RENEB Inter-Laboratory Comparison 2021: Inter-Assay Comparison of Eight Dosimetry Assays

Abstract

Tools for radiation exposure reconstruction are required to support the medical management of radiation victims in radiological or nuclear incidents. Different biological and physical dosimetry assays can be used for various exposure scenarios to estimate the dose of ionizing radiation a person has absorbed. Regular validation of the techniques through inter-laboratory comparisons (ILC) is essential to guarantee high quality results. In the current RENEB inter-laboratory comparison, the performance quality of established cytogenetic assays [dicentric chromosome assay (DCA), cytokinesis-block micronucleus assay (CBMN), stable chromosomal translocation assay (FISH) and premature chromosome condensation assay (PCC)] was tested in comparison to molecular biological assays [gamma-H2AX foci (gH2AX), gene expression (GE)] and physical dosimetry-based assays [electron paramagnetic resonance (EPR), optically or thermally stimulated luminescence (LUM)]. Three blinded coded samples (e.g., blood, enamel or mobiles) were exposed to 0, 1.2 or 3.5 Gy X-ray reference doses (240 kVp, 1 Gy/min). These doses roughly correspond to clinically relevant groups of unexposed to low exposed (0-1 Gy), moderately exposed (1-2 Gy, no severe acute health effects expected) and highly exposed individuals (>2 Gy, requiring early intensive medical care). In the frame of the current RENEB inter-laboratory comparison, samples were sent to 86 specialized teams in 46 organizations from 27 nations for dose estimation and identification of three clinically relevant groups. The time for sending early crude reports and more precise reports was documented for each laboratory and assay where possible. The quality of dose estimates was analyzed with three different levels of granularity, 1. by calculating the frequency of correctly reported clinically relevant dose categories, 2. by determining the number of dose estimates within the uncertainty intervals recommended for triage dosimetry (±0.5 Gy or ±1.0 Gy for doses <2.5 Gy or >2.5 Gy), and 3. by calculating the absolute difference (AD) of estimated doses relative to the reference doses. In total, 554 dose estimates were submitted within the 6-week period given before the exercise was closed. For samples processed with the highest priority, earliest dose estimates/categories were reported within 5-10 h of receipt for GE, gH2AX, LUM, EPR, 2-3 days for DCA, CBMN and within 6-7 days for the FISH assay. For the unirradiated control sample, the categorization in the correct clinically relevant group (0-1 Gy) as well as the allocation to the triage uncertainty interval was, with the exception of a few outliers, successfully performed for all assays. For the 3.5 Gy sample the percentage of correct classifications to the clinically relevant group (≥2 Gy) was between 89-100% for all assays, with the exception of gH2AX. For the 1.2 Gy sample, an exact allocation to the clinically relevant group was more difficult and 0-50% or 0-48% of the estimates were wrongly classified into the lowest or highest dose categories, respectively. For the irradiated samples, the correct allocation to the triage uncertainty intervals varied considerably between assays for the 1.2 Gy (29-76%) and 3.5 Gy (17-100%) samples. While a systematic shift towards higher doses was observed for the cytogenetic-based assays, extreme outliers exceeding the reference doses 2-6 fold were observed for EPR, FISH and GE assays. These outliers were related to a particular material examined (tooth enamel for EPR assay, reported as kerma in enamel, but when converted into the proper quantity, i.e. to kerma in air, expected dose estimates could be recalculated in most cases), the level of experience of the teams (FISH) and methodological uncertainties (GE). This was the first RENEB ILC where everything, from blood sampling to irradiation and shipment of the samples, was organized and realized at the same institution, for several biological and physical retrospective dosimetry assays. Almost all assays appeared comparably applicable for the identification of unexposed and highly exposed individuals and the allocation of medical relevant groups, with the latter requiring medical support for the acute radiation scenario simulated in this exercise. However, extreme outliers or a systematic shift of dose estimates have been observed for some assays. Possible reasons will be discussed in the assay specific papers of this special issue. In summary, this ILC clearly demonstrates the need to conduct regular exercises to identify research needs, but also to identify technical problems and to optimize the design of future ILCs.

FIG. 1.

Irradiation setup for the RENEB inter-laboratory comparison 2021 involving eight dosimetry Assays. Kerma homogeneity was determined using Gafchromic EBT3 films (mean of 5 irradiations shown. Homogeneity ±1.5% in the relevant field areas, in which samples were placed, reproducibility ±0.5%). Irradiation time and thus the dose was controlled by a Farmer chamber TM30010-1 placed next to all irradiated materials.

FIG. 2.

The earliest report times of dose categories (low, medium, high; upper part), as well as dose estimates (dose magnitude; lower part), are provided for all assays. Three categories in report time were defined and are expressed in bold gray letters. Asterisks refer to suspected dose estimates and are not actual reported dose estimates. Assays are ordered over report time of dose estimates. Abbreviations: optically or thermally stimulated luminescence [LUM], gamma-H2AX foci [gH2AX], gene expression assays [GE], electron paramagnetic resonance [EPR], dicentric chromosome assay [DCA], premature chromosome condensation assay [PCC], cytokinesis-block micronucleus assay [CBMN] and stable chromosomal translocation assay [FISH] assays.

FIG. 3.

Reporting times of dose estimates/categories cumulated (in percent) over time are depicted for all assays. Three categories in report time were defined and expressed in white letters (left side of the graph). Assays are ordered over report time of dose estimates. Abbreviations: gamma-H2AX foci [gH2AX], optically or thermally stimulated luminescence [LUM], gene expression assays [GE], electron paramagnetic resonance [EPR], dicentric chromosome assay [DCA], premature chromosome condensation assay [PCC], cytokinesis-block micronucleus assay [CBMN] and stable chromosomal translocation assay [FISH] assays.

FIG. 4.

Distribution of reported dose estimates (jitter plots to the left) and calculated absolute differences (box plots to the right) are shown for each team and assay regarding the 0 Gy irradiated samples (panels A andB), 1.2 Gy irradiated sample (panels C and D) and 3.5 Gy irradiated samples (panel E and F). Short-dashed horizontal lines refer to the corresponding uncertainty interval as recommended for triage dosimetry and percentages shown below the assay labels (x-scale) refer to the number of reported dose estimates lying within these dose intervals. Another horizontal solid line visualizes reference doses. EPR and LUM (comprising optically and thermally stimulated luminescence assays, OSL, TL) reported dose estimates refer to kerma in air except for enamel (white squares), which is kerma in enamel and cannot be compared with kerma in air reference dose estimates, but have been reported as that and are therefore shown. Resistor based dose estimates employing LUM (OSL) are shown as white-filled diamonds on the right side of the LUM data. All other inorganic based (e.g., glass) EPR and OSL reported dose estimates are depicted as transparent gray hexagons and TL results are depicted as translucent black hexagons on the left side of the LUM data. Reported and reference dose estimates regarding cytogenetic and molecular biological assays refer to kerma in water, respectively. Corresponding gray circles refer to irradiated blood samples. The conversion factor (Table 1) is 1.08 from kerma in air to kerma in water.

FIG. 5.

This cartoon reflects different radiation exposure geometries and corresponding assumed discrepancies between physical dose estimates based on mobile phone irradiation and mean whole-body dose as measured with biological dosimetry assays. Abbreviations: IR, irradiation; “mobile < body” refer to exposure differences (<, >, =) measured in mobile phones and the whole body depending on different exposure geometries.