First Tomographic Image of Neutron Captures Rate in a BNCT Facility

D.M. MINSKY; A.A. VALDA; A.J. KREINER; A.A. BURLON; S. GREEN; C. WOJNECKI; Z.GHANI

Buenos Aires

Congreso; 14th International Congress on Neutron Capture Therapy; 2010

International Society for NeutronCapture Therapy

Presentación oral - Charla Plenaria (D.M. Minsky)Introduction: Dosimetry in BNCT is a very complicated matter due to the several reactions that neutrons undergo with the nuclei of elements contained in different tissues. BNCT dose depends both on neutron beam quality and boron compound distribution. The actual method for evaluating the boron dose consists in calculating the neutron flux distribution in the body and convolute it with boron concentration estimations that are inferred from blood samples taken before, during and after the patient irradiation and previous biodistribution studies. This indirect method leads to large uncertainties. In 94% of the neutron captures in 10B, the residual nucleus 7Li is emitted in its first excited state that decays via a characteristic 478keV prompt ? ray. The attenuation coefficient for this photon in soft tissue is about 0.1cm-1 and hence it can be detected outside the body as a measure of the boron dose. A single photon emission tomography (SPECT) system has been proposed for obtaining boron dose maps during a BNCT irradiation. We show here some results with a prototype and the first SPECT boron dose image obtained in a BNCT facility.  Materials and Methods: A SPECT tomograph prototype based on four LaBr3(Ce) scintillation detectors has been constructed. The radiation has been collimated by means of 30cm long and 0.5cm in diameter lead collimators. The detectors have been shielded from the gamma and neutron background by successive layers of paraffin, cadmium, lead and 6Li. The system has been linearly moved to measure full projections by a computer controlled motor system. A 9cm in diameter water filled cylindrical head phantom with a 3cm in diameter cylindrical tumor model with 400ppm of 10B was used for the experiments. The phantom was positioned at the Birmingham University accelerator based BNCT facility and irradiated with the accelerator set at 800µA and 2.8MeV. In order to measure different projections, the phantom has been rotated around its axis; this movement and all the measurements have also been made in a computer controlled way. Although the original system was designed for the acquisition of 20 angular projections with 41 bins each, 13 projections were considered enough for this feasibility test. Symmetries have been used to reduce the number of measurements. An expectation maximization-maximum likelihood algorithm has been used to reconstruct the density of neutron captures rate in the phantom. The results have been compared with Monte Carlo N-Particle 5.140 code simulations.  Results and Discussions: We have obtained the first tomographic image of the boron dose of a phantom irradiated at a facility with a clinical BNCT neutron spectrum. The experimentally reconstructed neutron capture rate density in the tumor and the simulated one agree within errors. Further work must be performed in order to reduce the background and in detector optimization for detection limit improvement.