The medical-irradiation characteristics for neutron capture therapy at the Heavy Water Neutron Irradiation Facility of Kyoto University Research Reactor

Neutron flux evaluation algorithm with a combination of Monte Carlo and removal-diffusion calculation methods for boron neutron capture therapy

BACKGROUND

In Japan, the clinical treatment of boron neutron capture therapy (BNCT) has been applied to unresectable, locally advanced, and recurrent head and neck carcinomas using an accelerator-based neutron source since June of 2020. Considering the increase in the number of patients receiving BNCT, efficiency of the treatment planning procedure is becoming increasingly important. Therefore, novel and rapid dose calculation algorithms must be developed. We developed a novel algorithm for calculating neutron flux, which comprises of a combination of a Monte Carlo (MC) method and a method based on the removal-diffusion (RD) theory (RD calculation method) for the purpose of dose calculation of BNCT.

PURPOSE

We present the details of our novel algorithm and the verification results of the calculation accuracy based on the MC calculation result.

METHODS

In this study, the "MC-RD" calculation method was developed, wherein the RD calculation method was used to calculate the thermalization process of neutrons and the MC method was used to calculate the moderation process. The RD parameters were determined by MC calculations in advance. The MC-RD calculation accuracy was verified by comparing the results of the MC-RD and MC calculations with respect to the neutron flux distributions in each of the cubic and head phantoms filled with water.

RESULTS

Comparing the MC-RD calculation results with those of MC calculations, it was found that the MC-RD calculation accurately reproduced the thermal neutron flux distribution inside the phantom, with the exception of the region near the surface of the phantom.

CONCLUSIONS

The MC-RD calculation method is useful for the evaluation of the neutron flux distribution for the purpose of BNCT dose calculation, except for the region near the surface.

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.

Profile analysis of adverse events after boron neutron capture therapy for head and neck cancer: a sub-analysis of the JHN002 study

The purpose of this study was to outline the course and profile of adverse events specific to boron neutron capture therapy (BNCT) for head and neck cancer. This was a sub-analysis of the phase II JHN002 trial. Patients received 400 mg/kg borofalan(10B), followed by neutron irradiation. The course of adverse events after BNCT was documented in the JHN002 Look Up study. Patients were grouped into face/front (FF), face/lateral (FL) and neck (N) beam groups according to the point of skin incidence of the epithermal neutron beam axis, and the profile of adverse events dependent on beam incidence position was examined. The courses of adverse events in eight recurrent squamous cell carcinoma (R-SCC) and 13 recurrent or locally advanced non-SCC cases were analyzed. Median interval to complete recovery was 23 days (interquartile range (IQR), 14-48 days) for oral mucositis, 40 days (IQR, 24-56 days) for dermatitis, 58 days (IQR, 53-80 days) for dysgeusia and 156 days (IQR, 82-163 days) for alopecia. In the FF beam group, parotitis (P = 0.007) was less frequent, while oral mucositis (P = 0.032), fatigue (P = 0.002), conjunctivitis (P = 0.001), epistaxis (P = 0.001) and abdominal discomfort (P = 0.029) tended to be more frequent than in the FL and N beam groups. Courses and irradiation site-specific profiles of adverse events in BNCT for head and neck cancer were identified. This profile may be useful for considering interventions to prevent exacerbation of treatment-related adverse events on BNCT.

Neutron flux evaluation model provided in the accelerator-based boron neutron capture therapy system employing a solid-state lithium target

An accelerator-based boron neutron capture therapy (BNCT) system employing a solid-state Li target can achieve sufficient neutron flux for treatment although the neutron flux is reduced over the lifetime of its target. In this study, the reduction was examined in the five targets, and a model was then established to represent the neutron flux. In each target, a reduction in neutron flux was observed based on the integrated proton charge on the target, and its reduction reached 28% after the integrated proton charge of 2.52 × 10⁶ mC was delivered to the target in the system. The calculated neutron flux acquired by the model was compared to the measured neutron flux based on an integrated proton charge, and the mean discrepancies were less than 0.1% in all the targets investigated. These discrepancies were comparable among the five targets examined. Thus, the reduction of the neutron flux can be represented by the model. Additionally, by adequately revising the model, it may be applicable to other BNCT systems employing a Li target, thus furthering research in this direction. Therefore, the established model will play an important role in the accelerator-based BNCT system with a solid-state Li target in controlling neutron delivery and understanding the neutron output characteristics.

