Evaluating the Therapeutic Potential and the Suitability of Neutron Beams Designed for Clinical BNCT

Abstract The standard of neutron beam quality for Boron Neutron Capture Therapy (BNCT) of deep-seated tumours is currently defined by its physical characteristics in air: the epithermal neutron flux, the ratio of thermal and epithermal neutron flux, the fast neutron and photon dose contamination, and the beam collimation. Traditionally, the beam design consists in tailoring a Beam Shaping Assembly (BSA) able to deliver a neutron beam with the recommended values of these figures of merit (FOMs). This work investigated the possibility to produce an epithermal neutron beam able to guarantee the best clinical performance for deep-seated tumours, starting from a 5 MeV, 30 mA proton beam coupled to a beryllium target. Different Beam Shaping Assemblies were designed using those physical FOMs which, however, were not enough to establish a clear ranking of the different beams, nor to describe their clinical relevance. To go beyond this traditional approach, beams were then evaluated employing new criteria based on the dose distributions obtained in-phantom and on the calculation of the Uncomplicated Tumour Control Probability (UTCP). Such radiobiological FOM allows establishing the therapeutic potential of the beams. Moreover, we included the concept of suitability as a criterion to select the safest BSA design, calculating the in-patient out-of-beam dosimetry. The clinical relevance of the selected beam was finally tested in the treatment planning of a clinical case treated at the FiR 1 beam in Finland, where several patients have safely and successfully received BNCT in the last years. Despite the selected beam does not comply with all the standard physical recommendations, it shows a therapeutic potential comparable and even better than that of FiR 1. This confirms that establishing the performance of a beam cannot rely only on its physical characteristics, but requires additional criteria able to predict the clinical outcome of a BNCT treatment.