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

Efficacy and safety of boron neutron capture therapy for Hypopharyngeal/Laryngeal cancer patients with previous head and neck irradiation

BACKGROUND AND PURPOSE

Patients with hypopharyngeal cancer (HPC)/laryngeal cancer (LCA) with a history of head and neck irradiation are often difficult to treat with conventional radiotherapy. This study aimed to clarify the efficacy and safety of boron neutron capture therapy (BNCT) for HPC/LCA.

MATERIALS AND METHODS

In this retrospective study, HPC/LCA with local lesions were analyzed, including both recurrent cases after treatment and second primary cases. The primary endpoints were tumor response and incidence of adverse events (AEs) after BNCT. The secondary endpoints were local control (LC), progression-free survival (PFS), and overall survival (OS). Evaluation of tumor response was terminated when any additional treatment was administered, and only survival data were collected.

RESULTS

The analysis included 25 and 11 cases of HPC and LCA, respectively. All had a history of head and neck irradiation, and median dose of prior radiotherapy was 70 Gy. The complete response (CR) rate was 72%, overall response rate was 84%, and the 1-year LC and PFS were 63.1% and 53.7%, respectively. The median survival time was 15.5 months, and the 2-year OS was 79.8%. Of the 27 patients with CR, 11 cases recurred at a median of 6.0 months. The acute G3 AEs were oral mucositis (6%), pharyngeal mucositis (3%), and soft tissue infection (3%). Acutely, there were no G4-5 AEs, except hyperamylasemia, and in the late phase, there were no G3 or higher AEs.

CONCLUSION

BNCT can be achieve good tumor response while preserving the larynx without severe AEs.

Evaluation of calculation accuracy and computation time in a commercial treatment planning system for accelerator-based boron neutron capture therapy

This study aims to evaluate the feasibility of using a commercially available boron neutron capture therapy (BNCT) dose calculation program (NeuCure® Dose Engine) in terms of calculation accuracy and computation time. Treatment planning was simulated under the following calculation parameters: 1.5-5.0 mm grid sizes and 1-10% statistical uncertainties. The calculated monitor units (MUs) and computation times were evaluated. The MUs calculated on grid sizes larger than 2 mm were overestimated by 2% compared with the result of 1.5 mm grid. We established the two-step method for the routine administration of BNCT: multiple calculations involved in beam optimization should be done at a 5 mm grid and a 10% statistical uncertainty (the shortest computation time: 10.3 ± 2.1 min) in the first-step, and final dose calculations should be performed at a 2 mm grid and a 10% statistical uncertainty (satisfied clinical accuracy: 6.9 ± 0.3 h) in the second-step.

Evaluating optimal quality assurance and quality control conditions of activation measurements at the accelerator-based boron neutron capture therapy system employing a lithium target

Evaluating neutron output is important to ensure proper dose delivery for patients in boron neutron capture therapy (BNCT). It requires efficient quality assurance (QA) and quality control (QC) while maintaining measurement accuracy. This study investigated the optimal measurement conditions for QA/QC of activation measurements using a high-purity germanium (HP-Ge) detector in an accelerator-based boron neutron capture therapy (AB-BNCT) system employing a lithium target. The QA/QC uncertainty of the activation measurement was evaluated based on counts, reproducibility, and standard radiation source uncertainties. Measurements in a polymethyl methacrylate (PMMA) cylindrical phantom using aluminum-manganese (Al-Mn) foils and aluminum-gold (Al-Au) foils and measurements in a water phantom using gold wire with and without cadmium cover were performed to determine the optimal measurement conditions. The QA/QC uncertainties of the activation measurements were 4.5% for Au and 4.6% for Mn. The optimum irradiation proton charge and measurement time were determined to be 36 C and 900 s for measurements in a PMMA cylindrical phantom, 7.0 C and 900 s for gold wire measurements in a water phantom, and 54 C and 900 s at 0-2.2 cm depth and 3,600 s at deeper depths for gold wire measurements with cadmium cover. Our results serve as a reference for determining measurement conditions when performing QA/QC of activation measurements using HP-Ge detectors at an AB-BNCT employing a lithium target.

