simulated. Following the phantom simulation, a dummy patient CT dataset was used to simulate a boron neutron capture therapy (BNCT) irradiation of the brain. A mock tumor located at 4, 6, 8 cm along the central axis and 4-cm off-axis was set, and the dose distribution was simulated for a maximum total biologically weighted brain dose of 12.5 Gy with a beam entering from the vertex.

RESULTS

All three filters improved the beam penetration of the accelerator-based neutron source. Filter design C was found to be the most suitable filter, increasing the advantage depth from 9.1 to 9.9 cm. Compared with the unfiltered beam, the mean weighted dose in the tumor located at a depth of 8 cm along the beam axis was increased by ∼25%, and 34% for the tumor located at a depth of 8 cm and off-axis by 4 cm.

CONCLUSION

A neutron filtration system for an accelerator-based BNCT system was investigated using Monte Carlo simulation. The proposed filter design significantly improved the dose distribution for the treatment of deep targets in the brain.

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.

In Vivo Accelerator-Based Boron Neutron Capture Therapy for Spontaneous Tumors in Large Animals: Case Series

(1) Background: accelerator-based neutron sources are a new frontier for BNCT but many technical issues remain. We aimed to study such issues and results in larger-animal BNCT (cats and dogs) with naturally occurring, malignant tumors in different locations as an intermediate step in translating current research into clinical practice. (2) Methods: 10 pet cats and dogs with incurable, malignant tumors that had no treatment alternatives were included in this study. A tandem accelerator with vacuum insulation was used as a neutron source. As a boron-containing agent, 10B-enriched sodium borocaptate (BSH) was used at a dose of 100 mg/kg. Animal condition as well as tumor progression/regression were monitored. (3) Results: regression of tumors in response to treatment, improvements in the overall clinical picture, and an increase in the estimated duration and quality of life were observed. Treatment-related toxicity was mild and reversible. (4) Conclusions: our study contributes to preparations for human BNCT clinical trials and suggests utility for veterinary oncology.

Evaluation of a treatment planning system developed for clinical boron neutron capture therapy and validation against an independent Monte Carlo dose calculation system

Boron neutron capture therapy (BNCT) for the treatment of unresectable, locally advanced, and recurrent carcinoma of the head and neck cancer has been approved by the Japanese government for reimbursement under the national health insurance as of June 2020. A new treatment planning system for clinical BNCT has been developed by Sumitomo Heavy Industries, Ltd. (Sumitomo), NeuCure® Dose Engine_. To safely implement this system for clinical use, the simulated neutron flux and gamma ray dose rate inside a water phantom was compared against experimental measurements. Furthermore, to validate and verify the new planning system, the dose distribution inside an anthropomorphic head phantom was compared against a BNCT treatment planning system SERA and an in-house developed Monte Carlo dose calculation program. The simulated results closely matched the experimental results, within 5% for the thermal neutron flux and 10% for the gamma ray dose rate. The dose distribution inside the head phantom closely matched with SERA and the in-house developed dose calculation program, within 3% for the tumour and a difference of 0.3 Gy_w for the brain.

Biodistribution and radiation dosimetry of D-isomer of 4-borono-2-[18F]fluoro-phenylalanine: A comparative PET/CT study with L-isomer in healthy human volunteers

INTRODUCTION

L-isomer of 4-borono-2-18F-fluoro-phenylalanine (L-[18F]FBPA) was generally applied in clinic for boron neutron capture therapy (BNCT). With radiotracer validation, D-isomer of [18F]FBPA (D-[18F]FBPA) was found a higher tumor to normal brain tissue ratio (TBR) than its L-isomer on positron emission tomography (PET) in rat brain glioma. The present study was conducted as a first-in-human study to explore the biodistribution and radiation dosimetry of D-[18F]FBPA in healthy human volunteers, compared with L-[18F]FBPA.

METHODS

D-[18F]FBPA or L-[18F]FBPA was injected intravenously. Five whole-body PET scans were performed for each subject in the next 2 h. Organ time-activity curves were drawn by measuring SUVmean in volumes of interest. Absorbed dose coefficient of target organs and effective dose (ED) were estimated on OLINDA/EXM.

