Appendix C
Glossary and Acronyms
Σ  Risk cross section. 
absolute risk (AR)  The rate of disease among a population. 
absorbed dose (D)  The fundamental dose quantity is absorbed dose, D, defined as the quotient of by dm, where is the mean energy imparted by ionizing radiation to matter of mass dm; thus 
Unit: J kg^{−1}. 

The special name for the unit of absorbed dose is gray (Gy). A formerly used (nonSI) unit is rad. 1 Gy = 1 J/kg = 100 rad. 1 rad = 0.01 J/kg. In radiation protection and epidemiology, absorbed dose averaged over organs and tissues is used.  
activity (A)  Variation dN of number of nuclei N in a particular energy state, in a sample of a radionuclide, due to spontaneous nuclear transitions from this state during an infinitesimal time interval, divided by its duration dt. thus: 
A = dN/dt. Unit: s^{−1}. 

The special name for the unit of activity is becquerel (Bq). A formerly used (nonSI) unit is curie (Ci). 1 Bq = 1 s^{−1} = 2.7 × 10^{−11} Ci. 1 Ci = 3.7 × 10^{10} Bq.  
additive effect  When two agents do not interact, the combined effect is equal to the sum of the effects of the two agents acting alone. 
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Appendix C
Glossary and Acronyms
Σ Risk cross section.
absolute risk (AR) The rate of disease among a population.
_
The fundamental dose quantity is absorbed dose, D, defined as the quotient of de by dm,
absorbed dose (D) _
where de is the mean energy imparted by ionizing radiation to matter of mass d m; thus
.
Unit: J kg–l.
The special name for the unit of absorbed dose is gray (Gy). A formerly used (nonSI)
unit is rad. 1 Gy = 1 J/kg = 100 rad. 1 rad = 0.01 J/kg. In radiation protection and epi
epi
demiology, absorbed dose averaged over organs and tissues is used.
activity (A) Variation dN of number of nuclei N in a particular energy state, in a sample of a radio
nuclide, due to spontaneous nuclear transitions from this state during an infinitesimal
time interval, divided by its duration dt, thus:
A = −dN/dt.
Unit: s−1.
The special name for the unit of activity is becquerel (Bq). A formerly used (nonSI) unit
is curie (Ci). 1 Bq = 1 s−1 = 2.7 × 10−11 Ci. 1 Ci = 3.7 × 1010 Bq.
additive effect When two agents do not interact, the combined effect is equal to the sum of the effects
of the two agents acting alone.
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64 TECHNICAL EVALUATION OF THE NASA MODEL FOR CANCER RISK TO ASTRONAUTS
AML Acute myeloid leukemia.
apoptosis Programmed cell death. The cell death is characterized by a distinctive fragmentation of
DNA that is regulated by cellular functions.
artificial radioactivity Manmade radioactivity produced by fission, fusion, particle bombardment, or electro 
magnetic irradiation.
attributable risk (AR) The estimated rate of a disease (such as lung cancer) that could, in theory, be prevented
if all exposures to a particular causative agent (such as radon) were eliminated.
AU Astronomical unit.
background radiation The radiation to which a member of the population is exposed from natural sources,
composed of terrestrial radiation due to naturally occurring radionuclides in the soil and
building material, cosmic radiation originating in outer space, and naturally occurring
radionuclides in the human body.
baseline rate of cancer The annual cancer incidence observed in a population in the absence of the specific agent
being studied; the baseline rate includes cancers from a number of other causes, such as
smoking and occupational exposures to chemicals.
SI unit of activity. 1 Bq = 1 s−1 = 2.7 × 10−11 Ci.
becquerel (Bq)
BEIR Biological Effects of Ionizing Radiation. Refers to the reports by the National Research
Council’s Committee on Biological Effects of Ionizing Radiation and its successor com 
mittees. The most recent of these reports is BEIR VII, published in 2006.
beta particle An electron or positron emitted from a nucleus during radioactive decay.
bias Factors that influence the outcome of data collection, such as causing certain measure 
ments to have a greater chance of being included than others.
