Boron neutron capture therapy (BNCT) for malignant melanoma with special reference to absorbed doses to the normal skin and tumor

Effects of neutron sources and moderator materials on therapeutic dose delivery to deep-seated lesions in proton accelerator-based boron neutron capture therapy

This study aimed to identify the optimal conditions for delivering sufficient doses to deep-seated lesions within short irradiation times for two boron carriers of different T/N ratios. The therapeutic depth and irradiation time of a neutron beam for beam shaping assemblies (BSAs) with a Li or Be target and a MgF2 or CaF2 moderator were examined with the fast-neutron dose per epithermal neutron (FNR) as a parameter. When T/N = 3.61, the therapeutic depth was almost saturated at an FNR of about 10 × 10-13 Gy cm2; when the FNR value was about 10 × 10-13 Gy cm2, the therapeutic depth of the neutron beam for the BSA with a Be target and a MgF2 moderator was almost identical to that for the neutron beam for the BSA with a Be target and a CaF2 moderator, and slightly greater than those for the neutron beams for the BSAs with a Li target and a MgF2 or CaF2 moderator; moreover, the irradiation time of the neutron beam for the BSA with a Be target and a MgF2 moderator was shorter than that for the neutron beam for the BSA with a Be target and a CaF2 moderator. When T/N = 100, the therapeutic depths of the neutron beams for the BSAs varied greatly depending on the FNR, and were greater than the corresponding values for T/N = 3.61. We therefore concluded that the BSA with a Be target and a MgF2 moderator that produced a neutron beam with an FNR of about 10 × 10-13 Gy cm2 is optimal for delivering sufficient doses to deep-seated lesions in short irradiation times when T/N = 3.61, and stricter control over FNR is required when T/N = 100.

Acral cutaneous malignant melanoma treated with linear accelerator-based boron neutron capture therapy system: a case report of first patient

This study reports the first patient treatment for cutaneous malignant melanoma using a linear accelerator-based boron neutron capture therapy (BNCT) system. A single-center open-label phase I clinical trial had been conducted using the system since November 2019. A patient with a localized node-negative acral malignant melanoma and the largest diameter of the tumor ≤ 15 cm who refused primary surgery and chemotherapy was enrolled. After administering boronophenylalanine (BPA), a single treatment of BNCT with the maximum dose of 18 Gy-Eq delivered to the skin was performed. The safety and efficacy of the accelerator-based BNCT system for treating localized cutaneous malignant melanoma were evaluated. The first patient with cutaneous malignant melanoma in situ on the second finger of the left hand did not develop dose-limiting toxicity in the clinical trial. After BNCT, the treatment efficacy was gradually observed, and the patient achieved PR within 6 months and CR within 12 months. Moreover, during the follow-up period of 12 months after BNCT, the patient did not exhibit a recurrence without any treatment-related grade 2 or higher adverse events. Although grade 1 adverse events of dermatitis, dry skin, skin hyperpigmentation, edema, nausea, and aching pain were noted in the patient, those adverse events were relieved without any treatment. This case report shows that the accelerator-based BNCT may become a promising treatment modality for cutaneous malignant melanoma. We expect further clinical trials to reveal the efficacy and safety of the accelerator-based BNCT for cutaneous malignant melanoma.

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.

Gadolinium Neutron Capture Therapy for Cats and Dogs with Spontaneous Tumors Using Gd-DTPA

We conducted a clinical veterinary study on neutron capture therapy (NCT) at a neutron-producing accelerator with seven incurable pets with spontaneous tumors and gadolinium as a neutron capture agent (gadolinium neutron capture therapy, or GdNCT). Gadolinium-containing dimeglumine gadopentetate, or Gd-DTPA (Magnevist®, 0.6 mL/kg b.w.), was used. We observed mild and reversible toxicity related to the treatment. However, no significant tumor regression in response to the treatment was observed. In most cases, there was continued tumor growth. Overall clinical improvement after treatment was only temporary. The use of Gd-DTPA for NCT had no significant effects on the life expectancy and quality of life of animals with spontaneous tumors. Further experiments using more advanced gadolinium compounds are needed to improve the effect of GdNCT so that it can become an alternative to boron neutron capture therapy. Such studies are also necessary for further NCT implementation in clinical practice as well as in veterinary medicine.

SARU: A self-attention ResUNet to generate synthetic CT images for MR-only BNCT treatment planning

PURPOSE

Despite the significant physical differences between magnetic resonance imaging (MRI) and computed tomography (CT), the high entropy of MRI data indicates the existence of a surjective transformation from MRI to CT image. However, there is no specific optimization of the network itself in previous MRI/CT translation works, resulting in mistakes in details such as the skull margin and cavity edge. These errors might have moderate effect on conventional radiotherapy, but for boron neutron capture therapy (BNCT), the skin dose will be a critical part of the dose composition. Thus, the purpose of this work is to create a self-attention network that could directly transfer MRI to synthetical computerized tomography (sCT) images with lower inaccuracy at the skin edge and examine the viability of magnetic resonance (MR)-guided BNCT.

