Dose evaluation on gadolinium neutron capture therapy

Gadolinium, which has high neutron capture cross section, is proposed to apply as an agent of neutron capture therapy in order to extend clinical applications. On such gadolinium neutron capture therapy (GdNCT), it is supposed that internal conversion electrons, Auger electrons (containing Coster-Kronig electrons) and X-rays, produced in consequence of neutron capture reaction, play important roles on microscopic energy deposition. At Heavy Water Facility in Kyoto University Research Reactor Institute, irradiation experiments were performed to measure thermal neutron flux distributions and gamma-ray absorbed dose distributions inside water phantom which contained 500 ppm gadolinium region by gold wire and TLD (Mg2SiO4(Tb)), respectively. Here, internal conversion electrons and Auger electrons with short range could not be measured due to the sheath thickness of TLD. The experiments were compared with the calculations which were obtained using two-dimensional transport code DOT 3.5. Consequently, the calculational results were in good agreement with experiments for thermal neutron flux distributions, but for gamma-ray absorbed dose distributions, were overestimated by about 30% in gadolinium region. Furthermore, the dose contributions of internal conversion electrons and Auger electrons were evaluated by adding components of those electrons to neutron Kerma factor of gadolinium. The results estimated that those electrons give about 15% of total average dose in gadolinium region. (author)