Properties of paraffin wax as a bolus material in accelerator-based boron neutron capture therapy.

Background and purpose: Boron neutron capture therapy (BNCT) is targeted radiation therapy enabling cellular-level cancer treatment. With epithermal neutrons, the dose maximum typically occurs ~2 cm deep in tissue, challenging superficial tumor control. As in external beam radiation therapy, surface dose can be increased using a bolus. However, in BNCT, tissue equivalency is complex and strongly dependent on elemental composition. This study examined a paraffin wax bolus's effect on the epithermal neutron beam in accelerator-based BNCT (AB-BNCT) and evaluated agreement between treatment planning system (TPS) calculations and measurements.

Materials and methods: Beam characterization used the neutron activation method with gold and manganese foils. Due to its high cross-section for thermal neutrons, manganese activation serves as a surrogate for boron dose estimation. Irradiations were conducted in a 3D water tank and in a head-shaped phantom with 5 and 10 mm boluses. Dose calculation utilized the newly commissioned RayStation TPS with a Monte Carlo-based engine built on the GEANT4 toolkit.

Results: Calculated and measured results agree within 5% accuracy in significant dose region (>50% dose). Near the surface and at greater depths, agreement remains within 10%. The bolus shifts the activation depth curve toward the surface by 4-13 mm depending on its thickness. Manganese surface activation increases from 30% without a bolus to ~70% and ~ 90% with 5 and 10 mm boluses, respectively.

Interpretation: Paraffin wax effectively moderates neutron energy, making it a suitable bolus material for AB-BNCT treatments requiring increased surface dose.