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Design of a [gamma]-ray analysis system for determination of boron in a patient's head, during neutron irradiation
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ECN-I--97-057 Overig
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Boron Neutron Capture Therapy (BNCT) is a new radiation therapy in whichthermal neutron capture by "1"0B is used for the selective destruction of a cancer tumour. At the High Flux Reactor (HFR) in Petten, Netherlands, a therapy facility is built for the neutron irradiations. In first instance, patients with a brain tumour 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. Yet, both concentrations change in time after the administration of the tumour-seeking boron compound. An accurate determination of the patient's dose requires the knowledge of these time dependent concentrations during the therapy. For this reason, a gamma-ray telescope system, together with a reconstruction tool, are developed. Two HPGe-detectors measure the 478 keV prompt gamma-rays which are emitted at the boron neutron capture reaction, in a large background of gamma-rays and neutrons. By using the detectors in a telescope configuration, only gamma-rays emitted by a small specific region are detected. The best shielding of the detectors is obtained by performing the measurements through a small hole in the iron roof. A reconstruction tool is developed to 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 over 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-line for normalisation 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 optimisation of the detector configuration. Experiments on a head phantom with 5 ppm 10B in healthy tissue and 62 ppm in the tumour proved that the minimum measuring time is 2 minutes live time. The reconstruction provided deviations of the boron concentrations of 5% for the healthy tissue and 15% for the tumour. 13 figs., 1 tab., 10 refs., 10 appendices

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