Boron Neutron Capture Enhancement (BNCE) of fast neutron irradiation for glioblastoma: increase of thermal neutron flux with heavy material collimation, a theoretical evaluation

Despite the fact that fast neutron irradiation of glioblastoma has shown on autopsies an ability to sterilize tumors, no therapeutic windows have been found for these particles due to their toxicity toward normal brain. There fore, the Boron Neutron Capture Enhancement (BNCE) of fast neutron beam has been suggested. This paper addresses the problem of fast neutron beam coll imation, which induces a dramatic decrease of the thermal neutron flux in t he depth of the tissues when smaller irradiation fields are used. Thermolum inescent dosimeter TLD-600 and TLD-700 were used to determine the thermal n eutron flux within a Plexiglas phantom irradiated under the Nice Biomedical Cyclotron p(60)+Be(32) fast neutron beam. A BNCE of 4.6% in physical dose was determined for a 10 x 10 cm(2) field, and of 10.4% for a 20 x 20 cm(2) one. A Dose Modification Factor of 1.19 was calculated for CAL 58 glioblast oma cells irradiated thanks to the larger field. In order to increase the t hermal flux in depth while shaping the beam, heavy material collimation was studied with Monte Carlo simulations using coupled FLUKA and MCNP-4A codes . The use of 20 cm width lead blocks allowed a 2 fold thermal neutron flux increase in the depth of the phantom, while shielding the fast neutron beam with a fast neutron dose transmission of 23%. Using the DMF of 1.19, a BNC E of 40% was calculated in the beam axis. This enhancement might be suffici ent to open, at least theoretically, a therapeutic window.