Demonstration of a high-intensity neutron source based on a liquid-lithium target for Accelerator based Boron Neutron Capture Therapy

Long-term beam output stability of an accelerator-based boron neutron capture therapy system

BACKGROUND

Accelerator-based boron neutron capture therapy (AB-BNCT) systems are becoming commercially available and are expected to be widely used in hospitals. To ensure the safety of BNCT, establishing a quality assurance (QA) program and properly managing the stability of the system are necessary. In particular, a high level of beam output stability is required to avoid accidents because beam output is a major factor in patient dose. However, no studies have analyzed the long-term beam output stability of AB-BNCT systems.

PURPOSE

This study aimed to retrospectively analyze the long-term stability of the beam output by statistical process control (SPC) based on the QA results over 3 years.

METHODS

The data analyzed are the results of daily QA (DQA) and weekly QA (WQA) in an AB-BNCT system and were taken between June 2020 and September 2023. The evaluation of the stability of the beam output was based on the reaction rate between gold and neutrons calculated using the activation foil method using a gold foil. In DQA, which can be performed in a short time, the gold foil was applied directly to the beam irradiation aperture in air. In WQA, measurements were performed at the phantom surface, 2-cm depth, and 6-cm depth using a dedicated water phantom. The acquired data were retrospectively analyzed by individuals and a moving range chart (I-MR chart), exponentially weighted moving average control chart (EWMA chart), and several process capability indexes (PCIs).

RESULTS

Over 99% of the DQA I-MR chart results were within control limits, whereas the WQA I-MR chart results showed that 1.8%, 4.1%, and 2.0% of the measurements exceeded the control limits at the surface, 2-cm depth, and 6-cm depth, respectively. The variation in the reaction rate of the gold foil before and after the replacement of the target was <0.5%. The EWMA chart results revealed no significant beam output drift for either DQA or WQA. Most measured data were normal based on the results of the Anderson-Darling test and met the requirements for PCI evaluation; most PCI values were >1.0; however, the Cpmk of DQA and the 2- and 6-cm depth WQAs between August 2021 and November 2022 in treatment course 2 were 0.83, 0.77, and 0.87, respectively, which were <1.0.

CONCLUSIONS

The long-term stability of beam output was confirmed using SPC in an AB-BNCT system. The results of the control chart revealed no significant variation or drift in the beam output, and the quantitative evaluation using PCI revealed high stability. A routine QA program will enable us to provide safe BNCT.

Comprehensive evaluation of dosimetric impact against position errors in accelerator-based BNCT under different treatment parameter settings

BACKGROUND

Patients who undergo accelerator-based (AB) boron neutron capture therapy (BNCT) for head and neck cancer in the sitting position are generally uncomfortably immobilized, and patient motion during this treatment may be greater than that in other radiotherapy techniques. Furthermore, the treatment time of BNCT is relatively long (up to approximately 1 h), which increases the possibility of patient movement during treatment. As most BNCT irradiations are performed in a single fraction, the dosimetric error due to patient motion is of greater consequence and needs to be evaluated and accounted for. Several treatment parameters are required for BNCT dose calculation.

PURPOSE

To investigate the dosimetric impacts (DIs) against position errors using a simple cylindrical phantom for an AB-BNCT system under different treatment parameter settings.

METHODS

The treatment plans were created in RayStation and the dose calculation was performed using the NeuCure® dose engine. A cylindrical phantom (16 cm diameter × 20 cm height) made of soft tissue was modeled. Dummy tumors in the form of a 3-cm-diameter sphere were arranged at depths of 2.5 and 6.5 cm (denoted by T2.5 and T6.5 , respectively). Reference plans were created by setting the following parameters: collimator size = 10, 12, or 15 cm in diameter, collimator-to-surface distance (CSD) = 4.0 or 8.0 cm, tumor-to-blood ratio (T/B ratio) using 18 F-fluoro-borono-phenylalanine = 2.5 or 5.0, and 10 B concentration in blood = 20, 25, or 30 ppm. The prescribed dose was D95%  ≥ 20 Gy-eq for both T2.5 and T6.5 . Based on the reference plans, phantom-shifted plans were created in 26 directions [all combinations of left-right (LR), anterior-posterior (AP), and superior-inferior (SI) directions) and three distances (1.0, 2.0, and 3.0 cm). The DIs were evaluated at D80% of the tumors. The shift direction dependency of the DI in the LR, AP, and SI directions was evaluated by conducting a multiple regression analysis (MRA) and other analyses where required.

RESULTS

The coefficients of the MRA of the DIs for LR, AP, and SI shifts were -0.08, 2.16, and -0.04 (p-values = 0.084, <0.01, and 0.334) for T2.5 and -0.05, 2.08, and 0.15 (p-values = 0.526, <0.01, and 0.065) for T6.5 , respectively. The analysis of variance showed that DIs due to the AP shift were significantly greater for smaller collimator sizes on T2.5 and smaller CSD on T6.5 . Dose reduction due to SI or LR (lateral) shifts was significantly greater for smaller collimator sizes on both T2.5 and T6.5 and smaller CSD on T2.5 , according to the Student's t-test. There were no significant differences in the DIs against both the AP shift and the lateral shift between the different T/B ratios and 10 B concentrations.

CONCLUSION

The DIs were largely affected by the shift in the AP direction and were influenced by the different treatment parameters.

Status of low-energy accelerator-based BNCT worldwide and in Argentina

Existing and active low-energy Accelerator-Based BNCT programs worldwide will be reviewed and compared. In particular, the program in Argentina will be discussed which consists of the development of an Electro-Static-Quadrupole (ESQ) Accelerator-Based treatment facility. The facility is conceived to operate with the deuteron-induced reactions ⁹Be(d,n)¹⁰B and ¹³C(d,n)¹⁴N at 1.45 MeV deuteron energy, as neutron sources. Neutron production target development status is specified. The present status of the construction of the new accelerator development laboratory and future BNCT centre is shown.

Investigation of the neutron spectrum measurement method for dose evaluation in boron neutron capture therapy

In boron neutron capture therapy, it is important to evaluate the dose administered to a patient's body outside the tumour area. The exposure dose is evaluated by calculation; however, the calculated value must be validated using a measured value. The dose evaluations based on the measured neutron spectrum are investigated. Multi-foil activation, combined with a LiCaAlF₆ scintillation detector and an imaging plate, is proposed as a measurement method. The proposed method can measure the neutron spectrum at various points quickly.