SHIELDING DESIGN AND DOSE ASSESSMENT FOR ACCELERATOR BASED NEUTRON-CAPTURE THERAPY

Preparations are ongoing to test the viability and usefulness of an ac celerator source of epithermal neutrons for ultimate use in a clinical environment. This feasibility study is to be conducted in a shielded room located on the Massachusetts Institute of Technology campus and w ill not involve patient irradiations. The accelerator production of ne utrons is based on the Li-7(p, n)Be-7 reaction, and a maximum proton b eam current of 4 mA at an energy of 2.5 MeV is anticipated. The result ant 3.58 x 10(12) neutrons s(-1) have a maximum energy of 800 keV and will be substantially moderated. This paper describes the Monte Carlo methods used to estimate the neutron and photon dose rates in a variet y of locations in the vicinity of the accelerator, as well as the shie lding configuration required when the device is run at maximum current . Results indicate that the highest absorbed dose rate to which any in dividual will be exposed is 3 mu Sv h(-1) (0.3 mrem h(-1)). The highes t possible yearly dose is 0.2 mu Sv (2 X 10(-2) mrem) to the general p ublic or 0.9 mSv (90 mrem) to a radiation worker in close proximity to the accelerator facility. The shielding necessary to achieve these do se levels is also discussed.