METHODS

A retrospective analysis was undertaken on 104 patients with brain malignancies who had both CT and MRI as part of their radiation treatment plan. The CT images were deformably registered to the MRI. In the U-shaped generation network, we introduced spatial and channel attention modules, as well as a versatile "Attentional ResBlock," which reduce the parameters while maintaining high performance. We employed five-fold cross-validation to test all patients, compared the proposed network to those used in earlier studies, and used Monte Carlo software to simulate the BNCT process for dosimetric evaluation in test set.

RESULTS

Compared with UNet, Pix2Pix, and ResNet, the mean absolute error (MAE) of self-attention ResUNet (SARU) is reduced by 12.91, 17.48, and 9.50 HU, respectively. The "two one-sided tests" show no significant difference in dose-volume histogram (DVH) results. And for all tested cases, the average 2%/2 mm gamma index of UNet, ResNet, Pix2Pix, and SARU were 0.96 ± 0.03, 0.96 ± 0.03, 0.95 ± 0.03, and 0.98 ± 0.01, respectively. The error of skin dose from SARU is much less than the results from other methods.

CONCLUSIONS

We have developed a residual U-shape network with an attention mechanism to generate sCT images from MRI for BNCT treatment planning with lower MAE in six organs. There is no significant difference between the dose distribution calculated by sCT and real CT. This solution may greatly simplify the BNCT treatment planning process, lower the BNCT treatment dose, and minimize image feature mismatch.

Accelerator based epithermal neutron source for clinical boron neutron capture therapy

The world’s first accelerator based epithermal neutron source for clinical boron neutron capture therapy (BNCT) was designed, developed, and commissioned between 2008 and 2010 by Sumitomo Heavy Industries in collaboration with Kyoto University at the Kyoto University Institute for Integrated Radiation and Nuclear Science. The accelerator system is cyclotron-based and accelerates a proton up to an energy of approximately 30 MeV. The proton strikes a beryllium target, which produces fast neutrons that traverse a beam shaping assembly composed of a combination of lead, iron, aluminum, and calcium fluoride to reduce the neutron energy down to the epithermal range (∼10 keV) suitable for BNCT. The system is designed to produce an epithermal neutron flux of up to 1.4 × 10 9 n · cm − 2 · s − 1 (exiting from the moderator of a 12 cm diameter collimator) with a proton current of 1 mA. In 2017, the same type of accelerator was installed at the Kansai BNCT Medical Center and in March 2020 the system received medical device approval in Japan (Sumitomo Heavy Industries, NeuCure® BNCT system). Soon after, BNCT for unresectable, locally advanced, and recurrent carcinoma of the head and neck region was approved by the Japanese government for reimbursement covered by the national health insurance system.

Molecular Events in the Melanogenesis Cascade as Novel Melanoma-Targeted Small Molecules: Principle and Development

Malignant melanoma is one of the most malignant of all cancers. Melanoma occurs at the epidermo-dermal interface of the skin and mucosa, where small vessels and lymphatics are abundant. Consequently, from the onset of the disease, melanoma easily metastasizes to other organs throughout the body via lymphatic and blood circulation. At present, the most effective treatment method is surgical resection, and other attempted methods, such as chemotherapy, radiotherapy, immunotherapy, targeted therapy, and gene therapy, have not yet produced sufficient results. Since melanogenesis is a unique biochemical pathway that functions only in melanocytes and their neoplastic counterparts, melanoma cells, the development of drugs that target melanogenesis is a promising area of research. Melanin consists of small-molecule derivatives that are always synthesized by melanoma cells. Amelanosis reflects the macroscopic visibility of color changes (hypomelanosis). Under microscopy, melanin pigments and their precursors are present in amelanotic melanoma cells. Tumors can be easily targeted by small molecules that chemically mimic melanogenic substrates. In addition, small-molecule melanin metabolites are toxic to melanocytes and melanoma cells and can kill them. This review describes our development of chemo-thermo-immunotherapy based on the synthesis of melanogenesis-based small-molecule derivatives and conjugation to magnetite nanoparticles. We also introduce the other melanogenesis-related chemotherapy and thermal medicine approaches and discuss currently introduced targeted therapies with immune checkpoint inhibitors for unresectable/metastatic melanoma.

Boron Neutron Capture Therapy: Clinical Application and Research Progress

Boron neutron capture therapy (BNCT) is a binary modality that is used to treat a variety of malignancies, using neutrons to irradiate boron-10 (¹⁰B) nuclei that have entered tumor cells to produce highly linear energy transfer (LET) alpha particles and recoil ⁷Li nuclei (¹⁰B [n, α] ⁷Li). Therefore, the most important part in BNCT is to selectively deliver a large number of ¹⁰B to tumor cells and only a small amount to normal tissue. So far, BNCT has been used in more than 2000 cases worldwide, and the efficacy of BNCT in the treatment of head and neck cancer, malignant meningioma, melanoma and hepatocellular carcinoma has been confirmed. We collected and collated clinical studies of second-generation boron delivery agents. The combination of different drugs, the mode of administration, and the combination of multiple treatments have an important impact on patient survival. We summarized the critical issues that must be addressed, with the hope that the next generation of boron delivery agents will overcome these challenges.