Boltzmann transport Describes the trajectory and interactions of particles traversing a medium. So called
equation because of its similarity to the expression obtained by Boltzmann in connection with the
kinetic theory of gases.
BRYNTRN Computational model of baryon transport.
cancer A malignant tumor of potentially unlimited growth, capable of invading surrounding
tissue or spreading to other parts of the body by metastasis.
carcinogen An agent that can cause cancer. Ionizing radiation is a physical carcinogen; there are
also chemical and biological carcinogens; biological carcinogens may be extrinsic (e.g.,
viruses) or intrinsic (genetic defects).
carcinoma A malignant tumor (cancer) of epithelial origin.
casecontrol study An epidemiologic study in which people with disease and a similarly composed control
group are compared in terms of exposures to a putative causative agent.
cell culture The growing of cells in vitro (in a glass or plastic container, or in suspension) in such a
manner that the cells are no longer organized into tissues.
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APPENDIX C
CME Coronal mass ejection.
CNS Central nervous system.
cohort study An epidemiologic study in which groups of people (the cohort) are identified with respect
to the presence or absence of exposure to a diseasecausing agent, and in which the out 
comes of disease rates are compared; also called a followup study.
collective effective For a specified group of individuals: the sum of individual effective doses from a specified
dose source within a specified time period. Unit: j/kg; special name used for unit: personSv.
competing risks Causes other than the agent under study that contribute to the mortality rate. The mor
tality rate from these other causes is not included in the risk of dying from the factor
under study.
confidence interval An interval estimate of an unknown parameter, such as a risk. A 95 percent confidence
(CI) interval, as an example, is constructed from a procedure that is theoretically successful
in capturing the parameter of interest in 95 percent of its applications. Confidence limits
are the end points of a confidence interval.
constant relative risk A risk model which assumes that the ratio of the risk at a specific dose and the risk in
(CRR) the absence of the dose remains constant after a certain time.
coronal mass ejections Large regions of plasma ejected outward from the Sun by an electromagnetic process in
the solar atmosphere.
Former special unit of radioactivity. 1 Ci = 3.7 × 1010 Bq.
curie (Ci)
DDREF (dose and A judged factor by which the radiation effect, per unit of dose, caused by a given high
dose rate effectiveness or moderate dose of radiation received at high dose rates is reduced when doses are low
factor) or are received at low dose rates.
deletion Type of mutation in which sections of DNA are removed; term can refer to the removal
of a single base or many bases.
detriment The total harm to an exposed group (and its descendants) as a consequence of exposure
of the group to radiation. The principal components of detriment are probability of fatal
cancer attributable to radiation, weighted probability of nonfatal cancer attributable to
radiation, weighted probability of severe heritable effects, and length of life lost from
any harm induced.
detrimentadjusted The probability of the occurrence of cancer or heritable effect, adjusted to allow for the
risk different components of detriment to express the severity of the consequences.
DNA Deoxyribonucleic acid; the genetic material of cells.
doseeffect (dose A mathematical formulation and description of the way that the effect (or biological
response) model response) depends on the dose.
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66 TECHNICAL EVALUATION OF THE NASA MODEL FOR CANCER RISK TO ASTRONAUTS
dose equivalent The dose equivalent, H, at a point is given by
H = Q D,
where D is the absorbed dose and Q is the quality factor at that point. The unit of dose
equivalent is joules per kilogram (J kg−1), and its special name is the sievert (Sv).
This quantity is used by the ICRU (see ICRU) in the definition of operational quantities,
including ambient dose equivalent and personal dose equivalent, applied in radiation
monitoring.
dose range Definitions of low, medium, and high doses vary widely in the literature. For the pur
poses of this report, dose ranges are defined as follows: Low dose: 0 to 100 mGy (mSv).
