Review of reactor-based neutron beam development for BNCT applications

The revival over the last decade of BNCT as a potentially successful form of radiotherapy, especially for the treatment of otherwise untreatable, deep-seated tumours such as glioma, has led to a variety of neutron beam designs becoming available and being proposed at a number of nuclear research centres throughout the world. Present designs focus on producing beams of predominately epithermal neutrons which can penetrate healthy tissue to give the tumour at depth lethal doses whilst leaving the healthy tissue relatively unscathed. As well as the established design route using nuclear research reactors, recent work is also looking at accelerators and radioisotope sources. Despite this proliferation of designs for epithermal neutron beams, BNCT is performed currently, albeit in Japan only, using thermal neutron beams. Epithermal neutron beams have therefore, as yet, still not been used for the treatment of patients. Nevertheless, it is expected that within the next 1-2 years, clinical trials using epithermal neutrons will begin in both Europe and the USA. This paper reviews those issues that confront the beam designer in developing a neutron beam, including: the merits of epithermal neutrons over thermal neutrons; design methods utilising the nuclear characteristics of moderating and filter materials to optimise the beam parameters; and the engineering aspects of installing and operating such beams. With a beam available, it is essential that the beam is characterised. The dosimetry techniques used in performing this task are presented accordingly and shown how the measured beam parameters both the free beam and phantoms, are used to qualify or rank a beam. A brief history of beam development for BNCT applications is also given, with further comments and concluding remarks on the current status and future prognoses for beam development at other centres throughout the world