Design and construction of an accelerator-based boron neutron capture therapy (AB-BNCT) facility with multiple treatment rooms at the Southern Tohoku BNCT Research Center

Installation of an accelerator-based boron neutron capture therapy (AB-BNCT) system was started in April 2014 at the Southern Tohoku BNCT Research Center (STBRC), and clinical trials began in January 2016. There are two treatment rooms, which have same specifications, and the beam quality equivalency was confirmed both rooms. Here, we describe the design and construction of the first hospital-based AB-BNCT facility in the world with multiple treatment rooms.

Dependence of neutrons generated by 7Li(p,n) reaction on Li thickness under free-air condition in accelerator-based boron neutron capture therapy system employing solid-state Li target

PURPOSE

An accelerator-based boron neutron capture therapy (BNCT) system with a solid-state Li target is reported to have degradation of the Li target. The degradation reduces the Li thickness, which may change spectra of the generated neutrons corresponding to the Li thickness. This study aims to examine the relationship between the Li thickness and the generated neutrons and to investigate the effects of the Li thickness on the absorbed dose in BNCT.

METHOD

The neutron energy spectra were calculated via Monte Carlo simulation for Li thicknesses ranging from 20 to 150 μm. Using the system, the saturated radioactivity of gold induced by reactions between ¹⁹⁷Au and the generated neutrons was evaluated with the simulation and the measurement, and those were compared. Additionally, for each Li thickness, the saturated radioactivity was compared with the number of generated neutrons. The absorbed doses delivered by ¹⁰B(n,α)⁷Li, ¹⁴N(n,p)¹⁴C, ¹H(n, g)²H, and (n,n') reactions in water were also calculated for each Li thickness.

RESULTS

The measurement and simulation indicated a reduction in the number of neutrons due to the degradation of the Li target. However, the absorbed doses were comparable for each Li thickness when the requisite number of neutrons for BNCT was delivered. Additionally, the saturated radioactivity of ¹⁹⁸Au could be a surrogate for the number of neutrons even if the Li thickness was varied.

CONCLUSIONS

No notable effect to the absorbed dose was observed when required neutron fluence was delivered in the BNCT even if the degradation of the Li was observed.

Cerebrospinal fluid dissemination of high-grade gliomas following boron neutron capture therapy occurs more frequently in the small cell subtype of IDH1R132H mutation-negative glioblastoma

We have used boron neutron capture therapy (BNCT) to treat patients in Japan with newly diagnosed or recurrent high-grade gliomas and have observed a significant increase in median survival time following BNCT. Although cerebrospinal fluid dissemination (CSFD) is not usually seen with the current standard therapy of patients with glioblastoma (GBM), here we report that subarachnoid or intraventricular CSFD was the most frequent cause of death for a cohort of our patients with high-grade gliomas who had been treated with BNCT. The study population consisted of 87 patients with supratentorial high-grade gliomas; 41 had newly diagnosed tumors and 46 had recurrent tumors. Thirty of 87 patients who were treated between January 2002 and July 2013 developed CSFD. Tumor histology before BNCT and immunohistochemical staining for two molecular markers, Ki-67 and IDH1^R132H, were evaluated for 20 of the 30 patients for whom pathology slides were available. Fluorescence in situ hybridization (FISH) was performed on 3 IDH1^R132H-positive and 1 control IDH1^R132H-negative tumors in order to determine chromosome 1p and 19q status. Histopathologic evaluation revealed that 10 of the 20 patients' tumors were IDH1^R132H-negative small cell GBMs. The remaining patients had tumors consisting of other IDH1^R132H-negative GBM variants, an IDH1^R132H-positive GBM and two anaplastic oligodendrogliomas. Ki-67 immunopositivity ranged from 2 to 75%. In summary, IDH1^R132H-negative GBMs, especially small cell GBMs, accounted for a disproportionately large number of patients who had CSF dissemination. This suggests that these tumor types had an increased propensity to disseminate via the CSF following BNCT and that these patients are at high risk for this clinically serious event.

Modeling the detection efficiency of an HP-Ge detector for use in boron neutron capture therapy

The multi-foil method is commonly used to determine upon an energy spectrum of neutrons in boron neutron capture therapy. The method requires to measure the radioactivation of the foils. This study develops a simple modeling procedure of a high-purity Ge detector, which is used to measure the radioactivation, in order to calculate the detection efficiency with GEANT4. By changing four parameters from their manufacturing specifications of the detector, the simulated detection efficiency is able to reproduce the actual detection efficiency.