A method for delivering the required neutron fluence in an accelerator-based boron neutron capture therapy system employing a lithium target

Accelerator-based boron neutron capture therapy (BNCT) systems employing a solid-state lithium target indicated the reduction of neutron flux over the lifetime of a target, and its reduction could represent the neutron flux model. This study proposes a novel compensatory approach for delivering the required neutron fluence and validates its clinical applicability. The proposed approach relies on the neutron flux model and the cumulative sum of real-time measurements of proton charges. The accuracy of delivering the required neutron fluence for BNCT using the proposed approach was examined in five Li targets. With the proposed approach, the required neutron fluence could be delivered within 3.0%, and within 1.0% in most cases. However, those without using the proposed approach exceeded 3.0% in some cases. The proposed approach can consider the neutron flux reduction adequately and decrease the effect of uncertainty in neutron measurements. Therefore, the proposed approach can improve the accuracy of delivering the required fluence for BNCT even if a neutron flux reduction is expected during treatment and over the lifetime of the Li target. Additionally, by adequately revising the approach, it may apply to other type of BNCT systems employing a Li target, furthering research in this direction.

Performance evaluation of a 71Ga(n,γ)72Ga reaction-based epithermal neutron flux detector at an AB-BNCT device

The 71Ga(n,γ)72Ga reaction-based epithermal neutron flux detectors are novel instruments developed to measure the epithermal neutron flux of boron neutron capture therapy (BNCT) treatment beams. In this study, a spherical epithermal neutron flux detector using 71Ga(n,γ)72Ga reaction was prototyped. The performance of the detector was experimentally evaluated at an accelerator-based BNCT (AB-BNCT) device developed by Lanzhou University, China. Based on the experimental results and related analysis, we demonstrated that the detector is a reliable tool for the quality assurance of BNCT treatment beams.

Feasibility study of optical imaging of the boron-dose distribution by a liquid scintillator in a clinical boron neutron capture therapy field

BACKGROUND

Evaluation of the boron dose is essential for boron neutron capture therapy (BNCT). Nevertheless, a direct evaluation method for the boron-dose distribution has not yet been established in the clinical BNCT field. To date, even in quality assurance (QA) measurements, the boron dose has been indirectly evaluated from the thermal neutron flux measured using the activation method with gold foil or wire and an assumed boron concentration in the QA procedure. Recently, we successfully conducted optical imaging of the boron-dose distribution using a cooled charge-coupled device (CCD) camera and a boron-added liquid scintillator at the E-3 port facility of the Kyoto University Research Reactor (KUR), which supplies an almost pure thermal neutron beam with very low gamma-ray contamination. However, in a clinical accelerator-based BNCT facility, there is a concern that the boron-dose distribution may not be accurately extracted because the unwanted luminescence intensity, which is irrelevant to the boron dose is expected to increase owing to the contamination of fast neutrons and gamma rays.

PURPOSE

The purpose of this research was to study the validity of a newly proposed method using a boron-added liquid scintillator and a cooled CCD camera to directly observe the boron-dose distribution in a clinical accelerator-based BNCT field.

METHOD

A liquid scintillator phantom with 10 B was prepared by filling a small quartz glass container with a commercial liquid scintillator and boron-containing material (trimethyl borate); its natural boron concentration was 1 wt%. Luminescence images of the boron-neutron capture reaction were obtained in a water tank at several different depths using a CCD camera. The contribution of background luminescence, mainly due to gamma rays, was removed by subtracting the luminescence images obtained using another sole liquid scintillator phantom (natural boron concentration of 0 wt%) at each corresponding depth, and a depth profile of the boron dose with several discrete points was obtained. The obtained depth profile was compared with that of calculated boron dose, and those of thermal neutron flux which were experimentally measured or calculated using a Monte Carlo code.

