Radiation Hazards to Crews of Interplanetary Missions: Biological Issues and Research Strategies (1996)

Absolute risk:

An expression of excess risk based on the assumption that the excess risk from exposure to radiation adds to the underlying (baseline) risk by an increment dependent on dose but independent of the underlying natural risk.

Absorbed dose:

The mean energy imparted by ionizing radiation to an irradiated object per unit mass. Units: Gray, rad.

Acute effects of radiation:

Effects that occur shortly after exposure to radiation, usually within a week. They result from exposure to radiation at relatively high doses, usually greater than 1 Gy. They are usually due to the killing of cells in critical tissues in the body.

Alpha radiation:

A type of particulate radiation released from a radioactive atom. Alpha particles are helium nuclei. Alpha particle radiation is densely ionizing (high-LET) radiation and can be very damaging, but it is very limited in its ability to penetrate tissue. For example, alpha radiation will not penetrate the outer layers of the skin.

Ataxia telangiectasia:

A disorder inherited as a recessive trait that is characterized by neurological changes, such as cerebellar ataxia, immunological deficiency, an increased susceptibility to cancer, especially lymphomas, and an increased cellular radiosensitivity.

Background radiation:

Radiation received by the entire human population due to naturally occurring radiation and radioactive materials in the environment. The three major sources of natural background radiation are (1) cosmic radiation, (2) radiation from naturally occurring radioactive elements in Earth's surface, and (3) internal radiation arising from radioactive atoms normally present in foodstuffs or in the air.

Becquerel:

International System (SI) unit of activity of a radionuclide, equal to 1 radioactive decay per second (equals 2.7 × 10^-11 Ci).

Cancer:

A malignant tumor of potentially unlimited growth, capable of invading surrounding tissue or spreading to other parts of the body by metastasis.

Carcinogen:

A physical agent such as ionizing radiation or ultraviolet radiation or chemical agents, such as vinyl chloride, that may cause cancer.

Cell-killing effect:

The cessation of cell division and/or metabolism. Sufficient doses of radiation may kill cells in the body. Cell death is responsible for most of the acute effects of radiation.

Cell sensitivity to radiation:

The relative sensitivity (radiosensitivity) of individual cell types to the cell-killing or mutagenic effects of radiation.

Cell transformation:

A process by which cells in vitro, which have a limited ability to divide, are altered by radiation or chemicals so as to have an unlimited division potential. (See Neoplastically transformed cells.)

Curie:

A unit of radioactivity equal to 3.7 × 10¹⁰ disintegrations per second.

Deterministic effects:

Effects formerly known as nonstochastic effects that may appear early or late after irradiation. There is a threshold dose above which both the probability of occurrence and the severity of the effect increase. Most deterministic effects involve cell killing.

Dose:

See Absorbed dose.

Dose effect (dose response) model:

A mathematical formulation used to predict the magnitude of an effect that would be produced by a given dose of radiation.

Dose equivalent:

See Equivalent Dose.

Dose rate:

The quantity of absorbed dose delivered per unit of time.

Doubling dose:

Amount of radiation needed to double the natural incidence of a genetic or somatic abnormality.

DREF:

A factor by which the effect caused by a specific dose of radiation changes at low dose rates as compared with high dose rates.

Electron volt:

A unit of energy (1.6 × 10^-19 J). 1 eV is equivalent to the amount of energy gained by an electron passing through a potential difference of 1 volt.

Epidemiologic study:

The study of human populations designed to establish the relationships among various factors that determine the frequency and distribution of a disease. For example, a number of such studies have examined the role of radiation (the factor) in the induction of cancer (the disease). The science of epidemiology is derived from the word “epidemic.”

Epilation:

Loss of hair.

Equivalent dose:

Absorbed dose averaged over an organ or tissue and weighted for the radiation quality for the type of radiation of concern.

Erythema:

Redness of the skin. A transient erythema can occur a few hours after irradiation due to increased permeability of the capillaries. The main erythematous reaction occurs some weeks after exposure to radiation and is due to loss of cells in the basal layer. A late phase 8 to 20 weeks after irradiation is associated with damage to the dermis.

Excess cancers:

The number of individuals in a population who develop cancer over and above the number that would be expected to do so normally. Normally, about one out of every four people will develop cancer during his or her lifetime, and cancer will strike two out of every three families.

Fractionation:

The delivery of a given total dose of a radiation as several smaller doses, separated by intervals of time.

Gamma rays:

Short-wavelength electromagnetic radiation of nuclear origin with an energy range of about 10 keV to 9 MeV.

Gene:

The biological unit of heredity in a cell. Each gene determines and controls a specific characteristic or function of the cell. Genes are composed of DNA and are arranged linearly at definite positions on the chromosomes in the nucleus of each cell. Because genes are duplicated at the time of cell division, the characteristics for which they encode are heritable and are transmitted to future generations of cells.

