X Radiation & Gamma Radiation And Neutrons

06/18/03RoC Background Document for X & Gamma Radiation and NeutronsNeutronsNeutrons are electrically neutral particles that, together with positively charged protons,make up atomic nuclei. The number of neutrons defines the isotope of an element.Neutrons have mass and energy and may be produced by humans with machines such asa cyclotron. The neutron decays to a proton by beta emission. As uncharged particles,neutrons do not interact with atomic electrons in the matter through which they pass, butthey do interact with the nuclei of the atoms present. The nuclear force, which leads tothese interactions, is very short ranged, which means that neutrons have to pass close to anucleus for an interaction to take place. Because of the small size of the nucleus inrelation to the atom as a whole, neutrons will have a low probability of interaction, andcould thus travel considerable distances in matter. Neutrons are capable of generating amuch denser ion path and damage to human tissue than electrons. Interactions of neutronswith biological material may result in the production of gamma radiation, protons, andalpha particles.Alpha and beta particlesAlpha and beta particles, which also are identified as part of ionizing radiation, are notincluded with these nominations but may be reviewed separately in the future for possiblelisting in the Report on Carcinogens.Human ExposureExposure to ionizing radiation comes from a variety of natural (environmental exposure)and anthropogenic sources, including exposure for military, medical, and occupationalpurposes.X radiation and gamma radiationEnvironmental exposure. Environmental exposure to X and gamma rays results fromterrestrial sources, particularly the radioactive nuclei chemically bound in the upper 25cm of the earth’s crust and in building construction materials. Radioactivity also has beenreleased into the environment from nuclear accidents, primarily from the largest nuclearaccident to date that occurred in Chernobyl, Ukraine in 1986. The worldwide annual percapita dose for residual radioactivity from Chernobyl was estimated to be 0.002millisievert (mSv) in 2000, down from a maximum of 0.04 mSv in 1986. Environmentalexposure also can come from nuclear power generation.Occupational exposure. Occupational exposure to X and gamma rays affectsapproximately 5 million workers worldwide with most being employed as coal miners orother underground miners in non-coal mines. Other occupationally exposed workersinclude medical workers, nuclear industry workers, and airline crews. The NuclearRegulatory Commission limits the occupational dose to 5 rem/year [1 rem 0.01 Sv].Medical uses. Exposure to medical radiation occurs for a large portion of the populationof more developed countries, such as the United States, that have a high level of medicalcare. However, medical exposures are very small compared to the exposure to naturalvi

06/18/03RoC Background Document for X & Gamma Radiation and Neutronssources of radiation with an annual collective dose of about 2 x 106 person-Sv/year formedical procedures compared to 14 x 106 person-Sv/year from background exposures.Medical exposures also differ from other exposures to radiation since the exposedindividual receives a direct benefit from the procedures, which include diagnosticradiology and radiation therapy.Military uses. Major past exposures to X and gamma radiation have resulted frommilitary uses of atomic weapons with the detonation of two atomic bombs overHiroshima and Nagasaki, Japan in 1945 and additional atmospheric testing of nuclearweapons that were carried out between 1945 and 1980. Survivors of the bombings ofHiroshima and Nagasaki were exposed to approximately 300 mSv on average while thelocal population near the nuclear test site in Nevada was estimated to have received anaverage dose of about 3 mSv.NeutronsExposure to neutrons derives from many of the same sources as those causing exposureto X and gamma radiation. However, neutron exposure from the atomic bombs atHiroshima and Nagasaki, Japan is now considered to have contributed only 1% to 2% ofthe total dose of ionizing radiation. Similarly, medical uses of neutrons are very limitedcurrently, and occupation exposure to neutrons in the nuclear industry accounts for onlyabout 3% of the total annual effective dose to nuclear plant workers. Occupationalexposure to neutrons can occur for aircraft crews and for oil-field workers when the lateruse neutron radiation for well logging. Most environmental exposure to neutrons is fromcosmic radiation, which has been estimated to result in an annual effective dose of 80 to200 μSv at sea level.Dosimetric methodsA variety of dosimetric methods are used for monitoring X and gamma rays inenvironmental and medical settings. X and gamma ray detectors include gas detectors,scintillators, and semiconductors. Individual personnel monitors of many types are in use,including film badges, thermoluminescent dosimeters, optically stimulated luminescencetechnology, and self-reading pocket dosimeters. Monitoring methods for neutrons aredivided into detectors of slow neutrons and fast neutrons. Detectors of slow neutronsinclude proportional counters using 10B or 3He, scintillators with 6Li or 10B, ionizationchambers lined with 235U, and semiconductors attached to a 6Li or 10B radiator. Detectorsof fast neutrons may be based on tissue-equivalent ionization chambers, recoil protontechniques, capture reactions or moderated detectors. Rem meters and neutronspectrometry, either proton recoil based or “Bonner sphere,” also can be used to detectfast neutrons. A wide variety of personnel monitors for neutrons are in use, e.g., nuclearemulsions, thermoluminescent detectors, track-etch detectors, electronic pocketdosimeters, activation detectors, and bubble detectors. Exposure to ionizing radiation alsomay be measured through the use of biological indices that may be either in vivo, i.e.,measurement of radioactivity in the human body, or in vitro, i.e., measurement ofradioactivity in urine, excreta, or other material taken from the body.vii