RESULTS

Two healthy volunteers (both males) and three healthy volunteers (2 males, 1 female) were intravenously injected with D-[18F]FBPA (5.5-7.2 MBq/kg) and L-[18F]FBPA (3.9-6.8 MBq/kg) respectively. Only limited accumulation of D-[18F]FBPA was observed in healthy human brain, pancreas, liver, spleen and skeleton. The ED was calculated to be 0.026 mSv/MBq. Urinary bladder wall received the highest dose of 0.28 mGy/MBq, followed by kidneys (0.06 mGy/MBq), and all the other organs received less than 0.03 mGy/MBq. For L-[18F]FBPA, higher uptake in brain, pancreas, liver, spleen and skeleton could be visualized, compared with D-[18F]FBPA. The ED of L-[18F]FBPA was 0.020 ± 0.001 mSv/MBq. Urinary bladder wall and kidneys still received the highest dose among organs but with lower values than those of D-[18F]FBPA.

CONCLUSIONS

D-[18F]FBPA had lower activity in normal brain, liver, spleen, pancreas and skeleton, compared with its L-isomer. D-[18F]FBPA is safe from a radiological standpoint.

ADVANCES IN KNOWLEDGE AND IMPLICATIONS FOR PATIENT CARE

D-[18F]FBPA was safe from a radiological standpoint, and had lower activity in normal brain, liver, spleen, pancreas and skeleton than its L-isomer. This study ensures the safety and validity of D-[18F]FBPA for further clinical trials in patients with cancer.

Boron Neutron Capture Therapy: A Review of Clinical Applications

Boron neutron capture therapy (BNCT) is an emerging treatment modality aimed at improving the therapeutic ratio for traditionally difficult to treat tumors. BNCT utilizes boronated agents to preferentially deliver boron-10 to tumors, which, after undergoing irradiation with neutrons, yields litihium-7 and an alpha particle. The alpha particle has a short range, therefore preferentially affecting tumor tissues while sparing more distal normal tissues. To date, BNCT has been studied clinically in a variety of disease sites, including glioblastoma multiforme, meningioma, head and neck cancers, lung cancers, breast cancers, hepatocellular carcinoma, sarcomas, cutaneous malignancies, extramammary Paget's disease, recurrent cancers, pediatric cancers, and metastatic disease. We aim to provide an up-to-date and comprehensive review of the studies of each of these disease sites, as well as a review on the challenges facing adoption of BNCT.

Single-dose toxicity study by intra-arterial injection of 10BSH entrapped water-in-oil-in-water emulsion for boron neutron capture therapy to hepatocellular carcinoma

We developed a mixing medical device by attaching Shirasu porous glass Millipore membrane to prepare water-in-oil-in-water (WOW) emulsion in a shorter time to be applied as ¹⁰B-entrapped WOW emulsion for hepatocellular carcinoma (HCC) treatment. Single-dose toxicity studies by intra-arterial injection of ¹⁰BSH-entrapped WOW were performed in rabbits and pig, and no side effects were observed. We hope to proceed to the preclinical and clinical studies for further evaluation of ¹⁰B compound as multidisciplinary treatments for HCC.

Reevaluation of CBE value of BPA for hepatocytes

The compound biological effectiveness (CBE) value of boronophenylalanine (BPA) for hepatocytes was experimentally determined for the purpose of boron neutron capture therapy (BNCT) for liver tumors. In this study, the critical reevaluation of previous value was performed. In previous experimental studies, the contribution of β component of dose was ignored in the response curve to X-ray. X-ray dose cell survival curves were estimated by combining the α/β values obtained in the ordinary micronucleus (MN) assay with the curve of MN-negative cell fraction (MN(-)F) to dose. This curve was compared to the boron neutron capture reaction (BNCR) dose curve. As a result, the CBE value was 4 at doses close to 0 Gy, decreasing to about 1.0 at doses close to 4.5 Gy. The new value is smaller than the previous value 4.2. This indicates that the bioequivalent dose to normal liver is lower than previously expected. Therefore, higher doses can be given to the tumor.

