Evaluations for prompt-gamma imaging of boron and hydrogen neutron capture reactions in boron neutron capture therapy (BNCT)

Boron neutron capture therapy (BNCT) is used for malignant brain tumours, with a tumour selective 10B-compound as a targeting agent of the radiation dose [1]. In this project, the possibility to measure on-line distribution of the boron neutron capture vents in patient's head during a BNCT irradiation is studied. The boron neutron capture reaction produces high-LET alpha and 7Li ion particles. The recoiling Li-nucleus emits a 478 keV gamma. The number of 478 keV photons gives information of the number of capture reactions, which is important for the dosimetry. Also hydrogen atoms, distributed quite evenly in the tissue, capture thermal neutrons emitting a 2223 keV gamma. This emission distribution is descriptive of the thermal neutron distribution in the tissue. The ratio of the boron capture gammas to the hydrogen capture gammas gives the boron concentration distribution, which is of primary importance in evaluating the success of the targeting of the tumour with the boron carrier molecule. Prompt gamma radiation can be measured with gamma detectors located around the patient's head and be used to image the boron distribution in patient's head during the treatment. Several alternative detector materials have been considered. Only two detec or types have been approved for testing phase: cadmium-zinc-telluride (CdZnTe) and silicon (Si) detectors. First results of prompt gamma measurements at the FiR 1 BNCT facility [2] using two CdZnTe detectors from eV PRODUCTS (USA) courtesy of Labtronic y (Finland) and a Si detector from Institute for Physics and Power Engineering, Obninsk (Russia) will be reported. A water filled head phantom (RSVP, Phantom Factory, USA) with a 10 ml insert filled with boric acid solution simulating a tumour was place in the 14 cm aperture epithermal neutron beam to simulate the situation in the final application. In order to determine neutron induced damage to the detectors, Monte Carlo simulations have been carried out with MCNP4C [3]. A detailed MCNP model of the beam collimator structures and the beam surroundings in the irradiation room has been constructed. Neutron spectra have been calculated in several different locations inside the irradiation room. Calculations for detector shielding have also been performed. [1] I. Auterinen et al. in these proceedings. [2] I. Auterinen et al., Frontiers in Neutron Capture Therapy. M. Frederick Hawthorne, Kenneth Shelly & Richard J. Wiersema (eds.). Vol. 1. Kluwer Academic / Plenum Publishers (2001), p. 267 - 275. [3] J. F. Briesmeister, LA-13709-M (2000)