Medium dose: in excess of 100 mGy up to a maximum of 1 Gy. High dose: in excess of 1
Gy up to the very high total doses used in radiation therapy (on the order of 20 to 60 Gy).
dose rate The absorbed dose delivered per unit time.
dose rate effectiveness The factor by which the effect per unit dose caused by a specific type of radiation changes
factor (DREF) at low doses or low dose rates (protracted or fractionated delivery of dose) as compared
to high doses delivered at high (or acute) dose rates.
dosimetric model A method for estimating risk based on the use of physical models for doses to target cells
and the use of results from epidemiologic studies of exposures to humans from other
types of radiation.
EB Empirical Bayes.
ecological fallacy The fact that two populations differ in many factors other than the one being evaluated
and that one or more of these other factors may be the underlying reason for any differ
ence noted in their morbidity or mortality experience.
ecologic study A method of epidemiologic study in which rates of health effects outcome based on
population rather than individual data are related to the measure of population radiation
exposure.
effective dose As defined by the ICRP (see ICRP): the sum of the equivalent doses HT in all specified
tissues and organs T of the body of a reference person, each weighted by wT, the tissue
weighting factor after sexaveraging of the equivalent doses.
� � � �T � �R �T,R � � �T �T ,
T R T
where
.
Unit: J kg−1.
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APPENDIX C
The special name for the unit of effective dose is sievert (Sv). Effective dose is used to
set exposure limits for radiation protection of stochastic effects and for implementing
the optimization principle (ALARA, or “as low as reasonably achievable”) in radiation
protection. The quantity enables the summation of doses from internal emitters and
external radiation fields in order to provide a single numerical value.
A special unit of energy: 1 eV = 1.6 × 10−19 J = 1.6 × 10−12 erg; 1 eV is equivalent
electron volt (eV)
to the energy gained by an electron in passing through a potential difference of 1 V;
1 keV = 1,000 eV; 1 MeV = 1,000,000 eV.
empirical model Derived from measurements in populations, as opposed to a theoretical model.
EPA Environmental Protection Agency.
epidemiology The study of the determinants of the frequency of disease in humans. The two main
types of epidemiologic studies of chronic disease are cohort (or followup) studies and
casecontrol studies.
equivalent dose The equivalent dose in an organ or tissue, HT, defined by
�T � � � �R �TR ,
R
where
.
and wR is the radiation weighting factor for radiation R. The sum is performed over all
types of radiations involved.
Unit: J kg−1.
The special name for the unit of equivalent dose is sievert (Sv).
1 Sv = 1 J/kg = 100 rem. 1 rem = 0.01 Sv (“rem” was the formerly used special name
for equivalent dose).
ESP Emission of solar protons.
etiology The science or description of cause(s) of disease.
EVA Extravehicular activity.
excess absolute risk The rate of disease in an exposed population minus the rate of disease in an unexposed
(EAR) population. Also termed “attributable risk” or “risk difference.”
excess relative risk The rate of disease in an exposed population divided by the rate of disease in an unex 
(ERR) posed population minus 1.0.
exposure (E) The condition of having contact with a physical or chemical agent.
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fibrosis Damage to normal tissue that results in the formation of excess fibrous connective tissue
in an organ or tissue.
fractionation The delivery of a given dose of radiation as several smaller doses separated by intervals
of time.
galactic cosmic ray Very energetic charged particles that have their origin inside our Galaxy and have speeds
(GCR) approaching the speed of light, propagating in the rarefied plasmas of space.
gamma radiation Also gamma rays; shortwavelength electromagnetic radiation originating from the decay
of radioactive nuclei, from bremsstrahlung, and from cosmic sources.
GCR Galactic cosmic ray.
geometric mean The geometric mean of a set of positive numbers is the exponential of the arithmetic mean
of their logarithms. The geometric mean of a lognormal distribution is the exponential
of the mean of the associated normal distribution.
geometric standard The geometric standard deviation of a lognormal distribution is the exponential of the
deviation (GSD) standard deviation of the associated normal distribution.
germ cells Reproductive cells such as the sperm and egg and their progenitor cells.
GMIR Global merged interaction regions.