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.

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.

Improvement in the neutron beam collimation for application in boron neutron capture therapy of the head and neck region

In June 2020, the Japanese government approved boron neutron capture therapy for the treatment of head and neck cancer. The treatment is usually performed in a single fraction, with the neutron irradiation time being approximately 30–60 min. As neutrons scatter in air and loses its intensity, it is preferable to bring the patient as close to the beam port as possible to shorten the irradiation time. However, this can be a challenge, especially for patients with head and neck cancer, as the shoulders are an obstacle to a clean positioning. In this study, a novel neutron collimation system for an accelerator based neutron source was designed to allow for a more comfortable treatment, without compromising the irradiation time. Experimental measurements confirmed the simulation results and showed the new collimator can reduce the irradiation time by approximately 60% (under the same condition where the distance between the source and the patient surface was kept the same). The dose delivered to the surrounding healthy tissue was reduced with the new collimator, showing a 25% decrease in the D₅₀ of the mucosal membrane. Overall, the use of the newly designed collimator will allow for a more comfortable treatment of the head and neck region, reduce the treatment time, and reduce the dose delivered to the surrounding healthy tissue.

Determining a methodology of dosimetric quality assurance for commercially available accelerator-based boron neutron capture therapy system

The irradiation field of boron neutron capture therapy (BNCT) consists of multiple dose components including thermal, epithermal and fast neutron, and gamma. The objective of this work was to establish a methodology of dosimetric quality assurance (QA), using the most standard and reliable measurement methods, and to determine tolerance level for each QA measurement for a commercially available accelerator-based BNCT system. In order to establish a system of dosimetric QA suitable for BNCT, the following steps were taken. First, standard measurement points based on tissue-administered doses in BNCT for brain tumors were defined, and clinical tolerances of dosimetric QA measurements were derived from the contribution to total tissue relative biological effectiveness factor-weighted dose for each dose component. Next, a QA program was proposed based on TG-142 and TG-198, and confirmed that it could be assessed whether constancy of each dose component was assured within the limits of tolerances or not by measurements of the proposed QA program. Finally, the validity of the BNCT QA program as an evaluation system was confirmed in a demonstration experiment for long-term measurement over 1 year. These results offer an easy, reliable QA method that is clinically applicable with dosimetric validity for the mixed irradiation field of accelerator-based BNCT.

Multimodal Applications of Zinc Gallate-Based Persistent Luminescent Nanoparticles in Cancer Treatment: Tumor Margining, Diagnosis, and Boron Neutron Capture Therapy

On the basis of the boron neutron capture therapy (BNCT) modality, we have designed and synthesized a zinc gallate (ZnGa₂O₄)-based nanoformulation for developing an innovative theranostic approach for cancer treatment. Initially, the (ZnGa_1.995Cr_0.005O₄ or ZnGa₂O₄:(0.5%)Cr persistent luminescence nanoparticles (PLNPs) embedded on silica matrix were synthesized. Their surface functionalization was performed using organic synthesis strategies to attach the amine functional moieties which were further coupled with poly(vicinal diol). These diols were helpful for conjugation with ¹⁰B(OH)₃, which subsequently served to couple with an in-house-synthesized variant of pH-(low)-insertion peptide (pHLIP) finally giving a tumor-targeting nanoformulation. Most importantly, the polymeric diols helped in conjugation of a substantial number of ¹⁰B to provide the therapeutic dose required for effective BNCT. This nanoformulation internalized substantially (∼80%) to WEHI-164 cancer cells within 6 h. Tumor homing studies indicated that the accumulation of this formulation at the acidic tumor site was within 2 h. The in vitro evaluation of the formulation against WEHI-164 cancer cells followed by neutron irradiation revealed its potent cytotoxicity with IC₅₀ ∼ 25 μM. In the case of studies on animal models, the melanoma-induced C57BL/6 and fibrosarcoma-induced BALB/c mice were treated with formulations through intratumoral and intravenous injections, respectively, followed by neutron irradiation, leading to a significant killing of the cancer cells, which was evidenced by a reduction in tumor volume (75-80%) as compared with a control tumor. Furthermore, the histopathological studies confirmed a damaging effect only on tumor cells, while there was no sign of damage to the vital organs in treated mice as well as in controls.