RESULTS

The depth profile evaluated from the subtracted images indicated reasonable agreement with the calculated boron-dose profile and thermal neutron flux profiles, except for the shallow region. This discrepancy is thought to be due to the contribution of light reflected from the tank wall. The simulation results also demonstrated that the thermal neutron flux would be severely perturbed by the 10 B-containing phantom if a relatively larger container was used to evaluate a wide range of boron-dose distributions in a single shot. This indicates a trade-off between the luminescence intensity of the 10 B-added phantom and its perturbation effect on the thermal neutron flux.

CONCLUSIONS

Although a partial discrepancy was observed, the validity of the newly proposed boron-dose evaluation method using liquid-scintillator phantoms with and without 10 B was experimentally confirmed in the neutron field of an accelerator-based clinical BNCT facility. However, this study has some limitations, including the trade-off problem stated above. Therefore, further studies are required to address these limitations.

Dose Rate Effect on Cell Survival in BNCT

The output constancy of the accelerator used for boron neutron capture therapy (BNCT) is essential to ensuring anti-tumor efficacy and safety. BNCT as currently practiced requires a wide variety of beam quality assessments to ensure that RBE dose errors are maintained within 5%. However, the necessity of maintaining a constant beam dose rate has not been fully discussed. We therefore clarified the effect of different physical dose rates of the accelerator BNCT on biological effects. SAS and A172 cells exposed to 10B-boronophenylalanine were irradiated using a neutron beam (normal operating current, 100 μA) at the Aomori Quantum Science Center. Thermal neutron flux was attenuated to 50.0 ± 0.96% under 50 μA irradiation compared to that under 100 μA irradiation. Cells were given physical doses of 1.67 and 3.36 Gy at 30 and 60 mC, respectively, and survival was significantly increased after 50 μA irradiation for both cell types (p = 0.0052 for SAS; p = 0.046 for A172, for 60 mC). Differences in accelerator BNCT beam dose rates have non-negligible effects on biological effects. Dose rate fluctuations and differences should not be easily permitted to obtain consistent biological effects.

Characterization and clinical utility of different collimator shapes in accelerator-based BNCT systems for head and neck cancer

NeuCure® is the only accelerator-based boron neutron capture therapy (BNCT) system in the world with pharmaceutical approval. Until now, only flat collimators (FCs) on the patient side have been installed. However, in some cases of head and neck cancer patients, positioning the patient close enough to the collimator when using FCs was difficult. Thus, there are concerns about the prolongation of the irradiation time and overdose to normal tissues. To address these issues, a collimator with a convex-extended section on the patient side (extended collimators [ECs]) was developed, and its pharmaceutical approval was obtained in February 2022. This study evaluated the physical characterization and usefulness of each collimator using a simple geometry water phantom model and human model. In the water phantom model, the thermal neutron fluxes at 2 cm depth on the central axis were 5.13 × 108, 6.79 × 108, 1.02 × 109, and 1.17 × 109n/cm2/s for FC(120), FC(150), EC50(120), and EC100(120), respectively, when the distance from the irradiation aperture was kept constant at 18 cm. With ECs, the relative off-axis thermal neutron flux decreased steeply. In the hypopharyngeal cancer human model, the tumor dose changes were within <2%, but the maximum oral mucosa doses were 7.79, 8.51, 6.76, and 4.57 Gy-Eq, respectively. The irradiation times were 54.3, 41.3, 29.2, and 24.8 min, respectively. In cases where positioning the patient close to the collimator is difficult, the use of ECs may reduce the dose to normal tissues and shorten the irradiation time.

Initial evaluation of accelerator-based neutron source system at the Shonan Kamakura General Hospital

An accelerator-based boron neutron capture therapy (AB-BNCT) system was installed at the Shonan Kamakura General Hospital (SKGH). We confirmed that a stable operation was possible for 1 h at a current of 30 mA. The evaluated thermal neutron flux was 2.8 × 10⁹ cm^-2 s^-1 and in good agreement (±5%) with the calculated values. The daily variation was within ±2%. The ambient dose rate due to residual radioactivity after irradiation was approximately 5 μSv/h using a lead shutter.