Synthesis and characterization of boron fenbufen and its F-18 labeled homolog for boron neutron capture therapy of COX-2 overexpressed cholangiocarcinoma

Boron neutron capture therapy (BNCT) is a binary therapy that employs neutron irradiation on the boron agents to release high-energy helium and alpha particles to kill cancer cells. An optimal response to BNCT depends critically on the time point of maximal ¹⁰B accumulation and highest tumor to normal ratio (T/N) for performing the neutron irradiation. The aggressive cholangiocarcinoma (CCA) representing a liver cancer that overexpresses COX-2 enzyme is aimed to be targeted by COX-2 selective boron carrier, fenbufen boronopinacol (FBPin). Two main works were performed including: 1) chemical synthesis of FBPin as the boron carrier and 2) radiochemical labeling with F-18 to provide the radiofluoro congener, m-[¹⁸F]fluorofenbufen ester boronopinacol (m-[¹⁸F]FFBPin), to assess the binding affinity, cellular accumulation level and distribution profile in CCA rats. FBPin was prepared from bromofenbufen via 3 steps with 82% yield. The binding assay employed [¹⁸F]FFBPin to compete FBPin for binding to COX-1 (IC₅₀=0.91±0.68μM) and COX-2 (IC₅₀=0.33±0.24μM). [¹⁸F]FFBPin-derived 60-min dynamic PET scans predict the ¹⁰B-accumulation of 0.8-1.2ppm in liver and 1.2-1.8ppm in tumor and tumor to normal ratio=1.38±0.12. BNCT was performed 40-55min post intravenous administration of FBPin (20-30mg) in the CCA rats. CCA rats treated with BNCT display more tumor reduction than that by NCT with respect of 2-[¹⁸F]fluoro-2-deoxy glucose uptake in the tumor region of interest, 20.83±3.00% (n=12) vs. 12.83±3.79% (n=10), P=0.05. The visualizing agent [¹⁸F]FFBPin resembles FBPin to generate the time-dependent boron concentration profile. Optimal neutron irradiation period is thus determinable for BNCT. A boron-substituted agent based on COX-2-binding features has been prepared. The moderate COX-2/COX-1 selectivity index of 2.78 allows a fair tumor selectivity index of 1.38 with a mild cardiovascular effect. The therapeutic effect from FBPin with BNCT warrants a proper COX-2 targeting of boron NSAIDs.

Toward a clinical application of ex situ boron neutron capture therapy for lung tumors at the RA-3 reactor in Argentina

PURPOSE

Many types of lung tumors have a very poor prognosis due to their spread in the whole organ volume. The fact that boron neutron capture therapy (BNCT) would allow for selective targeting of all the nodules regardless of their position, prompted a preclinical feasibility study of ex situ BNCT at the thermal neutron facility of RA-3 reactor in the province of Buenos Aires, Argentina. (l)-4p-dihydroxy-borylphenylalanine fructose complex (BPA-F) biodistribution studies in an adult sheep model and computational dosimetry for a human explanted lung were performed to evaluate the feasibility and the therapeutic potential of ex situ BNCT.

METHODS

Two kinds of boron biodistribution studies were carried out in the healthy sheep: a set of pharmacokinetic studies without lung excision, and a set that consisted of evaluation of boron concentration in the explanted and perfused lung. In order to assess the feasibility of the clinical application of ex situ BNCT at RA-3, a case of multiple lung metastases was analyzed. A detailed computational representation of the geometry of the lung was built based on a real collapsed human lung. Dosimetric calculations and dose limiting considerations were based on the experimental results from the adult sheep, and on the most suitable information published in the literature. In addition, a workable treatment plan was considered to assess the clinical application in a realistic scenario.

RESULTS

Concentration-time profiles for the normal sheep showed that the boron kinetics in blood, lung, and skin would adequately represent the boron behavior and absolute uptake expected in human tissues. Results strongly suggest that the distribution of the boron compound is spatially homogeneous in the lung. A constant lung-to-blood ratio of 1.3 ± 0.1 was observed from 80 min after the end of BPA-F infusion. The fact that this ratio remains constant during time would allow the blood boron concentration to be used as a surrogate and indirect quantification of the estimated value in the explanted healthy lung. The proposed preclinical animal model allowed for the study of the explanted lung. As expected, the boron concentration values fell as a result of the application of the preservation protocol required to preserve the lung function. The distribution of the boron concentration retention factor was obtained for healthy lung, with a mean value of 0.46 ± 0.14 consistent with that reported for metastatic colon carcinoma model in rat perfused lung. Considering the human lung model and suitable tumor control probability for lung cancer, a promising average fraction of controlled lesions higher than 85% was obtained even for a low tumor-to-normal boron concentration ratio of 2.