Evaluation of radioactivity in the bodies of mice induced by neutron exposure from an epi-thermal neutron source of an accelerator-based boron neutron capture therapy system

This study aimed to evaluate the residual radioactivity in mice induced by neutron irradiation with an accelerator-based boron neutron capture therapy (BNCT) system using a solid Li target. The radionuclides and their activities were evaluated using a high-purity germanium (HP-Ge) detector. The saturated radioactivity of the irradiated mouse was estimated to assess the radiation protection needs for using the accelerator-based BNCT system. ²⁴Na, ³⁸Cl, ^80mBr, ⁸²Br, ⁵⁶Mn, and ⁴²K were identified, and their saturated radioactivities were (1.4 ± 0.1) × 10², (2.2 ± 0.1) × 10¹, (3.4 ± 0.4) × 10², 2.8 ± 0.1, 8.0 ± 0.1, and (3.8 ± 0.1) × 10¹ Bq/g/mA, respectively. The ²⁴Na activation rate at a given neutron fluence was found to be consistent with the value reported from nuclear-reactor-based BNCT experiments. The induced activity of each nuclide can be estimated by entering the saturated activity of each nuclide, sample mass, irradiation time, and proton current into the derived activation equation in our accelerator-based BNCT system.

Development of Monte Carlo based real-time treatment planning system with fast calculation algorithm for boron neutron capture therapy

PURPOSE

We simulated the effect of patient displacement on organ doses in boron neutron capture therapy (BNCT). In addition, we developed a faster calculation algorithm (NCT high-speed) to simulate irradiation more efficiently.

METHODS

We simulated dose evaluation for the standard irradiation position (reference position) using a head phantom. Cases were assumed where the patient body is shifted in lateral directions compared to the reference position, as well as in the direction away from the irradiation aperture. For three groups of neutron (thermal, epithermal, and fast), flux distribution using NCT high-speed with a voxelized homogeneous phantom was calculated. The three groups of neutron fluxes were calculated for the same conditions with Monte Carlo code. These calculated results were compared.

RESULTS

In the evaluations of body movements, there were no significant differences even with shifting up to 9mm in the lateral directions. However, the dose decreased by about 10% with shifts of 9mm in a direction away from the irradiation aperture. When comparing both calculations in the phantom surface up to 3cm, the maximum differences between the fluxes calculated by NCT high-speed with those calculated by Monte Carlo code for thermal neutrons and epithermal neutrons were 10% and 18%, respectively. The time required for NCT high-speed code was about 1/10th compared to Monte Carlo calculation.

CONCLUSIONS

In the evaluation, the longitudinal displacement has a considerable effect on the organ doses. We also achieved faster calculation of depth distribution of thermal neutron flux using NCT high-speed calculation code.

Evaluation of thermal neutron irradiation field using a cyclotron-based neutron source for alpha autoradiography

It is important to measure the microdistribution of (10)B in a cell to predict the cell-killing effect of new boron compounds in the field of boron neutron capture therapy. Alpha autoradiography has generally been used to detect the microdistribution of (10)B in a cell. Although it has been performed using a reactor-based neutron source, the realization of an accelerator-based thermal neutron irradiation field is anticipated because of its easy installation at any location and stable operation. Therefore, we propose a method using a cyclotron-based epithermal neutron source in combination with a water phantom to produce a thermal neutron irradiation field for alpha autoradiography. This system can supply a uniform thermal neutron field with an intensity of 1.7×10(9) (cm(-2)s(-1)) and an area of 40mm in diameter. In this paper, we give an overview of our proposed system and describe a demonstration test using a mouse liver sample injected with 500mg/kg of boronophenyl-alanine.

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.

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.

Genomic instability induced by ionizing radiation in human hepatocytes

The aim of this study was to characterize genomic instability induced by ionizing radiation (IR) in human hepatocytes as reflected by alterations in cloning efficiency, micronucleus (MN) frequency, and apoptosis. The human normal liver 7702 cell line (HL7702) was subjected to initial irradiation of (60)Co-γ ray at doses of 0 (control group), 2, 4, 6, 8, or 10 Gy in each group. Progeny of surviving cells from a second irradiation at dose of 2 Gy were cultured for 15 passages until they were transferred. The cloning efficiency, MN frequency, and apoptotic rate were measured after the initial irradiation, and repeated at passage 15 before and after the second irradiation. The initial irradiation resulted in a dose-dependent decline in cloning efficiency and an increase in MN frequency and apoptotic rate. At passage 15 in progeny of initially irradiated cells, cloning efficiency, MN frequency returned to control levels while apoptotic rate rose. After the second irradiation, cloning efficiency fell while a rise in MN frequency and apoptosis occurred. Our results show that the second irradiation may further enhance cell progeny injury induced by initial irradiation, such that genomic instability that may be difficult to detect after one irradiation is more apparent with subsequent irradiation.