Genetic effects of radiation:

Effects arising from damage to genes in the germ cells of the mother or father, which are thus passed on to their children. The genetic effects of radiation will therefore not be seen in an irradiated individual but may occur in his or her offspring or in future generations.

Gray:

SI unit of absorbed dose, equal to the energy transferred by ionizing radiation to a mass of matter corresponding to 1 J/kg (equals 100 rad).

HZE particles:

Heavy (high-atomic-number) high-energy particles, such as carbon or iron nuclei, with an energy range in cosmic rays between approximately 10² to 10³ MeV per nucleon.

In vitro study:

Study carried out in individual cells grown in a flask or test tube in the laboratory.

In vivo study:

Study carried out in a living organism.

Ionizing radiation:

Radiation that is able to penetrate and deposit its energy at random within cells and tissues by ejecting electrons from atoms, thus “ionizing” them. Alpha and beta radiation and protons are examples of charged particles that can ionize. X rays, gamma rays, and neutrons are not charged, but they too may ionize.

Kinase:

An enzyme that transfers a phosphate group to a substrate, such as the side chain of a protein (and in doing so changes the reactivity of the protein).

Latent period:

The period of time between exposure and expression of a disease. After exposure to radiation, there may be a delay of several years (the minimum latent period) before any cancers are seen.

Linear dose response model:

A dose response model that predicts a direct (linear) straight-line relationship between cause and effect over a wide range of doses. That is, for each increase in dose there would be a corresponding increase in the effect. For example, if 1 Gy caused cancer to develop in 5% of a group of animals in an experiment, then 2 Gy would lead to 10%, 3 Gy to 15%, and so on.

Linear energy transfer:

Average amount of energy lost per unit of particle track length. Low linear energy transfer (LET) radiation is characterized by light charged particles such as electrons produced by x rays and gamma rays where the distance between ionizing events is large on the scale of a cellular nucleus. High linear energy transfer (LET) radiation is characterized by heavy charged particles such as alpha particles and heavy nuclei, where the distance between ionizing events is small on the scale of a cellular nucleus.

Linear-quadratic model:

Also, linear-quadratic dose-effect relationship; expresses the effect (e.g., mutation or cancer) as partly proportional to the dose (linear term) and partly proportional to the square of the dose (quadratic term). The linear term predominates at lower doses, the quadratic term at higher doses.

Minisatellite regions:

Regions of small repetitive DNA sequences. A hypervariable locus, also known as minisatellite DNA, consists of a block of tandem repeats of short “core” sequences. Core sequences range in size from 11 to 60 base pairs. The number of repeats varies among individuals and this variation is a basis for DNA fingerprinting.

Modal energy:

Energy of the maximum number of particles.

Monte Carlo calculation:

A statistical method that evaluates a probability distribution by means of random sampling.

Multiplicative interaction model:

A model in which the relative risk (the relative excess risk plus one) resulting from exposure to two risk factors is taken to be the product of the relative risks from the two factors taken separately.

Neoplasm:

Any new and abnormal growth, such as a tumor; neoplastic disease refers to any disease that forms tumors, whether malignant or benign.

Neoplastically transformed cells:

Tissue culture cells changed in vitro from growing in an orderly pattern and exhibiting contact inhibition to growing in a pattern more like that of cancer cells, due to the loss of contact inhibition.

Neutron:

Uncharged subatomic particle capable of producing ionization in matter by collision with charged particles.

Nonstochastic effect:

See Deterministic effects.

Nuclide:

A species of atom characterized by the constitution of its nucleus, which is specified by the atomic mass (M) and atomic number (Z).

Photon:

A unit of electromagnetic radiation. One gamma ray is a photon of electromagnetic radiation.

Polymerase chain reaction:

An enzymatic method used to amplify minute amounts of specific DNA sequences in order to analyze the sequences precisely.

Promoter:

An agent that is not by itself carcinogenic but that can amplify the effect of a true carcinogen by increasing the probability of late-stage cellular changes needed to complete the carcinogenic process.

Protraction:

The spreading out of a radiation dose over time by continuous or periodic delivery at a lower dose rate.

Quadratic dose response model:

A model that predicts that a biological effect continually increases out of proportion to an increase in dose. Effects at low doses would thus be relatively small.

Quality factor:

A LET-dependent factor, established by consensus, by which absorbed doses are multiplied to obtain for radiation-protection purposes the equivalent dose, a quantity that expresses the effectiveness of an absorbed dose on a common scale for types of ionizing radiation.

Rad:

Unit of absorbed dose of ionizing radiation, based on the amount of energy absorbed in a given mass of tissue; replaced by the Gray, an SI unit (equals 0.01 Gy or 100 erg/g).

Radiation weighting factor (W_R):

A factor, established by consensus and used in radiation protection, to weight the absorbed dose averaged over an organ to obtain the equivalent dose for the radiation quality of interest.

Radiation quality:

The radiation environment described in terms of the distribution of rays and particles and their energies or the distribution of LET.

