EVALUATION OF NEUTRON-IRRADIATION FIELD FOR BORON NEUTRON-CAPTURE THERAPY BY USING ABSORBED DOSE IN A PHANTOM

Purpose:: The method of an evaluation for the dose characteristics of BNCT is presented, and an eccentric core design for the TRIGA-II react or is proposed. Methods and Materials:: We have defined the ''irradiat ion time'' as the time of irradiation in which the ''maximum 1 mu g/g dose'' becomes 3,000 RBE-cGy, because we assumed that the normal tissu e contained 1 mu g/g B-10. We have also changed the RBE values and cal culated the absorbed dose in the ''irradiation time'' by using an arra ngement including both a facility structure and a body phantom. Moreov er, we have modified the dose criteria for BNCT as follows: The ''eye dose,'' ''total body dose,'' and ''except-head dose'' should be less t han 200, 100, and 50 RBE-cGy, respectively. We have added one more cri teria for BNCT-that the thermal neutron fluence at the tumor position (5 cm from the surface) should be over 2.5 X 10(12) n/cm(2) in the ''i rradiation time.'' Results:: The distance from the core side to the ir radiation port is a very important factor to design a neutron irradiat ion field for BNCT. Conclusion:: We can get the acceptable dose for BN CT with only 1-h irradiation by using a 100 kW reactor if we can get t he irradiation port at the distance of 120 cm from the core side.