CONCLUSIONS

This work reports for the first time data supporting the validity of the ovine model as an adequate human surrogate in terms of boron kinetics and uptake in clinically relevant tissues. Collectively, the results and analysis presented would strongly suggest that ex situ whole lung BNCT irradiation is a feasible and highly promising technique that could greatly contribute to the treatment of metastatic lung disease in those patients without extrapulmonary spread, increasing not only the expected overall survival but also the resulting quality of life.

Improvement of depth dose distribution using multiple-field irradiation in boron neutron capture therapy

It is important that improvements are made to depth dose distribution in boron neutron capture therapy, because the neutrons do not reach the innermost regions of the human body. Here, we evaluated the dose distribution obtained using multiple-field irradiation in simulation. From a dose volume histogram analysis, it was found that the mean and minimum tumor doses were increased using two-field irradiation, because of improved dose distribution for deeper-sited tumors.

FBPA PET in boron neutron capture therapy for cancer: prediction of (10)B concentration in the tumor and normal tissue in a rat xenograft model

Background

Boron neutron capture therapy (BNCT) is a molecular radiation treatment based on the ¹⁰B (n, α) ⁷Li nuclear reaction in cancer cells, in which delivery of ¹⁰B by 4-borono-phenylalanine conjugated with fructose (BPA-fr) to the cancer cells is of critical importance. The PET tracer 4-borono-2-¹⁸ F-fluoro-phenylalanine (FBPA) has been used to predict the accumulation of BPA-fr before BNCT. However, because of the difference in chemical structure between BPA-fr and FBPA and the difference in the dose administered between BPA-fr (therapeutic dose) and FBPA (tracer dose), the predictive value of FBPA PET for BPA-fr accumulation in the tumor and normal tissues is not yet clearly proven. We conducted this study to validate FBPA PET as a useful test to predict the accumulation of BPA-fr in the tumor and normal tissues before BNCT.

Methods

RGC-6 rat glioma cells (1.9 × 10⁷) were implanted subcutaneously in seven male F344 rats. On day 20 after the tumor implantation, dynamic PET scan was performed on four rats after injection of FBPA for 1 h. Whole-body PET/CT was performed 1 h after intravenous injection of the FBPA solution (30.5 ± 0.7 MBq, 1.69 ± 1.21 mg/kg). PET accumulation of FBPA in the tumor tissue and various normal tissues was estimated as a percentage of the injected dose per gram (%ID/g). One hour after the PET/CT scan, BPA-fructose (167.32 ± 18.65 mg/kg) was injected intravenously, and the rats were dissected 1 h after the BPA-fr injection. The absolute concentration of ¹⁰B in the autopsied tissues and blood was measured by inductively coupled plasma optical emission spectrometry (ICP-OES).

Results

The highest absolute concentration of ¹⁰B determined by ICP-OES was found in the kidney (4.34 ± 0.84 %ID/g), followed by the pancreas (2.73 ± 0.63 %ID/g), and the tumor (1.44 ± 0.44 %ID/g). A significant positive correlation was found between the accumulation levels of BPA-fr and FBPA (r = 0.91, p < 0.05).

Conclusions

FBPA PET can reliably predict accumulation of BPA-fr in the tumor as well as normal tissues.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-014-0070-2) contains supplementary material, which is available to authorized users.

Dose estimation for internal organs during boron neutron capture therapy for body-trunk tumors

Radiation doses during boron neutron capture therapy for body-trunk tumors were estimated for various internal organs, using data from patients treated at Kyoto University Research Reactor Institute. Dose-volume histograms were constructed for tissues of the lung, liver, kidney, pancreas, and bowel. For pleural mesothelioma, the target total dose to the normal lung tissues on the diseased side is 5Gy-Eq in average for the whole lung. It was confirmed that the dose to the liver should be carefully considered in cases of right lung disease.

Pilot clinical study of boron neutron capture therapy for recurrent hepatic cancer involving the intra-arterial injection of a (10)BSH-containing WOW emulsion

A 63-year-old man with multiple HCC in his left liver lobe was enrolled as the first patient in a pilot study of boron neutron capture therapy (BNCT) involving the selective intra-arterial infusion of a (10)BSH-containing water-in-oil-in-water emulsion ((10)BSH-WOW). The size of the tumorous region remained stable during the 3 months after the BNCT. No adverse effects of the BNCT were observed. The present results show that (10)BSH-WOW can be used as novel intra-arterial boron carriers during BNCT for HCC.

