Radiation exposure, whether from an external source or from radioactive material taken into the body, requires immediate and specialized testing to determine the potential for harm. Testing for internal radiation is complex because it must accomplish two distinct goals: physically detecting and quantifying the radioactive substance itself, and measuring the resulting biological damage to the body. This comprehensive assessment is necessary to determine the absorbed dose and to guide medical professionals toward the most appropriate treatment decisions.
Methods for Detecting Internal Contamination
Internal contamination occurs when radioactive material is inhaled, ingested, absorbed through the skin, or introduced through a wound. Identifying this material involves bioassay techniques, which are divided into in vitro (in glass) and in vivo (in life) measurements to locate and quantify the substance.
Bioassay samples involve the collection of biological materials such as urine, feces, sweat, and nasal swabs for laboratory analysis. Measuring the concentration of radionuclides in these excreta allows scientists to calculate the total amount of radioactive material taken into the body. This in vitro analysis often uses methods like gamma counting or liquid scintillation counting to identify and quantify the specific isotopes present.
In contrast, in vivo measurements directly measure the radiation emitted from within the body using specialized detection equipment. Whole-Body Counters (WBCs) are large, heavily shielded devices that use sensitive detectors, often sodium iodide or germanium, to measure gamma radiation. These measurements are particularly useful for gamma-emitting radionuclides, such as Cesium-137, and help determine the location and quantity of the contaminant.
Organ-specific counters are a type of in vivo measurement that focuses on areas where certain radionuclides are known to concentrate. For example, a thyroid counter is used specifically to measure radioactive iodine (Iodine-131) because the thyroid gland naturally collects iodine. These direct measurements provide a rapid, non-invasive way to check for the presence of radioactive material.
Assessing Radiation-Induced Biological Damage
While detection methods quantify the contaminant, biodosimetry techniques assess the severity of the exposure by measuring the biological damage caused by the radiation. This approach provides an estimate of the absorbed radiation dose based on the body’s reaction, which is a more accurate indicator of potential health effects than contaminant quantity alone.
Hematological analysis is one of the quickest methods for initial dose assessment following an acute exposure event. Radiation rapidly damages dividing cells, including those in the bone marrow, leading to a noticeable drop in circulating white blood cells. Tracking the decline in lymphocyte counts, known as lymphocyte depletion kinetics, offers a rapid, though less precise, indication of the absorbed dose for initial triage decisions.
The gold standard for biological dose estimation is cytogenetic analysis, specifically the Dicentric Chromosome Assay (DCA). This method involves culturing and analyzing lymphocytes from a blood sample to count the number of dicentric chromosomes—abnormal chromosomes with two centromeres—which are uniquely induced by ionizing radiation. The frequency of these aberrations is directly correlated with the absorbed radiation dose, providing a reliable and accurate retrospective dose estimate.
Advanced biodosimetry research explores the use of gene expression markers to create faster, high-throughput dose assessment tools. These methods analyze changes in the patterns of specific proteins or RNA molecules that are rapidly altered following radiation exposure. While not yet a standard clinical tool, these ‘omics’ approaches, including proteomics and transcriptomics, hold promise for quickly screening large numbers of people after a mass-casualty radiation incident.
Interpreting Dose and Guiding Medical Action
The final step is combining data from physical detection and biological damage assessment into a comprehensive dose calculation, or dosimetry. The absorbed dose, measured in Gray (Gy), represents the energy deposited per unit mass of tissue and is the foundation for assessing immediate health risks. This absorbed dose is then converted into the equivalent dose or effective dose, measured in Sievert (Sv), by applying weighting factors to account for the type of radiation and the sensitivity of the exposed organs.
The calculated dose is the most important factor for guiding medical action, differentiating between exposures requiring minimal monitoring and those demanding immediate intervention. For internal contamination, an estimated dose may trigger the use of decorporation agents like chelation therapy. This involves administering drugs that bind to and help remove radioactive materials from the body, such as Prussian Blue for Cesium or DTPA for Plutonium.
For individuals who received a high whole-body dose, typically exceeding 4 Gray, the primary concern is damage to the hematopoietic system, which produces blood cells in the bone marrow. Medical teams initiate supportive care, including blood transfusions, antibiotics to prevent infection, and growth factors like G-CSF to stimulate remaining bone marrow stem cells. In the most severe cases, stem cell transplantation may be considered to replace the destroyed bone marrow and restore blood cell production.