RENEB Inter-Laboratory Comparison 2021: The Dicentric Chromosome Assay.

After large-scale radiation accidents where many individuals are suspected to be exposed to ionizing radiation, biological and physical retrospective dosimetry assays are important tools to aid clinical decision making by categorizing individuals into unexposed/minimally, moderately or highly exposed groups. Quality-controlled inter-laboratory comparisons of simulated accident scenarios are regularly performed in the frame of the European legal association RENEB (Running the European Network of Biological and Physical retrospective Dosimetry) to optimize international networking and emergency readiness in case of large-scale radiation events. In total 33 laboratories from 22 countries around the world participated in the current RENEB inter-laboratory comparison 2021 for the dicentric chromosome assay. Blood was irradiated in vitro with X rays (240 kVp, 13 mA, ∼75 keV, 1 Gy/min) to simulate an acute, homogeneous whole-body exposure. Three blood samples (no. 1: 0 Gy, no. 2: 1.2 Gy, no. 3: 3.5 Gy) were sent to each participant and the task was to culture samples, to prepare slides and to assess radiation doses based on the observed dicentric yields from 50 manually or 150 semi-automatically scored metaphases (triage mode scoring). Approximately two-thirds of the participants applied calibration curves from irradiations with γ rays and about 1/3 from irradiations with X rays with varying energies. The categorization of the samples in clinically relevant groups corresponding to individuals that were unexposed/minimally (0-1 Gy), moderately (1-2 Gy) or highly exposed (>2 Gy) was successfully performed by all participants for sample no. 1 and no. 3 and by ≥74% for sample no. 2. However, while most participants estimated a dose of exactly 0 Gy for the sham-irradiated sample, the precise dose estimates of the samples irradiated with doses >0 Gy were systematically higher than the corresponding reference doses and showed a median deviation of 0.5 Gy (sample no. 2) and 0.95 Gy (sample no. 3) for manual scoring. By converting doses estimated based on γ-ray calibration curves to X-ray doses of a comparable mean photon energy as used in this exercise, the median deviation decreased to 0.27 Gy (sample no. 2) and 0.6 Gy (sample no. 3). The main aim of biological dosimetry in the case of a large-scale event is the categorization of individuals into clinically relevant groups, to aid clinical decision making. This task was successfully performed by all participants for the 0 Gy and 3.5 Gy samples and by 74% (manual scoring) and 80% (semiautomatic scoring) for the 1.2 Gy sample. Due to the accuracy of the dicentric chromosome assay and the high number of participating laboratories, a systematic shift of the dose estimates could be revealed. Differences in radiation quality (X ray vs. γ ray) between the test samples and the applied dose effect curves can partly explain the systematic shift. There might be several additional reasons for the observed bias (e.g., donor effects, transport, experimental conditions or the irradiation setup) and the analysis of these reasons provides great opportunities for future research. The participation of laboratories from countries around the world gave the opportunity to compare the results on an international level.

Tonotopic map of potassium currents in chick auditory hair cells using an intact basilar papilla.

The avian basilar papilla is tonotopically organized such that hair cells along the sensory epithelium respond best to acoustic stimulation at differing frequencies. This specificity arises due to the mechanics of the cochlea itself and intrinsic electrical properties of the hair cells. Tall hair cells show membrane voltage oscillations in response to step current injection that may allow cells to act as electrical resonators, boosting the response at the resonant frequency. These oscillations and the underlying currents have been studied in enzymatically isolated cells. This study uses a whole chick (Gallus domesticus) basilar papilla preparation where the entire epithelium and its afferent connections are intact. With this preparation, a map of changes in potassium currents of tall hair cells was produced. All cells recorded from expressed two K+ currents, a calcium-activated K+ current, I(K(Ca)), and a voltage-activated K+ current, I(K). Also, apical cells expressed an inward rectifier K+ current, I(IR). The amplitude of total outward current increases in a gradient along the tonotopic axis. Pharmacological blockers were used to separate the outward K+ currents. These experiments showed that both currents individually increase in magnitude along a gradient from apex to base. Finally, measurements of oscillation frequency in response to current steps suggest a discontinuous change in the electrical resonances at about 33% from the apex. This study demonstrates a new preparation to study the electrical properties of hair cells in more detail along the tonotopic axis of the chick basilar papilla.