An investigation of the feasibility of gadolinium for neutron capture synovectomy

Neutron capture synovectomy (NCS) has been proposed as a possible treatment modality for rheumatoid arthritis. Neutron capture synovectomy is a two-pa rt modality, in which a compound containing an isotope with an appreciable thermal neutron capture cross section is injected directly into the joint, followed by irradiation with a neutron beam. Investigations to date for NCS have focused on boron neutron capture synovectomy (BNCS), which utilizes t he B-10(n,alpha)Li-7 nuclear reaction to deliver a highly localized dose to the synovium. This paper examines the feasibility of gadolinium, specifica lly Gd-157, as an alternative to boron as a neutron capture agent for NCS. This alternative modality is termed Gadolinium Neutron Capture Synovectomy, or GNCS. Monte Carlo simulations have been used to compare B-10 and Gd-157 as isotopes for accelerator-based NCS. The neutron source used in these ca lculations was a moderated spectrum from the Be-9(p,n) reaction at a proton energy of 4 MeV. The therapy time to deliver the NCS therapeutic dose of 1 0 000 RBE-cGy, is 27 times longer when Gd-157 is used instead of B-10. The skin dose to the treated joint is 33 times larger when Gd-157 is used inste ad of B-10. Furthermore, the impact of using Gd-157 instead of B-10 was exa mined in terms of shielded whole-body dose to the patient. The effective do se is 202 mSv for GNCS, compared to 7.6 mSv for BNCS. This is shown to be a result of the longer treatment times required for GNCS; the contribution o f the high-energy photons emitted from neutron capture in gadolinium is min imal. Possible explanations as to the relative performance of Gd-157 and B- 10 are discussed, including differences in the RBE and range of boron and g adolinium neutron capture reaction products, and the relative values of the B-10 and Gd-157 thermal neutron capture cross section as a function of neu tron energy. (C) 2000 American Association of Physicists in Medicine. [S009 4-2405(00)01707-7].