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‐part 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 the10 B ( n , α ) 7 Li nuclear reaction to deliver a highly localized dose to the synovium. This paper examines the feasibility of gadolinium, specifically157 Gd , 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 compare10 B and157 Gd as isotopes for accelerator‐based NCS. The neutron source used in these calculations was a moderated spectrum from the9 Be ( p , n ) reaction at a proton energy of 4 MeV. The therapy time to deliver the NCS therapeutic dose of 10 000 RBE‐cGy, is 27 times longer when157 Gd is used instead of10 B . The skin dose to the treated joint is 33 times larger when157 Gd is used instead of10 B . Furthermore, the impact of using157 Gd instead of10 B was examined in terms of shielded whole‐body dose to the patient. The effective dose 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 of the high‐energy photons emitted from neutron capture in gadolinium is minimal. Possible explanations as to the relative performance of157 Gd and10 B are discussed, including differences in the RBE and range of boron and gadolinium neutron capture reaction products, and the relative values of the10 B and157 Gd thermal neutron capture cross section as a function of neutron energy.