GOES Geostationary operational environmental satellite.
gray (Gy) Special name of the SI unit for absorbed dose (see unit of dose). 1 Gy = 1 J/kg = 100 rads.
halflife, biological Time required to eliminate half of an incorporated amount of any substance from a body
by metabolic processes of elimination; it is approximately the same for both stable and
radioactive isotopes of a particular element.
halflife, radioactive Period of time that it takes for the amount of a radioactive substance to decrease by half
by decay.
heliosphere A vast, spheroidal cavity, approximately 200 times the mean SunEarth distance, created
in the interstellar gas by the outflowing solar wind.
highLET radiation Heavy, charged particles, such as protons or alphaparticles or heavier ions (such as are
encountered in galactic cosmic radiation), that produce ionizing events densely spaced
on a molecular scale (e.g., L > 10 keV/μm). Neutrons are also considered as highLET
radiation because the charged particles released in neutron interactions with matter are
highLET particles.
HMF Heliospheric magnetic field.
HRP Human Research Program (NASA).
HZE High atomic number (Z) and energy (E).
HZETRN High charge and energy transport code.
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APPENDIX C
ICRP (International An independent international organization that provides recommendations and guidance
Commission on on protection against ionizing radiation.
Radiological
Protection)
ICRU (International An independent international organization that provides recommendations and guidance
Commission on on radiation quantities, units, and measurements for all applications of ionizing radiation.
Radiation Units and
Measurements)
incidence Also, incidence rate; the rate of occurrence of a disease within a specified period of time,
often expressed as a number of cases per 100,000 individuals per year.
in utero In the womb: i.e., before birth.
inverse doserate An effect in which, for a given exposure, the probability of effect per unit dose increases
effect as the dose rate is lowered.
in vitro Cell culture conditions in glass or plastic containers.
in vivo In the living organism.
ionizing radiation Particles or electromagnetic radiation sufficiently energetic to dislodge electrons from
an atom or molecule, thereby creating an ion pair. Ionizing radiation includes x and
gamma radiation, electrons (beta radiation), alpha particles (helium nuclei), and heavier
charged atomic nuclei. Neutrons ionize indirectly by first interacting with components
of atomic nuclei.
ISS International Space Station.
kerma The kerma (kinetic energy released in matter) for ionizing uncharged particles (photons,
neutrons) is the quotient of the mean sum of the initial kinetic energies of all charged
particles liberated in a unit mass of a material by the uncharged particles incident on
that mass.
Unit: J kg−1.
The special name for the unit of kerma is gray (Gy). If all of the kinetic energy is absorbed
“locally,” the kerma is approximately equal to the absorbed dose.
latent period The period of time between exposure and expression of the disease. After exposure to
a dose of radiation, there typically is a delay of several years (the latent period) before
any cancer is observed.
LET The linear energy transfer (LET) for charged particles of a given type and energy in a
given material is the quotient of the mean energy lost by the charged particles due to
electronic interactions in traversing a unit track length. The unit often used for LET is
keV μm−1.
life table Shows the number of persons of a given number born or living at a specified age who
live to attain successive higher ages, together with the numbers who die in each interval.
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70 TECHNICAL EVALUATION OF THE NASA MODEL FOR CANCER RISK TO ASTRONAUTS
linear model or The linear model is a special case of the linearquadratic model, with the quadratic
relationship (also coefficient equal to zero; the linear model expresses the effect (e.g., cancer or mutation)
linear doseeffect as proportional to the dose (linear function of the dose).
relationship)
linearquadratic model Also, linearquadratic doseeffect relationship; expresses the effect (e.g., cancer) as the
sum of two components, one proportional to the dose (linear term) and one proportional
to the square of the dose (quadratic term). The linear term predominates at low doses;
the quadratic term, at high doses.
LNT model Linear nothreshold dose response for which any dose greater than zero has a finite prob
ability of producing an effect (e.g., mutation or cancer). The probability is calculated
either from the slope of a linear model or from the limiting slope, as the dose approaches
zero, of a linearquadratic model.
lognormal distribution When the logarithms of a randomly distributed quantity have a normal (Gaussian)
distribution.
lowLET radiation X rays and gamma rays, charged particles, such as electrons or highenergy protons that
produce low ionization density (e.g., L < 10 keV/μm).