Radiogenic:

Caused by radiation.

Radioisotope:

Radioactive species of an element with the same atomic number and identical chemical properties.

Radionuclide:

Radioactive species of an atom characterized by the constituents of its nucleus (atomic number).

Radiosensitivity:

Relative susceptibility of cells, tissues, organs, and organisms to the injurious action of radiation; radiosensitivity and its antonym, radioresistance, are used in a comparative sense rather than an absolute one.

Recessive gene disorder:

Requires that a pair of mutant genes, one from each parent, be present in order for a disease to be manifested. Examples are cystic fibrosis and ataxia telangiectasia.

Relative biological effectiveness:

Biological potency of one type of radiation compared with another that produces the same biological end point. It is numerically equal to the dose of gamma rays needed to produce a specific effect divided by the dose of particle radiation that produces the same effect. The reference radiation is often 200-keV x rays.

Relative risk:

An expression of risk relative to the underlying (baseline) risk. If the relative risk is 2, the excess risk equals the baseline risk.

Rem:

Unit of equivalent dose of ionizing radiation. The equivalent dose in rems is numerically equal to the absorbed dose in rads multiplied by the quality factor, the distribution factor, and any other necessary modifying factor. Replaced by Sievert in the SI system.

Repair processes:

Metabolic processes within a cell that can repair radiation damage before it is expressed as a biological effect such as cell killing.

Risk coefficient:

The increase in the incidence of disease or mortality per person exposed per unit equivalent dose: the relative-risk coefficient is the fractional increase in the baseline incidence or mortality rate for a unit dose.

Risk estimate:

The number of cases (or deaths) that are projected to occur in a specified exposed population per unit of collective dose, for a specified exposure regime and expression period, e.g., number of cases per person-Gray.

SI units:

The International System of Units as defined by the General Conference of Weights and Measures in 1960. These units are generally derived from the meter, kilogram, second, and SI-based units with special quantities for radiation including the Becquerel, Gray, and Sievert.

Sievert:

SI unit of radiation equivalent dose, equal to dose in Grays times a quality factor times other modifying factors, for example, a distribution factor; 1 Sievert equals 100 rem.

Solar flare:

A result of the explosive release of magnetic energy. Although solar flares were once thought to be the cause of solar particle events, coronal mass ejections are gaining favor as a key cause of solar particle events and certainly as a means of intensifying them.

Solar maximum:

Period of maximum probability of emission of solar event radiation, including protons, alpha radiation, and electromagnetic energy.

Solar minimum:

Period of minimum probability of emission of solar event radiation.

Solar particle event:

A flux of energetic ions and/or electrons of solar origin. Solar particle events substantially increase radiation above the background level set by galactic cosmic rays. The protons in such events are usually of greatest concern, although electrons sometimes dominate and a smaller component of heavier ions (especially alpha particles—doubly ionized helium) is also present. Solar particle events are associated

with very energetic solar flares or fast coronal mass ejections. Such events, which accelerate particles, tend to cluster in the more active phases of the approximately 11-year solar cycle.

Somatic effects of radiation:

Effects that occur in an irradiated individual due to damage produced in various tissues of the body, as opposed to genetic effects, which occur in the offspring of an irradiated individual owing to damage in germ cells. Potentially important somatic effects of radiation include induction of cancer and damage to the central nervous system. Cataract formation is also possible.

Specific energy:

The actual energy per unit mass deposited per unit volume in a given event. This is a stochastic quantity as opposed to the average value over a large number of instances (i.e., the absorbed dose).

Stochastic:

Random events leading to effects whose probability of occurrence in an exposed population of cells or individuals (rather than severity in an affected cell or individual) is a direct function of dose; these effects are commonly regarded as having no threshold. Hereditary effects are regarded as being stochastic; some somatic effects, especially carcinogenesis, are regarded as being stochastic.

Target theory (hit theory):

A theory explaining some biological effects of radiation on the basis that ionization, which occurs in a discrete volume (the target) within a cell, directly causes a lesion that later results in a physiological response to the damage at that location; one, two, or more hits (ionizing events within the target) may be necessary to elicit the response.

Threshold hypothesis:

The assumption that no radiation injury occurs below a specified dose.

Transport calculation:

Calculation of particle distributions and energy behind a specific shield, derived from the basic nuclear cross sections for interactions and fragmentation in shielding.

Whole-body external dose:

The dose of radiation from sources outside the body that irradiate the entire body. The dose from cosmic radiation is an example of a whole-body external dose.

x radiation:

Also x rays; penetrating electromagnetic radiation, usually produced by bombarding a metallic target with fast electrons in a high vacuum.

Xeroderma pigmentosum:

An inherited disease in which individuals are highly susceptible to cancer induced by exposure to solar radiation. Xeroderma pigmentosum cells have a defect in the ability to repair ultraviolet damage to their DNA, a defect that apparently accounts for the susceptibility.