[Development of high boron content liposomes and their promising antitumor effect for neutron capture therapy]

  High accumulation and selective delivery of boron into tumor tissue are the most important requirements to achieve the efficient cell-killing effect of boron neutron capture therapy (BNCT) that relies on the nuclear reaction of two essentially nontoxic species, boron-10 ((10)B) and thermal neutrons in boron-loaded tissues. Recent development of boron cluster lipids and their liposomal boron delivery system (BDS) are summarized in this article. Boron compounds that have no affinity to tumor can potentially be delivered to tumor tissues by liposomes, therefore, liposomal BDS would be one of the most attractive approaches for efficient BNCT of various cancers. There are two approaches for BDS: encapsulation of boron compounds into liposomes and incorporation of boron-conjugated lipids into the liposomal bilayer. The combination of both approaches has a potential for reduction of the total dose of liposomes without reducing the efficacy of BNCT.

Boron neutron capture therapy (BNCT) for liver metastasis in an experimental model: dose–response at five-week follow-up based on retrospective dose assessment in individual rats

Boron neutron capture therapy (BNCT) was proposed for untreatable colorectal liver metastases. Employing an experimental model of liver metastases in rats, we recently demonstrated that BNCT mediated by boronophenylalanine (BPA-BNCT) at 13 Gy prescribed to tumor is therapeutically useful at 3-week follow-up. The aim of the present study was to evaluate dose–response at 5-week follow-up, based on retrospective dose assessment in individual rats. BDIX rats were inoculated with syngeneic colon cancer cells DHD/K12/TRb. Tumor-bearing animals were divided into three groups: BPA-BNCT (n = 19), Beam only (n = 8) and Sham (n = 7) (matched manipulation, no treatment). For each rat, neutron flux was measured in situ and boron content was measured in a pre-irradiation blood sample for retrospective individual dose assessment. For statistical analysis (ANOVA), individual data for the BPA-BNCT group were pooled according to absorbed tumor dose, BPA-BNCT I: 4.5–8.9 Gy and BPA-BNCT II: 9.2–16 Gy. At 5 weeks post-irradiation, the tumor surface area post-treatment/pre-treatment ratio was 12.2 ± 6.6 for Sham, 7.8 ± 4.1 for Beam only, 4.4 ± 5.6 for BPA-BNCT I and 0.45 ± 0.20 for BPA-BNCT II; tumor nodule weight was 750 ± 480 mg for Sham, 960 ± 620 mg for Beam only, 380 ± 720 mg for BPA-BNCT I and 7.3 ± 5.9 mg for BPA-BNCT II. The BPA-BNCT II group exhibited statistically significant tumor control with no contributory liver toxicity. Potential threshold doses for tumor response and significant tumor control were established at 6.1 and 9.2 Gy, respectively.

Boron lipid-based liposomal boron delivery system for neutron capture therapy: recent development and future perspective

Recent development of boron cluster lipids and their liposomal boron delivery system (BDS) are summarized in this article. Boron compounds used in boron neutron capture therapy (BNCT) are, in general, nontoxic unless neutron capture reaction of boron takes place. Therefore, the boron compounds accumulated into other organs would not cause such side effects for patient. Selective and sufficient delivery of boron-10 to tumor results in the successful BNCT. There are two approaches for BDS: encapsulation of boron compounds into liposomes and incorporation of boron-conjugated lipids into the liposomal bilayer, both of which have been significantly investigated. The combination of both approaches displayed potency and, hence, the ability to reduce the total dose of liposomes without reducing the efficacy of BNCT. Boron compounds that have no affinity to tumor can potentially be delivered to tumor tissues by liposomes, therefore, liposomal BDS would be one of the most attractive approaches for efficient BNCT of various cancers.

