Are there 'common denominators' in different radiation exposure scenarios as a target for predictive assessment?

Mitigation of lung injury after accidental exposure to radiation

There is a serious need to develop effective mitigators against accidental radiation exposures. In radiation accidents, many people may receive nonuniform whole-body or partial-body irradiation. The lung is one of the more radiosensitive organs, demonstrating pneumonitis and fibrosis that are believed to develop at least partially because of radiation-induced chronic inflammation. Here we addressed the crucial questions of how damage to the lung can be mitigated and whether the response is affected by irradiation to the rest of the body. We examined the widely used dietary supplement genistein given at two dietary levels (750 or 3750 mg/kg) to Fischer rats irradiated with 12 Gy to the lung or 8 Gy to the lung + 4 Gy to the whole body excluding the head and tail (whole torso). We found that genistein had promising mitigating effects on oxidative damage, pneumonitis and fibrosis even at late times (36 weeks) when drug treatment was initiated 1 week after irradiation and stopped at 28 weeks postirradiation. The higher dose of genistein showed no greater beneficial effect. Combined lung and whole-torso irradiation caused more lung-related severe morbidity resulting in euthanasia of the animals than lung irradiation alone.

Hematologic consequences of exposure to ionizing radiation

From the early 1900s, it has been known that ionizing radiation (IR) impairs hematopoiesis through a variety of mechanisms. IR exposure directly damages hematopoietic stem cells and alters the capacity of bone marrow stromal elements to support and/or maintain hematopoiesis in vivo and in vitro. Exposure to IR induces dose-dependent declines in circulating hematopoietic cells not only through reduced bone marrow production, but also by redistribution and apoptosis of mature formed elements of the blood. Recently, the importance of using lymphocyte depletion kinetics to provide a "crude" dose estimate has been emphasized, particularly in rapid assessment of large numbers of individuals who may be exposed to IR through acts of terrorism or by accident. A practical strategy to estimate radiation dose and triage victims based upon clinical symptomatology is presented. An explosion of knowledge has occurred regarding molecular and cellular pathways that trigger and mediate hematologic responses to IR. In addition to damaging DNA, IR alters gene expression and transcription, and interferes with intracellular and intercellular signaling pathways. The clinical expression of these disturbances may be the development of leukemia, the most significant hematologic complication of IR exposure among survivors of the atomic bomb detonations over Japan. Those at greatest risk for leukemia are individuals exposed during childhood. The association of leukemia with chronic, low-dose-rate exposure from nuclear power plant accidents and/or nuclear device testing has been more difficult to establish, due in part to lack of precision and sensitivity of methods to assess doses that approach background radiation dose. Nevertheless, multiple myeloma may be associated with chronic exposure, particularly in those exposed at older ages.

Early identification of radiation accident victims for therapy of bone marrow failure

Ionizing radiation damages the lymphohematopoietic system via direct effects on viability and/or function of hematopoietic stem/progenitor cells and via abnormal production of cytokines (i.e., growth factors). Other tissues that have a rapid turnover (including the gastrointestinal tract and skin) are also profoundly affected by acute radiation exposure. A major issue in selection of appropriate therapy for bone marrow failure (i.e., the bone marrow syndrome) is early assessment of radiation dose. Although several biological markers are available for assessing dose received, the absolute polymorphonuclear neutrophil (PMN) and/or lymphocyte counts, together with clinical presentation (i.e., time to onset of nausea and vomiting, etc.) still provide the most practical and timely assessment of radiation dose. Limited information is available regarding CD34-positive cell frequency as a measure of radiation-induced damage to the bone marrow. Since a subpopulation of radioresistant hematopoietic stem cells may persist after exposure to high-dose radiation, the primary goal of therapy is to provide an adequate number of lymphohematopoietic stem cells for a finite (rather than indefinite) period, after which endogenous stem cells may reinstate lymphohematopoiesis. A model is presented which describes the hypothesis that stem cell clonal repopulation over time is distinct in transplant recipients who have received moderate compared to high-dose radiation exposures. Since some individuals receiving high levels of radiation and presenting with rapidly declining PMN counts spontaneously recover lymphohematopoiesis, better tools (including CD34-positive cell analysis) must be developed to select the appropriate therapy for exposed individuals.

Criteria for the selection of radiation accident victims for stem cell transplantation

The management of acute radiation syndrome in persons inadvertently exposed to total-body irradiation greatly benefits from the experience gained in previous clinical management situations. Within the framework of exposure to high doses, one of the most critical issues is the question of whether hematopoietic stem cell transplantation is indicated. A valid answer can be given with appropriate accuracy based on a thorough analysis and evaluation of all accessible case histories. Based on the clinical evaluation of patients severely affected with acute radiation syndrome, the following recommendations may be issued. There is limited indication for stem cell transplantation when there is accompanying severe radiation damage to organ systems other than the hematopoietic system. The optimum duration of pancytopenia can be estimated using the biomathematical model introduced in this article.