On-line reconstruction of low boron concentrations by in vivo gamma-ray spectroscopy for BNCT

Boron neutron capture therapy (BNCT) is a radiation therapy in which the ne utron capture reaction of B-10 is used for the selective destruction of tum ours. At the High Flux Reactor (HFR) in Petten, a therapy facility with an epithermal neutron beam has been built. Tn the first instance, patients wit h brain tumours will be treated. The doses delivered to the tumour and to t he healthy tissue depend on the thermal neutron fluence and on the boron co ncentrations in these regions. An accurate determination of the patient dos e during therapy requires knowledge of these time-dependent concentrations. For this reason, a gamma -ray telescope system, together with a reconstruc tion formalism, have been developed. By using a gamma -ray detector in a te lescope configuration, boron neutron capture gamma -rays of 478 keV emitted by a small specific region can be detected. The reconstruction formalism c an calculate absolute boron concentrations using the measured boron gamma - ray detection rates. Besides the baron gamma -rays, a large component of 2. 2 MeV gamma -rays emitted at thermal neutron capture in hydrogen is measure d. Since the hydrogen distribution is almost homogeneous within the head, t his component can serve as a measure of the total number of thermal neutron s in the observed volume. By using the hydrogen gamma -ray detection rate f or normalization of the boron concentration, the reconstruction tool elimin ates the greater part of the influence of the inhomogeneity of the thermal neutron distribution. MCNP calculations are used as a tool Tor the optimiza tion of the detector configuration. Experiments on a head phantom with 5 ppm B-10 in healthy tissue showed that boron detection with a standard deviation of 3% requires a minimum measuri ng time of 2 min live time. From two position-dependent measurements, boron concentrations in two compartments (healthy tissue and tumour) can be dete rmined. The reconstruction of the boron concentration in healthy tissue can be done with a standard deviation of 6%. The gamma -ray telescope can also be used for in vivo dosimetry.