LSS Life span study. Longterm study of health effects in the Hiroshima and Nagasaki atomic
bomb survivors.
mechanistic basis An explanation derived from a knowledge of the individual stages leading to an effect.
metaanalysis An analysis of epidemiologic data from several studies based on data included in
publications.
MLE Maximum likelihood estimate.
model A schematic description of a system, theory, or phenomenon that accounts for its known
or inferred properties and that may be used for further study of its characteristics.
Monte Carlo The method for evaluation of a probability distribution by means of random sampling.
calculation
mortality (rate) The frequency at which people die from a disease (e.g., a specific cancer), often expressed
as the number of deaths per 100,000 population per year.
MSL RAD Mars Science Laboratory Radiation Assessment Detector.
multiplicative effects The combined effect of two agents is equal to the product of the effects of the two agents
acting alone.
natural radioactivity The property of radioactivity exhibited by more than 50 naturally occurring radionuclides.
NCRP (National U.S. Council commissioned to formulate and disseminate information, guidance, and
Council on Radiation recommendations on radiation protection and measurements.
Protection and
Measurements)
NEO NearEarth object.
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APPENDIX C
neoplasm Any new and abnormal growth, such as a tumor; neoplastic disease refers to any disease
that forms tumors, whether malignant or benign.
NIH National Institutes of Health (U.S.).
nonstochastic A description of effects whose severity is a function of dose; for these, a threshold may
occur; some examples of somatic effects believed to be nonstochastic are cataract induc 
tion, nonmalignant damage to the skin, hematological deficiencies, and impairment of
fertility.
normal distribution The socalled bellshaped curve of randomly distributed quantities; also referred to as a
“Gaussian distribution.”
NS Neversmoker.
NSRL NASA Space Radiation Laboratory.
NTE Nontargeted effect. Biological response occurring in unirradiated cells as a result of
radiation exposure in other cells. Generally taken to include bystander responses, in
which biological changes are seen in unirradiated cells communicating with irradiated
(“targeted”) cells, and genomic instability in progeny of irradiated cells.
odds ratio (OR) The odds of being exposed among diseased persons divided by the odds of being exposed
among nondiseased persons.
oncogene An overexpressed or mutated version of a normal gene that can, in a dominant fashion,
release a cell from normal restraints controlling its proliferation and convert it into a tumor
cell, by itself or in combination with other cellular changes. (This should be contrasted
to a tumor suppressor gene whose product acts to suppress malignant transformation
of cells and operates in a recessive manner, where in classical terms, both alleles must
be lost or mutated to stop or curtail the supply of the growth suppressor gene product.)
PEL Permissible exposure limit.
phenotype The genetically and environmentally determined physical appearance of an organism.
photon An electromagnetic quantum whose energy (Eph) equals the product of the Planck
constant (h) and its frequency (v). With the convenient units eV and s, and with the
wavelength λ in μm: Eph = 4.136 10−15 v = 1.24/λ.
plasma A gas in which a significant number of the atoms and molecules have lost one or more
electrons and in which, therefore, electromagnetic effects are important.
pooled analysis An analysis of epidemiologic data from several studies based on original data from the
studies.
PRA Probabilistic risk assessment.
prevalence The number of cases of a disease in existence at a given time per unit of population,
usually 100,000 persons.
probability of A number that expresses the probability that a given cancer, in a specific tissue, has been
causation caused by a previous exposure to a carcinogenic agent, such as radiation.
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projection model A mathematical model that simultaneously describes the excess cancer risk at different
levels of some factor such as dose, time after exposure, or baseline level of risk, in terms
of a parametric function of that factor. It becomes a projection model when data in a
particular range of observations are used to assign values to the parameters in order to
estimate (or project) excess risk for factor values outside that range.
promoter An agent that is not by itself carcinogenic but which can amplify the effect of a true
carcinogen by increasing the probability of latestage cellular changes needed to complete
the carcinogenic process.
proportional mortality The ratio of the percentage of a specific cause of death among all deaths in the population
ratio being studied divided by the comparable percentage in a standard population.
protraction The spreading out of a radiation dose over time by continuous delivery at a lower dose
rate.