06/18/03RoC Background Document for X & Gamma Radiation and NeutronsHuman Cancer StudiesX radiation and gamma radiationIARC concluded in 1999 that all radiation studies taken together present a consistentbody of evidence for carcinogenicity of X radiation and gamma radiation in humans.IARC’s conclusion is corroborated by the newly published studies reviewed here.Recently published studies of second cancer occurrences after radiation treatment for firstcancers further supported the A-bomb survivor results concerning differences in latencyby type of cancer (higher risk of hematopoietic cancers appears in the first 10 years offollow-up compared to higher risks of solid cancers with increasing follow-up) and byage at exposure (higher risk for thyroid cancer after irradiation in childhood and forbreast cancer after irradiation in adolescence and during the reproductive years).Described below are the conclusions reached concerning which organ sites are to beconsidered radiosensitive and at what dose levels specific organs are affected.It is largely undisputed that leukemia and cancers of the thyroid, breast, and lung areassociated with radiation exposure, and that these associations have been found at dosesas low as 0.2 gray (Gy). The risk, however, depends to some extent on the age atexposure with exposure during childhood being mainly responsible for higher leukemiaand thyroid cancer risks and exposure during reproductive age for breast cancer. Asrecently suggested by some studies, lung cancer risk may be more strongly related toexposure later in life. Associations between radiation and cancers of the salivary glands,stomach, colon, bladder, ovary, central nervous system, and skin have been reported butare less well quantified. An exhaustive review by Ron (1998) noted that the relative risks(RR)for these cancer sites at 1 Gy exposure generally range from 1 to 2.5 for these sites.Some recent studies added additional evidence for cancers at these sites being caused byradiation exposures, i.e., by medical treatment with radiation (Garwicz et al. 2000, Bhatiaet al. 2002, Kleinerman et al. 1995, Brenner et al. 2000, Ron et al. 1999, Lichter et al.2000, Yeh et al. 2001), or by occupational low and protracted doses as reported for alarge Canadian worker cohort (Sont et al. 2001). The first large study of sarcomasconducted by Yap et al. (2002) added angiosarcomas to the list of radiation-inducedcancers occurring within the field of radiation at high therapeutic doses. In the IARCreport, associations of ionizing radiation exposures with cancers of the liver, esophagus,and, to a lesser extent multiple myeloma and non-Hodgkin’s lymphoma, were consideredinconsistent. Two recent studies, one conducted in a worker population (Gilbert et al.2000) and another among A-bomb survivors (Cologne et al. 1999), suggested that livercancers can be caused by radiation at doses above 100 mSv (in the worker populationespecially with concurrent exposure to radionuclides), and a linear dose-responserelationship for external radiation and liver cancers was calculated for the A-bombsurvivors (RR 1.81; 95% CI 1.32 to 2.43 per 1 Sv liver dose). A recent study byModan et al. (2000) added some evidence that radiation exposure during childhood mayaffect the incidence of lymphomas and melanomas.Finally, chronic lymphatic leukemia, Hodgkin’s disease, cancers of the cervix, prostrate,testis, and pancreas have rarely been related to radiation, although a recent large workerviii