Boron Neutron Capture Therapy: Current Status and Challenges

Boron neutron capture therapy (BNCT) is a re-emerging therapy with the ability to selectively kill tumor cells. After the boron delivery agents enter the tumor tissue and enrich the tumor cells, the thermal neutrons trigger the fission of the boron atoms, leading to the release of boron atoms and then leading to the release of the α particles (⁴He) and recoil lithium particles (⁷Li), along with the production of large amounts of energy in the narrow region. With the advantages of targeted therapy and low toxicity, BNCT has become a unique method in the field of radiotherapy. Since the beginning of the last century, BNCT has been emerging worldwide and gradually developed into a technology for the treatment of glioblastoma multiforme, head and neck cancer, malignant melanoma, and other cancers. At present, how to develop and innovate more efficient boron delivery agents and establish a more accurate boron-dose measurement system have become the problem faced by the development of BNCT. We discuss the use of boron delivery agents over the past several decades and the corresponding clinical trials and preclinical outcomes. Furthermore, the discussion brings recommendations on the future of boron delivery agents and this therapy.

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.

Dose-Dependent Suppression of Human Glioblastoma Xenograft Growth by Accelerator-Based Boron Neutron Capture Therapy with Simultaneous Use of Two Boron-Containing Compounds

Simple Summary

Accelerator-based boron neutron capture therapy (BNCT) has opened up new perspectives in increasing cancer treatment efficacy, including malignant brain tumors and particularly glioblastoma. We studied dosimetry control optimization, neutron beam parameter adjustment, and two boron compound combinations (along with single and double irradiation regimens) to assess safety and increase therapy efficacy, using a U87MG xenotransplant immunodeficient mouse model. In two sets of experiments, we achieved increases in tumor-growth inhibition (to 80–83%), a neutron capture therapy ratio of 2:1 (two times higher neutron capture therapy efficacy than neutron irradiation without boron), and increases in animal life expectancy, from 9 to 107 days, by treatment parameter adjustment. These results will contribute to the development of clinical-trial protocols for accelerator-based BNCT and further innovations in this cancer treatment method.

Abstract

(1) Background: Developments in accelerator-based neutron sources moved boron neutron capture therapy (BNCT) to the next phase, where new neutron radiation parameters had to be studied for the treatment of cancers, including brain tumors. We aimed to further improve accelerator-BNCT efficacy by optimizing dosimetry control, beam parameters, and combinations of boronophenylalanine (BPA) and sodium borocaptate (BSH) administration in U87MG xenograft-bearing immunodeficient mice with two different tumor locations. (2) Methods: The study included two sets of experiments. In Experiment #1, BPA only and single or double irradiation in higher doses were used, while, in Experiment #2, BPA and BSH combinations and single or double irradiation with dosage adjustment were analyzed. Mice without treatment or irradiation after BPA or BPA+BSH injection were used as controls. (3) Results: Irradiation parameter adjustment and BPA and BSH combination led to 80–83% tumor-growth inhibition index scores, irradiation:BNCT ratios of 1:2, and increases in animal life expectancy from 9 to 107 days. (4) Conclusions: Adjustments in dosimetry control, calculation of irradiation doses, and combined use of two ¹⁰B compounds allowed for BNCT optimization that will be useful in the development of clinical-trial protocols for accelerator-based BNCT.

Response of Normal Tissues to Boron Neutron Capture Therapy (BNCT) with 10B-Borocaptate Sodium (BSH) and 10B-Paraboronophenylalanine (BPA)

Boron neutron capture therapy (BNCT) is a cancer-selective radiotherapy that utilizes the cancer targeting ¹⁰B-compound. Cancer cells that take up the compound are substantially damaged by the high liner energy transfer (LET) particles emitted mainly from the ¹⁰B(n, α⁷Li reaction. BNCT can minimize the dose to normal tissues, but it must be performed within the tolerable range of normal tissues. Therefore, it is important to evaluate the response of normal tissues to BNCT. Since BNCT yields a mixture of high and low LET radiations that make it difficult to understand the radiobiological basis of BNCT, it is important to evaluate the relative biological effectiveness (RBE) and compound biological effectiveness (CBE) factors for assessing the responses of normal tissues to BNCT. BSH and BPA are the only ¹⁰B-compounds that can be used for clinical BNCT. Their biological behavior and cancer targeting mechanisms are different; therefore, they affect the CBE values differently. In this review, we present the RBE and CBE values of BPA or BSH for normal tissue damage by BNCT irradiation. The skin, brain (spinal cord), mucosa, lung, and liver are included as normal tissues. The CBE values of BPA and BSH for tumor control are also discussed.

Boron Neutron Capture Therapy (BNCT) for Cutaneous Malignant Melanoma Using 10B-p-Boronophenylalanine (BPA) with Special Reference to the Radiobiological Basis and Clinical Results

BNCT is a radiotherapeutic method for cancer treatment that uses tumor-targeting 10B-compounds. BNCT for cutaneous melanoma using BPA, a phenylalanine derivative, was first initiated by Mishima et al. in 1987. This article reviews the radiobiological basis of melanoma control and damage to normal tissues as well as the results of clinical studies. Experimental studies showed that the compound biological effectiveness (CBE) values of the 10B (n, α)7Li reaction for melanoma control ranged from 2.5 to 3.3. The CBE values of the 10B (n, α)7Li reaction for skin damage ranged from 2.4 to 3.7 with moist desquamation as the endpoint. The required single radiation dose for controlling human melanoma was estimated to be 25 Gy-Eq or more by analyzing the 50% tumor control dose data of conventional fractionated radiotherapy. From the literature, the maximum permissible dose to human skin by single irradiation was estimated to be 18 Gy-Eq. With respect to the pharmacokinetics of BPA in patients with melanoma treated with 85-350 mg/kg BPA, the melanoma-to-blood ratio ranged from 2.1-3.8 and the skin-to-blood ratio was 1.31 ± 0.22. Good local tumor control and long-term survival of the patients were achieved in two clinical trials of BNCT conducted in Japan.