QMSFRG Quantum multiple scattering fragmentation.
quadraticdose model A model that assumes that the excess risk is proportional to the square of the dose.
quality factor (Q) A LETdependent factor by which the absorbed dose is multiplied to obtain (for radiation
protection purposes) the dose equivalent. The currently used values of Q were chosen by
the ICRP on the basis of published RBE (see relative biologic effectiveness) values for a
range of radiation and biological end points. Absorbed dose in Gy × Q dose equivalent
in Sv.
quality factor (QF) A track structuredependent factor, for use in NASA’s proposed model, by which the
absorbed dose is multiplied to scale the cancer risk coefficients for lowLET radiation so
as to apply to space radiation. The proposed values of QF are chosen in the 2011 NASA
report on the basis of an empirical relationship and available data on RBE values for
HZE particles for a range of biological end points.
rad The formerly used special name for the unit of absorbed dose, now replaced by the SI
unit Gy (see unit of dose). 1 rad = 0.01 Gy = 100 erg/g.
radiation weighting wR, used by the ICRP to weight absorbed dose of an organ or tissue to obtain equivalent
factor dose for the organ or tissue.
radioactivity The property of nuclide decay in which particles or gamma radiations are usually emitted.
radiogenic Caused by radiation.
radioisotope A radioactive atomic species of an element with the same atomic number and usually
identical chemical properties.
radionuclide A radioactive species of an atom characterized by the constitution of its nucleus.
random errors Errors that vary in a nonreproducible way around a limiting mean. These errors can be
treated statistically by use of the laws of probability.
REIC Risk of exposureinduced cancer.
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APPENDIX C
REID Risk of exposureinduced death. The difference in a causespecific death rate for exposed
and unexposed populations of a given sex and a given age at exposure, as an additional
cause of death introduced into a population.
relative biological The ratio Dref/D, where D is the absorbed dose of a specified radiation and Dref is the
effectiveness (RBE) absorbed dose of a sparsely ionizing reference radiation (gamma rays or X rays) that
produces the same level of effect. When the magnitude of the dose D is not specified,
the RBE is meant to be the lowdose limit of the ratio Dref/D (this lowdose RBE equals
the lowdose effectiveness [initial slope] of the specified radiation to that of the refer
ence radiation).
relative risk (RR) The rate of disease in an exposed population divided by the rate of disease in an
unexposed population. Also termed “rate ratio.”
rem (rad equivalent man). A special unit of dose equivalent, now replaced by the SI unit sievert
(see unit of dose). 1 rem = 0.01 Sv.
RERF Radiation Effects Research Foundation (Japan).
risk A chance of injury, loss, or detriment. A measure of the deleterious effects that may be
expected as the result of an action or inaction.
risk assessment The process by which the risks associated with an action or inaction are identified and
quantified.
risk coefficient The increase in the annual incidence or mortality rate per unit dose: (1) absolute risk
coefficient is the increase of the incidence or mortality rate per unit dose; (2) relative
risk coefficient is the fractional increase above the baseline incidence or mortality rate
per unit dose.
risk estimate The increment of the incidence or mortality rate projected to occur in a specified exposed
population per unit dose for a specified exposure regime and expression period.
SEP Solar energetic particle.
SI units Units of the International System of Units as defined by the General Conference of
Weights and Measures in 1960. They are the base units, such as meter (m), kilogram (kg),
second (s), and their combinations, which have special names (e.g., the unit of energy,
1 J = 1 kg m2/s2, or absorbed dose, 1 Gy = 1 J/kg. (See unit of dose.)
sievert (Sv) Special name of the SI unit of dose equivalent, equivalent dose, and effective dose (see
unit of dose). 1 Sv = 1 J/kg = 100 rem.
solar cycle A periodic change of the Sun’s activity with a period of approximately 11 years; typically,
consists of 7 years of solar maximum and 4 years of solar minimum.
solar particle event Very energetic process from the Sun produced both by solar flares and by shocks driven
by fast coronal mass ejections; occurs when very strong magnetic fields in the solar
photosphere reach a critical instability.
solid cancers All malignant neoplasms other than those of the lymphatic and hematopoietic tissue.
somatic cells Nonreproductive cells.