06/18/03RoC Background Document for X & Gamma Radiation and Neutronscohort study (Sont et al. 2001) suggested otherwise for the latter two cancer types (testisand pancreatic cancers).NeutronsThere are no adequate epidemiological data available to evaluate the carcinogenicity ofneutrons in humans.Studies in Experimental AnimalsX radiation and gamma radiationX rays and gamma rays are clearly carcinogenic in all the species tested (see Table belowfor tumor sites that have been observed in animal studies), although tissues differ in theirsusceptibility to both radiation qualities i.e., low- and high-LET radiations. The degree ofsusceptibility for the induction of benign and malignant tumors is species-, strain-, ageand sex-dependent. Exposures in the early prenatal stages do not appear to increasecancer rates, but exposures in the later stages may do so. The question of whetherparental irradiation increases the susceptibility of offspring to radiogenic cancer iscontroversial, and conflicting results have been obtained in different experiments.NeutronsLow-energy neutrons, such as fission neutrons, are significantly more potent carcinogensthan low-LET radiations, such as X or gamma rays. There are some differences in theeffects among radiations of different quality, but none of the differences have beensufficient to reject the assumption made in risk estimation for radiation protectionpurposes, namely, that the effects of radiations of different LET differ quantitatively butnot qualitatively. There is no conclusive evidence of a signature alteration that mightdistinguish tumors induced by high-LET radiations from those induced by low-LETradiations. Tumors sites induced in experimental animals following exposure to neutronradiation are summarized in the Table below.ix

06/18/03RoC Background Document for X & Gamma Radiation and NeutronsSummary of tumors sites observed in experimental animals following exposure to Xrays, gamma rays, or neutronsTumor I TractHarderian gMammaryMultipleMyelomaOvaryPituitarySkinSoft TissuesSpinal CordThyroidVascular System* N NeutronsXMouseN*γ9XRatγ9Test AnimalType of RadiationRabbitN* XN* 99999999999999999999999999999999Genetic and Related EffectsX radiation and gamma radiationHuman in vivo studies. Studies in humans exposed to ionizing radiation following A bomb detonations and various radiation accidents and occupational exposures clearlyshow that low-LET radiations induce chromosomal alterations and gene mutations insomatic cells. The induction of genetic alterations in germ cells is less clear-cut. Studiesof males exposed as a consequence of the Chernobyl accident suggest that X rays andgamma rays may induce transmissible minisatellite mutations in male germ cells.Animal in vivo studies. Studies in normal mice, transgenic mice, and rhesus monkeyshave demonstrated that X and gamma rays induce mutations, chromosomal aberrations,micronuclei and DNA strand breaks in somatic cells. X rays and gamma rays inducegenetic damage in germ cells of mice, including dominant lethal mutations, recessivevisible mutations, and recessive lethal mutations.x