Gold Nanoparticles Permit In Situ Absorbed Dose Evaluation in Boron Neutron Capture Therapy for Malignant Tumors

Boron neutron capture therapy (BNCT) is an anticancer modality realized through ¹⁰B accumulation in tumor cells, neutron irradiation of the tumor, and decay of boron atoms with the release of alpha-particles and lithium nuclei that damage tumor cell DNA. As high-LET particle release takes place inside tumor cells absorbed dose calculations are difficult, since no essential extracellular energy is emitted. We placed gold nanoparticles inside tumor cells saturated with boron to more accurately measure the absorbed dose. T98G cells accumulated ~50 nm gold nanoparticles (AuNPs, 50 µg gold/mL) and boron-phenylalanine (BPA, 10, 20, 40 µg boron-10/mL), and were irradiated with a neutron flux of 3 × 10⁸ cm⁻²s⁻¹. Gamma-rays (411 keV) emitted by AuNPs in the cells were measured by a spectrometer and the absorbed dose was calculated using the formula D = (k × N × n)/m, where D was the absorbed dose (GyE), k—depth-related irradiation coefficient, N—number of activated gold atoms, n—boron concentration (ppm), and m—the mass of gold (g). Cell survival curves were fit to the linear-quadratic (LQ) model. We found no influence from the presence of the AuNPs on BNCT efficiency. Our approach will lead to further development of combined boron and high-Z element-containing compounds, and to further adaptation of isotope scanning for BNCT dosimetry.

Development of a dose distribution shifter to fit inside the collimator of a Boron Neutron Capture Therapy irradiation system to treat superficial tumours

The Kansai BNCT Medical Center has a cyclotron based epithermal neutron source for clinical Boron Neutron Capture Therapy. The system accelerates a proton to an energy of 30 MeV which strikes a beryllium target producing fast neutrons which are moderated down to epithermal neutrons for BNCT use. While clinical studies in the past have shown BNCT to be highly effective for malignant melanoma of the skin, to apply BNCT for superficial lesions using this system it is necessary to shift the thermal neutron distribution so that the maximum dose occurs near the surface. A dose distribution shifter was designed to fit inside the collimator to further moderate the neutrons to increase the surface dose and reduce the dose to the underlying normal tissue. Pure polyethylene was selected, and a Monte Carlo simulation was performed to determine the optimum thickness of the polyethylene slab. Compared with the original neutron beam, the shifter increased the thermal neutron flux at the skin by approximately 4 times. The measured and simulated central axis depth distribution and off axis distribution of the thermal neutron flux were found to be in good agreement. Compared with a 2 cm thick water equivalent bolus, a 26% increase in the thermal neutron flux at the surface was obtained, which would reduce the treatment time by approximately 29%. The DDS is a safe, simple and an effective tool for the treatment of superficial tumours for BNCT if an initially fast neutron beam requires moderation to maximise the thermal neutron flux at the tissue surface.

Long-term outcome of cutaneous melanoma patients treated with boron neutron capture therapy (BNCT)

Our aim was to assess the long-term clinical outcome of boron neutron capture therapy (BNCT) using 10B-para-boronophenylalanine (BPA) as the boron delivery agent for cutaneous melanoma. Eight patients (eight lesions) were treated between October 2003 and April 2014. Their ages ranged from 48 to 86 years at the time of treatment. All of the targets were primary lesions and they were located on the sole or face. No patient had evidence of regional lymph node involvement, distant metastases or an active secondary cancer. The clinical stage was cT1-2N0M0 and performance scores were <2. BNCT was carried out at the Kyoto University Research Reactor (KUR). The patients were irradiated with an epithermal neutron beam between the curative tumor dose and the tolerable skin dose. Eight patients were evaluated and six showed a complete response (CR), while two patients had a partial response (PR). Of the two patients with a PR, one has remained a PR with brown spots persisting for 7.5 years following BNCT. The tumor in the other patient recurred after 6 years at the site of persisting brown macula. The overall control rate (CR + PR without recurrence) for the cohort was 88% (7/8). There have never been any adverse events >Grade 2 for the long follow-up period. Our results suggest that BNCT may be a promising treatment modality in the management of early stage cutaneous melanoma when wide local excision is not feasible.

