Multiple parameter biodosimetry of exposed workers from the JCO criticality accident in Tokai-mura

Abstract

On 30 September 1999, three workers at the JCO (Japan Nuclear Fuel Conversion Corporation) uranium fuel processing facility in Tokai-mura, Ibaraki Prefecture, Japan, were severely exposed to neutrons and gamma rays. In this issue of the journal, Nishimura et al [1] report themeasurement of 32P in urine from the three victims for early estimation of neutron exposure levels. This is one of several reports that estimate doses received by the exposed workers and residents, based on biological responses [2–6], radiation monitoring/transport codes [7], and other opportunistic dosimetric approaches [8–9].The higher relative biological effectiveness (RBE) of neutrons forlife-threatening radiation injury justifies efforts to establish a neutron biodosimetry capability. Accurate estimation of the exposure dose by cytogenetic-based chromosome aberration assays, however, requires knowledge of the neutron component in the mixed neutron and gamma radiation scenario. Dose responses for dicentric and ring type chromosome aberration yields measured in peripheral blood lymphocytes are responsive to radiation quality. Protocols for dose assessment by cytogenetic–chromosome aberration assays are internationally accepted [10]. Analytic approaches using cytogenetic chromosome aberrations are established for dose assessment in mixed neutron and gamma radiation accidents. Use of the premature chromosome condensation, or PCC, assay now permits these measurements even at unusually high doses of gamma and neutron radiation [11–12]. Hayata and colleagues measured ring-type chromosome aberrations in interphase cells by use of the PCC-ring assay to estimate dose for the three severely exposed patients in this accident [2]. Ring-type aberrations are formed in higher yields after neutron versus gamma radiation but these ring-aberrations do not provide a unique signature response specific for neutron exposures. Intense research efforts are currently underway to identify more specific chromosome aberration [13] and molecular biomarkers for neutron exposures in order to overcome this limitation. Lymphocyte counts were used to estimate radiation doses for the same three severely exposed patients [5–6]. Haematological responses are an early response biomarker for radiation dose assessment. Goans and colleagues earlier reported lymphocyte depletion kinetic models for dose estimates based on human radiation accident registry data for whole-body acute gamma exposures [14] and more recently for criticality accidents [15]. Their data indicate a neutron RBE for lymphocyte depletion kinetics close to unity. Measurement of lymphocyte depletion kinetics is useful for initial dose estimation for radiation accidents. This biodosimetric approach alone, however, does not gauge the greater effectiveness of neutron radiation in criticality accidents involving mixed neutron and gamma radiation.Sodium-24 counts, with both blood and whole-body (Worker C) counts based on neutron activation of stable 23Na [1, 5–6], were also used to obtain early estimates of absorbed doses for the Tokai-mura victims. The measurement of 32P in blood and urine samples is a potentially useful alternative for estimation of severe neutron radiation exposures because, during interaction with biological materials, fast neutrons convert 32S to 32P and thermal neutrons convert 31P to 32P. Nishimura and colleagues report, in the accompanying article in this issue [1], blood and urine 32P values obtained from the three severely exposed patients in the JCO criticality accident. These data, combined with dose estimates derived from other approaches, can contribute to the establishment of an in vivo human calibration curve for neutron dose assessment based on urine 32P kinetics measurements.The current state of the art for dose assessment following radiation accidents involves use of multiple parameter biological dosimetry. Several of the radiation bioassays (i.e., chromosome aberrations, lymphocyte counts, prodromal symptoms) used for the Tokai-mura victims were calibrated against pure gamma exposures. Goans and colleagues' recent efforts extended human calibration curves for criticality accidents using both lymphocyte depletion kinetics [15] and the onset of vomiting prodromal symptom [16]. Nishimura and colleagues' findings show similar promise for urine 32P kinetics measurements. Finally, the medical radiation accident responders who contributed to the dose estimation effort during the JCO criticality accident are to be commended for the breadth of the biodosimetric approaches undertaken and the effectiveness of their actions.