A feasibility study of the post-irradiation dose estimation with SPECT technique for BNCT

Various radioactive nuclei are generated in and around the target volume after the irradiation for boron neutron capture therapy. By measuring and estimating the distributions of these nuclei with the technique of single photon emission computed tomography (SPECT), more accurate post-irradiation dose-estimation can be expected. The feasibility study was performed mainly by simulation. The radioactivity densities for Cl-38, Ca-49 and Na-24 just after the irradiation were calculated to be 100-1000 Bq/cm(3) in and around the target volume. It was confirmed that these nuclei could be detected by SPECT under some conditions. Using the density differences for these generated nuclei, discrimination between soft-tissue area and bone area can be achieved. In focusing on the shallower 1cm(3) voxel, the necessary counting-time for Na-24 was estimated to be a few tens of minutes when the distance between the SPECT detector and the voxel was shortened to 6 cm.

Evaluation of micronucleus induction in lymphocytes of patients following boron-neutron-capture-therapy: a comparison with thyroid cancer patients treated with radioiodine

This study evaluated micronuclei induction in peripheral T-lymphocytes after BNCT (boron neutron capture therapy) irradiation of 15 brain tumor and 20 head-and-neck cancer patients. In all of these patients, the micronucleus frequency increased after BNCT; the number of micronuclei per 1000 binucleated T-lymphocytes after BNCT increased by 33.0 +/- 12.2 for brain tumor patients and 22.8 +/- 10.3 for head-and-neck cancer patients. In 14 thyroid cancer patients who were administered radioiodine (3.3-5.6 GBq) treatment, the frequency of micronuclei after internal radioiodine therapy increased to 105.0 +/- 30.5 per 1000 binucleated T-lymphocytes. The increased micronucleus frequency of BNCT patients was less than one-third that seen for thyroid cancer patients after radioiodine treatment. These results demonstrate the usefulness of BNCT for selective high-LET radiotherapy, in association with a low irradiation effect of cytological radiation damage after BNCT, which provides a high tumor target dose for each cancer patient.

Improvement effect on the depth-dose distribution by CSF drainage and air infusion of a tumour-removed cavity in boron neutron capture therapy for malignant brain tumours

Boron neutron capture therapy (BNCT) without craniotomy for malignant brain tumours was started using an epi-thermal neutron beam at the Kyoto University Reactor in June 2002. We have tried some techniques to overcome the treatable-depth limit in BNCT. One of the effective techniques is void formation utilizing a tumour-removed cavity. The tumorous part is removed by craniotomy about 1 week before a BNCT treatment in our protocol. Just before the BNCT irradiation, the cerebro-spinal fluid (CSF) in the tumour-removed cavity is drained out, air is infused to the cavity and then the void is made. This void improves the neutron penetration, and the thermal neutron flux at depth increases. The phantom experiments and survey simulations modelling the CSF drainage and air infusion of the tumour-removed cavity were performed for the size and shape of the void. The advantage of the CSF drainage and air infusion is confirmed for the improvement in the depth-dose distribution. From the parametric surveys, it was confirmed that the cavity volume had good correlation with the improvement effect, and the larger effect was expected as the cavity volume was larger.

Dosimetric study of boron neutron capture therapy with borocaptate sodium (BSH)/lipiodol emulsion (BSH/lipiodol-BNCT) for treatment of multiple liver tumors

PURPOSE

We performed a computational study to investigate the feasibility of borocaptate sodium (BSH)/lipiodol-boron neutron capture therapy (BSH/lipiodol-BNCT) for multiple liver tumors using Simulation Environment for Radiotherapy Applications (SERA), a currently available BNCT treatment planning system.

METHODS AND MATERIALS

Three treatment plans for BSH/lipiodol-BNCT using two or three epithermal neutron beams in one fraction were generated for 4 patients with multiple liver tumors using the SERA system. The (10)B concentrations in the tumor and the liver assumed in the study were 197.3 and 15.3 ppm, respectively; and were obtained from experimental studies in animals. The therapeutic gain factors for the liver tumors, defined as the minimum dose to the tumor/maximum dose to the liver, and the inhomogeneity index of the thermal neutron fluence for the whole of the liver, defined as the maximum neutron fluence - minimum neutron fluence/mean neutron fluence, were evaluated in each plan.

RESULTS

Three epithermal neutron beams incident on the anterior, posterior, and right side of the patient can deliver the most homogeneous distribution of thermal neutron fluence to the whole of the liver and provide the greatest therapeutic gain factors for tumors in the right lobe and approximately equal therapeutic gain factors for tumors in the left lobe, compared with the two opposed (anterior-posterior) and two orthogonal (anterior-right) beams.

CONCLUSIONS

From a dosimetric viewpoint, the BSH/lipiodol-BNCT treatment plan using three epithermal neutron beams is the most suitable for the treatment of multiple liver tumors.