Boron Neutron Capture Therapy (BNCT) is a promising radiotherapy modality for treating a cancerous tumours, e.g. glioma - highly malignant cancer of the brain. It utilizes the reaction between the ¹⁰B-nucleus and slow (thermal) neutrons:
¹⁰B + n ® ⁷Li(0.84 MeV) + ⁴He(1.47 MeV) + g( 0.48 MeV)
The released ⁷Li atom and a-particle (⁴He) have the total energy of 2.31 MeV which is deposited within the range of approximately 5-9 mm i.e. at a distance corresponding to about one cell diameter. In principle, one boron-neutron interaction liberates enough energy to kill the cell in which the inter-action takes place. The short distance deposition of the released energy spares the surrounding boron-free tissue of the radiation damage.
The effect of BNCT is dependent on two preconditions. One is the selective accumulation of boron atoms in the target (tumour) and healthy cells, the other is that the tumour site is reached by a sufficient number of neutrons.
At present, LVR-15 reactor serves as the source of neutrons for BNCT. The thermal neutron beam and the beam of more energetic epithermal neutrons (1-10000 eV) have been tested for this purpose (Fig. 1). Thermal neutrons are rapidly attenuated in the irradiated tissues and therefore, it is difficult to obtain sufficient flux of thermal neutrons in inner parts of large or deeply sited tumours. However, more energetic epithermal neutron beam can be produced by using moderators and filters. Such a beam of neutrons peaks the thermal flux in deeply sited irradiated tissues.
Typical configuration for epithermal neutron beam in the LVR-15 geometry is demonstrated in Fig. 2. Fast neutrons escaping the core are transported through the inner shutter (can be filled by water) to a block of filters. Epithermal neutrons are collimated and transferred through the outer shutter to the irradiation point. Essential diameter of the beam is 11.5 cm with some possibility of reducing it. The shown configuration displays the core with the Be reflector, block of filters formed with B₄C-thin layer, Pb-5cm, graphite-4cm, Al-55cm, S-15cm, Pb-11cm. Behind the Al-C collimator are 1cm Ti + B₄C thin layer and the outer shutter.
For the study of all the material and configuration possibilities computer codes DORT (discrete ordinates) and MCNP (Monte Carlo) have been used. The DORT calculations were performed with the SAILOR - DLC 76 and new BUGLE 96 coupled cross-section libraries condensed to 47 neutron- and 21 gamma-energy groups.
Several filtered assemblies have been experimen-tally tested with the aim to determine parameters both of the free beam and the beam inside the phantom. The techniques available for measuring the neutron fluence include Si-Li semiconductor detectors, solid state nuclear track detectors, fission chambers and silicon diodes, gamma thermoluminescence detectors (TLD), scintillation spectrometer, Bonner spheres for neutron spectrum determination and proportional hydrogen counter. Good experience has been obtained with small semiconductor thermal neutron counters.
Various materials (Al₂O₃, AlF₃, SiO₂, SiC, C, S…) and their combinations are supposed to be studied for remodelling of filters and enhancement of the beam parameters.