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

Abstract

Boron neutron capture therapy (BNCT) is a radiationtherapy in which the neutron capture reaction of 10B is used forthe selective destruction of tumours. At the High Flux Reactor (HFR) inPetten, a therapy facility with an epithermal neutron beam has been built. Inthe first instance, patients with brain tumours will be treated. The dosesdelivered to the tumour and to the healthy tissue depend on the thermalneutron fluence and on the boron concentrations in these regions. Anaccurate determination of the patient dose during therapy requiresknowledge of these time-dependent concentrations. For this reason, aγ-ray telescope system, together with a reconstructionformalism, have been developed. By using a γ-ray detector in atelescope configuration, boron neutron capture γ-rays of 478 keVemitted by a small specific region can be detected. The reconstructionformalism can calculate absolute boron concentrations using themeasured boron γ-ray detection rates. Besides the boronγ-rays, a large component of 2.2 MeV γ-rays emitted atthermal neutron capture in hydrogen is measured. Since the hydrogendistribution is almost homogeneous within the head, this component canserve as a measure of the total number of thermal neutrons in theobserved volume. By using the hydrogen γ-ray detection rate fornormalization of the boron concentration, the reconstruction tooleliminates the greater part of the influence of the inhomogeneity ofthe thermal neutron distribution. MCNP calculations are used as a toolfor the optimization of the detector configuration.Experiments on a head phantom with 5 ppm 10B in healthy tissueshowed that boron detection with a standard deviation of 3% requires aminimum measuring time of 2 min live time. From two position-dependentmeasurements, boron concentrations in two compartments(healthy tissue and tumour) can be determined. The reconstruction ofthe boron concentration in healthy tissue can be done with a standarddeviation of 6%. The γ-ray telescope can also be used forin vivo dosimetry.