DOSE MONITORING FOR BORON NEUTRON-CAPTURE THERAPY USING A REACTOR-BASED EPITHERMAL NEUTRON BEAM

The aims of this study were (i) to determine the variation with time o f the relevant beam parameters of a clinical reactor-based epithermal neutron beam for boron neutron capture therapy (BNCT) and (ii) to test a monitoring system for its applicability to monitor the dose deliver ed to the dose specification point in a patient treated with BNCT. For this purpose two fission chambers covered with Cd and two GM counters were positioned in the beam-shaping collimator assembly of the epithe rmal neutron beam. The monitor count rates were compared with in-phant om reference measurements of the thermal neutron fluence rate, the gam ma-ray dose rate and the fast neutron dose rate, at a constant reactor power, over a period of 2 years. Differences in beam output, defined as the thermal neutron fluence rate at 2 cm depth in a phantom, of up to 15% were observed between various reactor cycles. A decrease in bea m output of about 5% was observed in each reactor cycle. An unacceptab le decrease of 50% in beam output due to malfunctioning of the beam fi lter assembly was detected. For safe and accurate treatment of patient s, on-line monitoring of the beam is essential. Using the calibrated m onitor system, the standard uncertainty in the total dose at depth due to variations with time of the beam output parameters has been reduce d to a clinically acceptable value of 1% (one standard deviation).