Biodistribution of sodium borocaptate (BSH) for boron neutron capture therapy (BNCT) in an oral cancer model

Boron neutron capture therapy (BNCT) is based on selective accumulation of ¹⁰B carriers in tumor followed by neutron irradiation. We previously proved the therapeutic success of BNCT mediated by the boron compounds boronophenylalanine and sodium decahydrodecaborate (GB-10) in the hamster cheek pouch oral cancer model. Based on the clinical relevance of the boron carrier sodium borocaptate (BSH) and the knowledge that the most effective way to optimize BNCT is to improve tumor boron targeting, the specific aim of this study was to perform biodistribution studies of BSH in the hamster cheek pouch oral cancer model and evaluate the feasibility of BNCT mediated by BSH at nuclear reactor RA-3. The general aim of these studies is to contribute to the knowledge of BNCT radiobiology and optimize BNCT for head and neck cancer. Sodium borocaptate (50 mg ¹⁰B/kg) was administered to tumor-bearing hamsters. Groups of 3-5 animals were killed humanely at nine time-points, 3-12 h post-administration. Samples of blood, tumor, precancerous pouch tissue, normal pouch tissue and other clinically relevant normal tissues were processed for boron measurement by optic emission spectroscopy. Tumor boron concentration peaked to therapeutically useful boron concentration values of 24-35 ppm. The boron concentration ratio tumor/normal pouch tissue ranged from 1.1 to 1.8. Pharmacokinetic curves showed that the optimum interval between BSH administration and neutron irradiation was 7-11 h. It is concluded that BNCT mediated by BSH at nuclear reactor RA-3 would be feasible.

Evaluation of performance of an accelerator-based BNCT facility for the treatment of different tumor targets

PURPOSE

Encouraging Boron Neutron Capture Therapy (BNCT) clinical results obtained in recent years have stimulated intense research to develop accelerator-based neutron sources to be installed in clinical facilities. In this work an assessment of an accelerator-based BNCT facility for the treatment of different tumor targets was performed, comparing the accelerator-derived results with reported reactor-based trials under similar conditions and subjected to the same clinical protocols.

MATERIALS AND METHODS

A set of real image studies was used to cover clinical-like cases of brain and head-and-neck tumors. In addition, two clinical cases of malignant nodular melanoma treated at the RA-6 BNCT facility in Argentina were used to thoroughly compare the clinical dosimetry with the accelerator-derived results.

RESULTS

The minimum weighted dose delivered to the clinical target volume was higher than 30 Gy and 14 Gy for the brain tumor and head-and-neck cases, respectively, in agreement with those achieved in clinical applications. For the melanoma cases, the minimum tumor doses were equal or higher than those achieved with the RA-6 reactor for identical field orientation and protocol. The whole-body dose assessment showed that the maximum photon-equivalent doses for those normal organs close to the beam direction were below the upper limits considered in the protocols used in the present work.

CONCLUSIONS

The obtained results indicate not only the good performance of the proposed beam shaping assembly design associated to the facility but also the potential applicability of accelerator-based BNCT in the treatment of both superficial and deep-seated tumors.

Development of high boron content liposomes and their promising antitumor effect for neutron capture therapy of cancers

Mercaptoundecahydrododecaborate (BSH)-encapsulating 10% distearoyl boron lipid (DSBL) liposomes were developed as a boron delivery vehicle for neutron capture therapy. The current approach is unique because the liposome shell itself possesses cytocidal potential in addition to its encapsulated agents. BSH-encapsulating 10% DSBL liposomes have high boron content (B/P ratio: 2.6) that enables us to prepare liposome solution with 5000 ppm boron concentration. BSH-encapsulating 10% DSBL liposomes displayed excellent boron delivery efficacy to tumor: boron concentrations reached 174, 93, and 32 ppm at doses of 50, 30, and 15 mg B/kg, respectively. Magnescope was also encapsulated in the 10% DSBL liposomes and the real-time biodistribution of the Magnescope-encapsulating DSBL liposomes was measured in a living body using MRI. Significant antitumor effect was observed in mice injected with BSH-encapsulating 10% DSBL liposomes even at the dose of 15 mg B/kg; the tumor completely disappeared three weeks after thermal neutron irradiation ((1.5-1.8) × 10(12) neutrons/cm(2)). The current results enabled us to reduce the total dose of liposomes to less than one-fifth compared with that of the BSH-encapsulating liposomes without reducing the efficacy of boron neutron capture therapy (BNCT).