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SPE Solar particle event.
specific activity Activity of a given nuclide per unit mass of a compound, element, or radioactive nuclide.
specific energy (z) The energy per unit mass actually deposited in a microscopic volume in a single energy
deposition event or at a given absorbed dose. This is a stochastic quantity as opposed to
its average, the absorbed dose, D. The mean energy imparted by ionizing radiation to a
medium per unit mass.
Unit: 1 Gy = 1 J/kg.
standardized mortality The ratio (multiplied by 100) of the mortality rate from a disease in the population being
ratio (SMR) studied divided by the comparable rate in a standard population. The ratio is similar to
a relative risk times 100.
stochastic Effects whose probability of occurrence in an exposed population (rather than severity
in an affected individual) depends on dose; stochastic effects are commonly regarded as
having no threshold; hereditary effects are stochastic; some somatic effects, especially
cancers, are regarded as being stochastic.
STS Space Transportation System.
suppressor gene A gene that can suppress another gene such as an oncogene. Changes in suppressor genes
can lead to expression by genes such as oncogenes.
synergistic effect Increased effectiveness results from an interaction between two agents, so that the total
effect is greater than the sum of the effects of the two agents acting alone.
systemic errors Errors that are reproducible and tend to bias a result in one direction. Their causes can
be assigned, at least in principle, and they can have constant and variable components.
Generally, these errors cannot be treated statistically.
target cells Cells in a tissue that have been determined to be the key cells in which changes occur in
order to produce an end point such as cancer.
TE Targeted effect; occurrence of biological responses in irradiated cells (in contrast to
NTE; see above).
threshold hypothesis The assumption that no radiation injury occurs below a specified dose.
transformed cells Tissue culture cells changed from growing in an orderly pattern exhibiting contact inhibi 
tion to growing in a pattern more like that of cancer cells.
transport The term used in the study of energetic particles (GCR, SEP, etc.) to describe quantita 
tively the motion of the particles and their interaction with matter. Generally, transport
calculations use an equation (see Boltzmann transport equation) that depends on the
medium (e.g., shielding material, structural material, human body, interplanetary space)
and particles being studied.
turbulence The essentially random fluctuations that often occur in a largescale fluid.
u Atomic mass unit. Kinetic energies are often expressed in units of MeV per
atomic mass unit (u), MeV/u, because particles with identical E then have the
same b.
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APPENDIX C
uncertainty The range of values within which the true value is estimated to lie. It is a best estimate
of possible inaccuracy due to both random and systemic errors.
units of dose Dosimetric units.
Unit conversion factors:
Gray (SI): 1 Gy = 1 J/kg = 100 rad: used for absorbed dose and kerma.
Sievert (SI): 1 Sv = 1 J/kg = 100 rem: used for equivalent dose, effective dose, and dose
equivalent.
Rem: 1 rem = 0.01 Sv.
UNSCEAR United Nations Scientific Committee on the Effects of Atomic Radiation; publishes
periodic reports on sources and effects of ionizing radiation.
variability The variation of a property or a quantity among members of a population. Such variation
is inherent in nature and is often assumed to be random; it can then be represented by a
frequency distribution.
weighted dose (d) The dose to the atomic bomb survivors, roughly adjusted to account for the increased
effectiveness of the small neutron absorbed dose contribution. The weighted dose equals
the gammaray absorbed dose to a specified organ plus the neutron absorbed dose mul 
tiplied by a weighting factor that has usually been set equal to 10 in the analyses by
RERF (see RERF).
x radiation Also x rays; penetrating electromagnetic radiation, usually produced by bombarding a
metallic target with fast electrons in a high vacuum.
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