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 neutron capture reaction of 10B is used for the selective destruction of tumours. At the High Flux Reactor (HFR) in Petten, a therapy facility with an epithermal neutron beam has been built. In the first instance, patients with brain tumours will be treated. The doses delivered to the tumour and to the healthy tissue depend on the thermal neutron fluence and on the boron concentrations in these regions. An accurate determination of the patient dose during therapy requires knowledge of these time-dependent concentrations. For this reason, a gamma-ray telescope system, together with a reconstruction formalism, have been developed. By using a gamma-ray detector in a telescope configuration, boron neutron capture gamma-rays of 478 keV emitted by a small specific region can be detected. The reconstruction formalism can calculate absolute boron concentrations using the measured boron gamma-ray detection rates. Besides the boron gamma-rays, a large component of 2.2 MeV gamma-rays emitted at thermal neutron capture in hydrogen is measured. Since the hydrogen distribution is almost homogeneous within the head, this component can serve as a measure of the total number of thermal neutrons in the observed volume. By using the hydrogen gamma-ray detection rate for normalization of the boron concentration, the reconstruction tool eliminates the greater part of the influence of the inhomogeneity of the thermal neutron distribution. MCNP calculations are used as a tool for the optimization of the detector configuration. Experiments on a head phantom with 5 ppm 10B in healthy tissue showed that boron detection with a standard deviation of 3% requires a minimum measuring time of 2 min live time. From two position-dependent measurements, boron concentrations in two compartments (healthy tissue and tumour) can be determined. 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.