Boron neutron capture therapy for clear cell sarcoma

Clear cell sarcoma of tendons and aponeuroses (CCS) is a rare, malignant tumor arising in lower extremities with no effective treatment other than wide surgical resection. Here described is a case of primary CCS in the peroneal tendon of the right foot of a 54-year-old woman enrolled to undergo BNCT. The tumor mass post-BNCT disappeared totally without damage to other normal tissue, demonstrating, for the first time, the potential efficacy of BNCT in complete local control of CCS.

The importance of optimal ROIs delineation for FBPA-PET before BNCT

One of the eligible criteria for patients to receive boron neutron capture therapy (BNCT) is based on the tumour-to-normal ratio (T/N) measured by FBPA-PET. However, there is no standard protocol for normal region-of-interested delineation. With comparison of contralateral cerebrum, our study revealed the consistency (p < 0.05) and high feasibility using the cerebellum as an alternative normal tissue baseline because of its homogeneous uptake. Following RECIST version 1.1, the standard-operating-procedure (SOP) for the BNCT fulfilled the expected tumour response and tumour shrinkage rate (p < 0.05). Our modified procedure can provide more precise information for BNCT within a reasonable time.

A realistic appraisal of boron neutron capture therapy as a cancer treatment modality

Boron neutron capture therapy (BNCT) is a binary therapeutic modality based on the nuclear capture and fission reactions that occur when the stable isotope boron-10 is irradiated with neutrons to produce high-energy alpha particles and recoiling lithium-7 nuclei. In this Commentary we will focus on a number of papers that were presented at a Symposium entitled "Current Clinical Status of Boron Neutron Capture Therapy and Paths to the Future", which was held in September 2017 at the China National Convention Center in Beijing. Results were presented by clinicians from Japan, Finland, the United States, the China mainland and Taiwan, China who have been working in the multiple disciplines that are required for carrying out clinical BNCT. The main focus was on the treatment of patients with malignant brain tumors, recurrent tumors of the head and neck region, and cutaneous melanomas. The results obtained in treating these patients were reported in detail and, although most of the patients with brain tumors and head and neck cancer were not cured, there was evidence of some clinical efficacy. Although there are a number of problems that must be addressed, further clinical studies to evaluate the efficacy of BNCT are warranted. First, despite considerable effort by numerous investigators over the past 40 years, there still are only two boron-containing drugs in clinical use, L-boronophenylalanine (BPA) and sodium borocaptate (BSH). Therefore, until new and more effective boron delivery agents are developed, efforts should be directed to improving the dosing and delivery of BPA and BSH. Second, due to a variety of reasons, nuclear reactor-based BNCT has ended except for its use in the China mainland and Taiwan. Therefore, the future of BNCT depends upon the results of the ongoing Phase II clinical trials that are being carried out in Japan and the soon to be initiated trials that will be carried out in Finland. If the results obtained from these clinical trials are sufficiently promising, then BNCT will have a clear path to the future, especially for patients with the therapeutically challenging malignancies that in the past have been treated with reactor-based BNCT.

Boron neutron capture therapy for vulvar melanoma and genital extramammary Paget's disease with curative responses

BACKGROUND

Although the most commonly recommended treatment for melanoma and extramammary Paget's disease (EMPD) of the genital region is wide surgical excision of the lesion, the procedure is highly invasive and can lead to functional and sexual problems. Alternative treatments have been used for local control when wide local excision was not feasible. Here, we describe four patients with genital malignancies who were treated with boron neutron capture therapy (BNCT).

METHODS

The four patients included one patient with vulvar melanoma (VM) and three with genital EMPD. They underwent BNCT at the Kyoto University Research Reactor between 2005 and 2014 using para-boronophenylalanine as the boron delivery agent. They were irradiated with an epithermal neutron beam between the curative tumor dose and the tolerable skin/mucosal doses.

RESULTS

All patients showed similar tumor and normal tissue responses following BNCT and achieved complete responses within 6 months. The most severe normal tissue response was moderate skin erosion during the first 2 months, which diminished gradually thereafter. Dysuria or contact pain persisted for 2 months and resolved completely by 4 months.

CONCLUSIONS

Treating VM and EMPD with BNCT resulted in complete local tumor control. Based on our clinical experience, we conclude that BNCT is a promising treatment for primary VM and EMPD of the genital region. Trial registration numbers UMIN000005124.

Understanding the potentiality of accelerator based-boron neutron capture therapy for osteosarcoma: dosimetry assessment based on the reported clinical experience

BACKGROUND

Osteosarcoma is the most frequent primary malignant bone tumour, and its incidence is higher in children and adolescents, for whom it represents more than 10% of solid cancers. Despite the introduction of adjuvant and neo-adjuvant chemotherapy that markedly increased the success rate in the treatment, aggressive surgery is still needed and a considerable percentage of patients do not survive due to recurrences or early metastases. Boron Neutron Capture Therapy (BNCT), an experimental radiotherapy, was investigated as a treatment that could allow a less aggressive surgery by killing infiltrated tumour cells in the surrounding healthy tissues. BNCT requires an intense neutron beam to ensure irradiation times of the order of 1 h. In Italy, a Radio Frequency Quadrupole (RFQ) proton accelerator has been designed and constructed for BNCT, and a suitable neutron spectrum was tailored by means of Monte Carlo calculations. This paper explores the feasibility of BNCT to treat osteosarcoma using this neutron source based on accelerator.