Boron neutron capture therapy (BNCT) for liver metastasis: therapeutic efficacy in an experimental model

Boron neutron capture therapy (BNCT) was proposed for untreatable colorectal liver metastases. The present study evaluates tumor control and potential radiotoxicity of BNCT in an experimental model of liver metastasis. BDIX rats were inoculated with syngeneic colon cancer cells DHD/K12/TRb. Tumor-bearing animals were divided into three groups: BPA-BNCT, boronophenylalanine (BPA) + neutron irradiation; Beam only, neutron irradiation; Sham, matched manipulation. The total absorbed dose administered with BPA-BNCT was 13 ± 3 Gy in tumor and 9 ± 2 Gy in healthy liver. Three weeks post-treatment, the tumor surface area post-treatment/pre-treatment ratio was 0.46 ± 0.20 for BPA-BNCT, 2.7 ± 1.8 for Beam only and 4.5 ± 3.1 for Sham. The pre-treatment tumor nodule mass of 48 ± 19 mg fell significantly to 19 ± 16 mg for BPA-BNCT, but rose significantly to 140 ± 106 mg for Beam only and to 346 ± 302 mg for Sham. For both end points, the differences between the BPA-BNCT group and each of the other groups were statistically significant (ANOVA). No clinical, macroscopic or histological normal liver radiotoxicity was observed. It is concluded that BPA-BNCT induced a significant remission of experimental colorectal tumor nodules in liver with no contributory liver toxicity.

The formulation of polyhedral boranes for the boron neutron capture therapy of cancer

The early promise of boron neutron capture therapy as a method for the treatment of cancer has been inhibited by the inherent toxicity associated with therapeutically useful doses of ¹⁰B-containing pharmacophores, the need for target-tissue specificity and the challenges imposed by biological barriers. Although developments in the synthetic chemistry of polyhedral boranes have addressed issues of toxicity to a considerable extent, the optimisation of the transport and the delivery of boronated agents to the site of action--the subject of this review--is a challenge that is addressed by the development of innovative formulation strategies.

Cationized gelatin-HVJ envelope with sodium borocaptate improved the BNCT efficacy for liver tumors in vivo

BACKGROUND

Boron neutron capture therapy (BNCT) is a cell-selective radiation therapy that uses the alpha particles and lithium nuclei produced by the boron neutron capture reaction. BNCT is a relatively safe tool for treating multiple or diffuse malignant tumors with little injury to normal tissue. The success or failure of BNCT depends upon the 10B compound accumulation within tumor cells and the proximity of the tumor cells to the body surface. To extend the therapeutic use of BNCT from surface tumors to visceral tumors will require 10B compounds that accumulate strongly in tumor cells without significant accumulation in normal cells, and an appropriate delivery method for deeper tissues.Hemagglutinating Virus of Japan Envelope (HVJ-E) is used as a vehicle for gene delivery because of its high ability to fuse with cells. However, its strong hemagglutination activity makes HVJ-E unsuitable for systemic administration.In this study, we developed a novel vector for 10B (sodium borocaptate: BSH) delivery using HVJ-E and cationized gelatin for treating multiple liver tumors with BNCT without severe adverse events.

METHODS

We developed cationized gelatin conjugate HVJ-E combined with BSH (CG-HVJ-E-BSH), and evaluated its characteristics (toxicity, affinity for tumor cells, accumulation and retention in tumor cells, boron-carrying capacity to multiple liver tumors in vivo, and bio-distribution) and effectiveness in BNCT therapy in a murine model of multiple liver tumors.

RESULTS

CG-HVJ-E reduced hemagglutination activity by half and was significantly less toxic in mice than HVJ-E. Higher 10B concentrations in murine osteosarcoma cells (LM8G5) were achieved with CG-HVJ-E-BSH than with BSH. When administered into mice bearing multiple LM8G5 liver tumors, the tumor/normal liver ratios of CG-HVJ-E-BSH were significantly higher than those of BSH for the first 48 hours (p < 0.05). In suppressing the spread of tumor cells in mice, BNCT treatment was as effective with CG-HVJ-E-BSH as with BSH containing a 35-fold higher 10B dose. Furthermore, CG-HVJ-E-BSH significantly increased the survival time of tumor-bearing mice compared to BSH at a comparable dosage of 10B.

CONCLUSION

CG-HVJ-E-BSH is a promising strategy for the BNCT treatment of visceral tumors without severe adverse events to surrounding normal tissues.

