# Development and dosimetric evaluation of a freely deformable 6Li‐based neutron shield for boron neutron capture therapy

## TL;DR

A flexible neutron shield was developed to reduce healthy tissue exposure during boron neutron capture therapy for head and neck cancer.

## Contribution

A freely deformable LiF-polyethylene neutron shield was developed and evaluated for clinical BNCT applications.

## Key findings

- The deformable shield reduced thermal neutron flux by 50%, close to the 60% reduction of a solid block.
- Clinical simulations showed up to 46.6% reduction in dose to the pharyngeal mucosa without affecting tumor coverage.
- Treatment delivery times were minimally impacted, and positional perturbations caused small dose variations.

## Abstract

Boron neutron capture therapy (BNCT) enables selective tumor irradiation by exploiting the high‐linear energy transfer particles generated from neutron interactions with 10B atoms. BNCT has been approved as an insurance‐covered medical treatment for recurrent head and neck cancer in Japan. Unlike photon radiotherapy, neutrons that come out of the collimator have an angular distribution. Therefore, it is necessary to keep the distance between the collimator and the patient as short as possible. However, for head and neck cancer treatments, patient anatomy often limits proximity to the collimator, creating an unwanted air gap. This ultimately increases the neutron exposure to surrounding healthy tissue.

To develop a freely deformable LiF‐polyethylene neutron shield and assess its impact on neutron/gamma attenuation and clinical organ at‐risk sparing in head and neck BNCT.

A freely deformable neutron shielding device was constructed using polyethylene beads loaded with lithium fluoride encapsulated in a vacuum‐sealed cushion. Neutron and gamma‐ray attenuation were measured in a water phantom under clinical conditions using an accelerator‐based BNCT system (NeuCure®, Kansai BNCT Medical Center). Measurements were compared with a solid LiF‐polyethylene block and validated through Monte Carlo–based simulations in a commercial treatment planning system. Three representative head and neck cases were further simulated to assess clinical dosimetric effects.

The deformable shielding device reduced the thermal neutron flux by approximately 50%, compared with 60% for the solid LiF‐polyethylene block. Simulated head and neck treatments demonstrated significant OAR dose reductions (up to 46.6% in pharyngeal mucosa D
50%) without compromising tumor dose coverage (D
80% ≥ 20 Gy‐eq). Treatment delivery times were minimally affected (< 2 min difference) across all plans. A 5 mm positional perturbation analysis showed ≤ 0.5 Gy‐eq variation in GTV D
min and pharyngeal mucosa D
50 and D
max.

The freely deformable LiF‐based neutron shielding device effectively attenuated stray neutron dose while maintaining target coverage in BNCT. Its adaptability and reusability make it a practical adjunct for patient‐specific dose optimization in clinical BNCT applications.