METHODS

The therapeutic efficacy of BNCT was analysed evaluating the dose distribution obtained in a clinical case of femur osteosarcoma. Mixed field dosimetry was assessed with two different formalisms whose parameters were specifically derived from radiobiological experiments involving in vitro UMR-106 osteosarcoma cell survival assays and boron concentration assessments in an animal model of osteosarcoma. A clinical case of skull osteosarcoma treated with BNCT in Japan was re-evaluated from the point of view of dose calculation and used as a reference for comparison.

RESULTS

The results in the case of femur osteosarcoma show that the RFQ beam would ensure a suitable tumour dose painting in a total irradiation time of less than an hour. Comparing the dosimetry between the analysed case and the treated patient in Japan it turns out that doses obtained in the femur tumour are at least as good as the ones delivered in the skull osteosarcoma. The same is concluded when the comparison is carried out taking into account osteosarcoma irradiations with photon radiation therapy.

CONCLUSIONS

The possibility to apply BNCT to osteosarcoma would allow a multimodal treatment consisting in neo-adjuvant chemotherapy, high-LET selective radiation treatment and a more conservative surgery.

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.

Boron neutron capture therapy for malignant melanoma: first clinical case report in China

A phase I/II clinical trial for treating malignant melanoma by boron neutron capture therapy (BNCT) was designed to evaluate whether the world’s first in-hospital neutron irradiator (IHNI) was qualified for BNCT. In this clinical trial planning to enroll 30 patients, the first case was treated on August 19, 2014. We present the protocol of this clinical trial, the treating procedure, and the clinical outcome of this first case. Only grade 2 acute radiation injury was observed during the first four weeks after BNCT and the injury healed after treatment. No late radiation injury was found during the 24-month follow-up. Based on positron emission tomography-computed tomography (PET/CT) scan, pathological analysis and gross examination, the patient showed a complete response to BNCT, indicating that BNCT is a potent therapy against malignant melanoma and IHNI has the potential to enable the delivery of BNCT in hospitals.

Induced radioactivity in the blood of cancer patients following Boron Neutron Capture Therapy

Since 1990, Boron Neutron Capture Therapy (BNCT) has been used for over 400 cancer patients at the Kyoto University Research Reactor Institute (KURRI). After BNCT, the patients are radioactive and their (24)Na and (38)Cl levels can be detected via a Na-I scintillation counter. This activity is predominantly due to (24)Na, which has a half-life of 14.96 h and thus remains in the body for extended time periods. Radioactive (24)Na is mainly generated from (23)Na in the target tissue that is exposed to the neutron beam in BNCT. The purpose of this study is to evaluate the relationship between the radioactivity of blood (24)Na following BNCT and the absorbed gamma ray dose in the irradiated field. To assess blood (24)Na, 1 ml of peripheral blood was collected from 30 patients immediately after the exposure, and the radioactivity of blood (24)Na was determined using a germanium counter. The activity of (24)Na in the blood correlated with the absorbed gamma ray doses in the irradiated field. For the same absorbed gamma ray dose in the irradiated field, the activity of blood (24)Na was higher in patients with neck or lung tumors than in patients with brain or skin tumors. The reasons for these findings are not readily apparent, but the difference in the blood volume and the ratio of bone to soft tissue in the irradiated field, as well as the dose that leaked through the clinical collimator, may be responsible.

A new understanding in the epidemiology of melanoma

The incidence of melanoma is continuing to increase worldwide. UV exposure is a known risk factor for melanoma. Geographic location is known to influence UV exposure and the distribution of the incidence of melanoma. Furthermore, epidemiologic data suggest that gender and genetics may influence the distribution of melanoma on the body surface and histopathologic characteristics of the lesion. This article describes what is known about the impact of gender, ethnicity and geography on the progression of melanoma. Advanced-stage cutaneous melanoma has a median survival time of less than 1 year. Surgical removal, radiotherapy, chemotherapy, targeted therapies and a variety of immunotherapies have been utilized in the treatment of melanoma. Current treatment strategies and the results of recent clinical trials are also discussed in this article.

Role of p53 mutation in the effect of boron neutron capture therapy on oral squamous cell carcinoma

Background

Boron neutron capture therapy (BNCT) is a selective radiotherapy, being effective for the treatment of even advanced malignancies in head and neck regions as well as brain tumors and skin melanomas. To clarify the role of p53 gene, the effect of BNCT on oral squamous cell carcinoma (SCC) cells showing either wild- (SAS/neo) or mutant-type (SAS/mp53) p53 was examined.