[Minimally invasive cytoselective radiation therapy using boron neutron capture reaction]

The cell-killing effect of boron neutron capture therapy (BNCT) is due to the nuclear reaction of two essentially nontoxic species, boron-10 ((10)B) and thermal neutrons, whose destructive effect is well observed in boron-loaded tissues. High accumulation and selective delivery of boron into tumor tissue are the most important requirements to achieve efficient neutron capture therapy of cancers. This review focuses on liposomal boron delivery system (BDS) as a recent promising approach that meet these requirements for BNCT. BDS involves two strategies: (1) encapsulation of boron in the aqueous core of liposomes and (2) accumulation of boron in the liposomal bilayer. In this review, recent development of liposomal boron delivery system is summarized.

Dodecaborate lipid liposomes as new vehicles for boron delivery system of neutron capture therapy

Closo-dodecaborate lipid liposomes were developed as new vehicles for boron delivery system (BDS) of neutron capture therapy. The current approach is unique because the liposome shell itself possesses cytocidal potential in combination with neutron irradiation. The liposomes composed of closo-dodecaborate lipids DSBL and DPBL displayed high cytotoxicity with thermal neutron irradiation. The closo-dodecaborate lipid liposomes were taken up into the cytoplasm by endocytosis without degradation of the liposomes. Boron concentration of 22.7 ppm in tumor was achieved by injection with DSBL-25% PEG liposomes at 20mg B/kg. Promising BNCT effects were observed in the mice injected with DSBL-25% PEG liposomes: the tumor growth was significantly suppressed after thermal neutron irradiation (1.8 x 10(12)neutrons/cm(2)).

Liposomal boron delivery for neutron capture therapy

Tumor cell destruction in boron neutron capture therapy (BNCT) is due to the nuclear reaction between (10)B and thermal neutrons. The thermal neutrons have an energy of 0.025 eV, clearly below the threshold energy required to ionize tissue components. However, neutron capture by (10)B produces lithium ion and helium (alpha-particles), which are high linear energy transfer (LET) particles, and dissipate their kinetic energy before traveling one cell diameter (5-9 microm) in biological tissues, ensuring their potential for precise cell killing. BNCT has been applied clinically for the treatment of malignant brain tumors, malignant melanoma, head and neck cancer, and hepatoma using two boron compounds: sodium borocaptate (Na(2)(10)B(12)H(11)SH; Na(2)(10)BSH) and l-p-boronophenylalanine (l-(10)BPA). These low molecular weight compounds are cleared easily from the cancer cells and blood. Therefore, high accumulation and selective delivery of boron compounds into tumor tissues are most important to achieve effective BNCT and to avoid damage of adjacent healthy cells. Much attention has been focused on the liposomal drug delivery system (DDS) as an attractive, intelligent technology of targeting and controlled release of (10)B compounds. Two approaches have been investigated for incorporation of (10)B into liposomes: (1) encapsulation of (10)B compounds into liposomes and (2) incorporation of (10)B-conjugated lipids into the liposomal bilayer. Our laboratory has developed boron ion cluster lipids for application of the latter approach. In this chapter, our boron lipid liposome approaches as well as recent developments of the liposomal boron delivery system are summarized.

Improvement of dose distribution in phantom by using epithermal neutron source based on the Be(p,n) reaction using a 30 MeV proton cyclotron accelerator

In order to generate epithermal neutrons for boron neutron capture therapy (BNCT), we proposed the method of filtering and moderating fast neutrons, which are emitted from the reaction between a beryllium target and 30 MeV protons accelerated by a cyclotron, using an optimum moderator system composed of iron, lead, aluminum, calcium fluoride, and enriched (6)LiF ceramic filter. At present, the epithermal-neutron source is under construction since June 2008 at Kyoto University Research Reactor Institute. This system consists of a cyclotron to supply a proton beam of about 1 mA at 30 MeV, a beam transport system, a beam scanner system for heat reduction on the beryllium target, a target cooling system, a beam shaping assembly, and an irradiation bed for patients. In this article, an overview of the cyclotron-based neutron source (CBNS) and the properties of the treatment neutron beam optimized by using the MCNPX Monte Carlo code are presented. The distribution of the RBE (relative biological effectiveness) dose in a phantom shows that, assuming a (10)B concentration of 13 ppm for normal tissue, this beam could be employed to treat a patient with an irradiation time less than 30 min and a dose less than 12.5 Gy-eq to normal tissue. The CBNS might be an alternative to the reactor-based neutron sources for BNCT treatments.