Experimental feasibility studies on a SPECT tomograph for BNCT dosimetry

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

Florencia, Italia. Exposición oral (D.M. Minsky)

Congreso; 13th Internacional Congress On Neutron Capture Therpay; 2008

Internacional Society for Neutron Capture Therapy

In 94% of neutron captures in boron, the resulting 7Li ion is emitted in an excited state and decays immediately through a characteristic 478 keV prompt gamma ray. The attenuation coefficient for this photon in tissue is about 0.1 cm-1, hence this g ray escapes from the body to a large extent. Its detection thus offers the possibility of a measurement of the boron dose. In previous work, we have designed a prototype of a Single Photon Emission Tomography system for this purpose. After taking 20 projections of 41 bins the system can reconstruct boron dose maps of 21x21 voxels of 1 cm3 size each. This dosimetry image would give spatial information not available with the current methods and will not depend on the great uncertainties in the boron concentration determination required to calculate the boron dose from the thermal neutron field. In this work we show experimental results obtained at the accelerator based facility of the University of Birmingham with a prototype consisting of four LaBr3(Ce) scintillator detectors and their collimators and shielding. The shielding consists of paraffin, cadmium and 6Li to thermalize and capture neutrons and lead to shield from g background. The lead collimators were designed in order to obtain a 1cm resolution at the beam axis. A water-filled cylindrical head phantom containing two 3cm in diameter cylinders filled with a 10B solution (tumor models) has been used for the experiments. A one-dimensional profile at fixed angle was measured across the tumors. The spectra show the difficulties in quantifying the 478 keV peak area due to its Doppler broadened shape, its overlapping with the low-energy tail of the intense 511 keV pair annihilation peak and the high g background. A least square algorithm has been used for fitting each spectrum. Although improvements must be made to reduce the background count-rate in the detectors, the acquired projection shows that the proposed system should be feasible.