Methods

Cells were exposed to neutron beams in the presence of boronophenylalanine (BPA) at Kyoto University Research Reactor. Treated cells were monitored for modulations in colony formation, proliferation, cell cycle, and expression of cell cycle-associated proteins.

Results

When SAS/neo and SAS/mp53 cells were subjected to BNCT, more suppressive effects on colony formation and cell viability were observed in SAS/neo compared with SAS/mp53 cells. Cell cycle arrest at the G1 checkpoint was observed in SAS/neo, but not in SAS/mp53. Apoptotic cells increased from 6 h after BNCT in SAS/neo and 48 h in SAS/mp53 cells. The expression of p21 was induced in SAS/neo only, but G2 arrest-associated proteins including Wee1, cdc2, and cyclin B1 were altered in both cell lines.

Conclusion

These results indicate that oral SCC cells with mutant-type are more resistant to BNCT than those with wild-type p53, and that the lack of G1 arrest and related apoptosis may contribute to the resistance. At a physical dose affecting the cell cycle, BNCT inhibits oral SCC cells in p53-dependent and -independent manners.

Early effects of boron neutron capture therapy on rat glioma models

Early effects of boron neutron capture therapy (BNCT) on malignant glioma are characterized by reduction of the enhancement area and regression of the peritumoral edema radiologically. The aim of this study was to investigate the early histological changes of tumors and inflammatory cells after BNCT in the rat brain. Rats were treated with BNCT using boronophenylalanine (BPA) 7 days after implantation of C6 glioma cells. The tumors were assessed with magnetic resonance imaging and histopathological examination at 4 days after BNCT. The mean tumor volumes were 39 +/- 2 mm3 in the BNCT group and 134 +/- 18 mm3 in the control group. In the BNCT group, tumor cells showed a less pleomorphic appearance with atypical nuclei and mitotic figures. The Ki-67 labeling index was 6.5% +/- 4.7% in the BNCT and 35% +/- 3.8% in the control group. The reactions of the inflammatory cells were examined with ED-1 as macrophage marker and OX42 as microglia marker. ED-1- and OX-42-positive cells were reduced both in the core and the marginal area of the tumor in the BNCT group. It is suggested that BNCT reduced tumor progression by suppression of proliferation. Inhibition of the activated macrophages may relate to reduced peritumoral edema in the early phase.

A Preliminary Experimental Study of Boron Neutron Capture Therapy for Malignant Tumors Spreading in Thoracic Cavity

BACKGROUND

The purpose of the present study is to verify the treatment effects of boron neutron capture therapy (BNCT) in ectopic tumors implanted in the thoracic cavity mimicking malignant pleural mesothelioma (MPM).

METHODS

The tumor model was created by implanting murine squamous cell carcinoma cells into the thoracic cavity. Mice were sorted into four groups: group I for non-treatment; group II for neutron irradiation; group III for gamma-ray irradiation; and group IV for BNCT irradiation. The antitumor effect was evaluated on the basis of the change in survival time. To assess the effects of BNCT on normal lung, non-tumor bearing mice were treated using the same method as done to the tumor-burdened mice.

RESULTS

The BNCT group had a longer survival time of 31 days (range 5 - 60), which was significantly longer than that of the non-treated control group (P = 0.011), but not significantly different from that of the neutron and gamma-ray groups (P = 0.067 and 0.094, respectively). In the BNCT and neutron groups, incidence of minimal lung fibrosis was significantly higher compared with the non-treated control group (P = 0.003 and 0.04, respectively).

CONCLUSIONS

BNCT is a potentially promising treatment for malignant tumors spreading in the thoracic cavity such as MPM.

First clinical case of boron neutron capture therapy for head and neck malignancies using 18F-BPA PET

BACKGROUND

We investigated the application of boron neutron capture therapy (BNCT) to suitable cancers other than glioblastoma and melanoma. Head and neck malignancies were consequently selected as adaptable cancers. We reported the clinical results of our first case treated and discussed several advantages to the application of BNCT to head and neck tumors.

METHODS

The patient was a 48-year-old woman with recurrent submandibular gland cancer. We confirmed the p-boronophenylalanine (BPA)-accumulating capacity of the tumor by fluorine-18-labeled p-boronophenylalanine positron emission tomography ((18)F-BPA PET) before BNCT. The tumor/normal tissue boron concentration ratio was 2.9. The patient underwent a preirradiation CT scan for treatment planning performed using the "SERA" software program. The tumor was irradiated at the Kyoto University Research Reactor with epithermal neutrons 5 MW for 90 minutes. The tumor dose and normal tissue dose calculated ranged from 20.0 to 25.2 Gy and from 3.2 to 5.8 Gy, respectively.

RESULTS

To date there has been continuous complete regression in the tumor and no acute and chronic complications for 1.5 years.

CONCLUSIONS

Although only 1 patient has shown complete regression and additional long-term follow-up should be required to assess this treatment, we believe that head and neck tumors are suitable for BNCT and that such excellent results will have a great impact on patients in the near future.