Feasibility of assessing the carcinogenicity of neutrons among neutron therapy patients

Nuclear workers, oil well loggers, astronauts, air flight crews, and frequent fliers can be exposed to low doses of neutrons, but the long-term human health consequences of neutron exposure are unknown. While few of these exposed populations are suitable for studying the effects of neutron exposure, patients treated with neutron-beam therapy might be a source of information. To assess the feasibility of conducting a multi-center international study of the late effects of neutron therapy, we surveyed 23 cancer centers that had used neutron beam therapy. For the 17 responding institutions, only 25% of the patients treated with neutrons (2,855 of 11,191) were alive more than 2 years after treatment. In a two-center U.S. pilot study of 484 neutron-treated cancer patients, we assessed the feasibility of obtaining radiotherapy records, cancer incidence and other follow-up data, and of estimating patient organ doses. Patients were treated with 42 MeV neutrons between 1972 and 1989. Applying a clinical equivalence factor of 3.2 for neutrons, total average organ doses outside the treatment beam ranged from 0.14 to 0.29 Gy for thyroid, 0.40 to 2.50 Gy for breast, 0.63 to 2.35 Gy for kidney, and 1.12 to 1.76 Gy for active bone marrow depending upon the primary cancer treatment site. We successfully traced 97% of the patients, but we found that patient survival was poor and that chemotherapy was not confirmable in a quarter of the patients. Based on our findings from the international survey and the feasibility study, we conclude that a large investigation could detect a fivefold or higher leukemia risk, but would be inadequate to evaluate the risk of solid cancers with long latent periods and therefore would likely not be informative with respect to neutron-related cancer risk in humans.