06/18/03RoC Background Document for X & Gamma Radiation and NeutronsHuman and animal in vitro studies. Evidence of chromosomal aberrations of varioustypes is well documented and constitutes the primary effect of ionizing radiationexposure. In human cells, ionizing radiation also induces mutations, micronuclei, andDNA strand breaks in somatic cells. Studies in animal somatic cells have shown thationizing radiation induces mutations, polyploidy, chromosomal instability, DNA damage,and cell transformation. Irradiation of human sperm resulted in chromosomal aberrationsand micronuclei, which were observed following fertilization of hamster oocytes.Mechanistic concerns. Double-strand DNA breaks and some base damage, quite possiblyat multiply damaged sites (or sites of clustered damage), appear to be most important forthe induction of chromosomal alterations and point mutations. These genetic end-pointsare largely the consequence of misrepair during one of several known DNA repairprocesses, although errors of DNA replication can occur for DNA damage remaining atthe time of replication. A number of cellular components and functions are involved inensuring efficient and accurate repair. Mutations in one or more of these processes willresult in increased sensitivity to the induction of genetic damage.NeutronsHuman in vivo and in vitro studies. Studies of individuals accidentally or medicallyexposed to neutron radiation show that induced chromosomal aberrations can persist fordecades, and some in vitro studies show genomic instability in progeny of irradiatedhuman cells. Many in vitro studies consistently demonstrate that neutron radiationinduces chromosomal aberrations in human peripheral lymphocytes more effectively thangamma radiation. Human data do not show statistically significant effects of parentalexposure on chromosomal abnormalities and mutations in subsequent generations.Animal in vivo and in vitro studies. DNA damage, chromosomal aberrations, genomicinstability, gene mutations, and cell transformations occurred in mammalian cellsexposed to neutrons in vitro. Germ-line instability in mice has persisted for at least twogenerations following irradiation. Somatic cell mutations have been detected at the hprtlocus and in ras oncogenes, and various cytogenetic effects, including chromosomalaberrations, sister chromatid exchanges, and micronuclei, have been reported in irradiatedmice. Reciprocal translocations in male germ cells were reported in rhesus monkeys andmarmosets.The genetic effects induced by neutron radiation are qualitatively similar to the effects ofX rays and gamma rays, but there are some quantitative differences. Several investigatorshave identified some potential cytogenetic fingerprints of neutron radiation based onthese quantitative differences. These include the ratios of simple translocations toinsertions (I-ratio), complete exchanges to incomplete rejoinings (S[I]-ratio), anddicentrics to interstitial deletions (H-ratio). In general, chromosomal aberrations,mutations, and DNA damage are induced more efficiently; DNA lesions are more severeand repaired less efficiently; and there are higher proportions of complex aberrationscompared to low-LET radiation.xi

06/18/03RoC Background Document for X & Gamma Radiation and NeutronsOther Relevant DataHealth effectsBiological effects of ionizing radiation are produced as the energy associated with theradiation penetrates and interacts with the atoms in the tissue. The effects from differenttypes of radiation differ quantitatively but are qualitatively similar. X rays, gamma rays,and neutrons are considered indirectly ionizing radiations because they most frequentlycause ionization of water molecules with production of reactive products that mayproduce modifications of DNA molecules. These reactive products include free electrons,ionized water molecules, hydroxyl ions, hydrogen free radicals, hydrogen ions, hydroxylradicals, and, in the presence of molecular oxygen, hydrogen peroxide, hydroperoxyradicals, and hydroperoxy ions. When reactions of these products with living cellsproduce unrepaired damage, deterministic (health effects in which the severity isdependent on the dose) and stochastic effects (health effects in which the severity isindependent of the dose but the probability is dependent of the dose, e.g. genetic effectsand cancer) may result.Early effects of ionizing radiation are deterministic effects that relate primarily to celldeath and vary with the radiosensitivity of cell populations. The prodromal syndromecomprises a set of acute symptoms of gastrointestinal and neuromuscular symptoms thatare seen as the initial response to whole-body irradiation. Increasing doses are associatedwith decreased survival time and with primary lethal effects that range from thehematopoietic syndrome through the gastrointestinal syndrome to the central nervoussystem syndrome. Neutrons have a higher relative biological effect compared to low-LETradiation.Radiation-sensitive disordersCertain genetic disorders predispose affected individuals to radiation sensitivity andcancer. These disorders include ataxia-telangiectasia (A-T), Nijmegen breakagesyndrome, Mre11 deficiency, and ligase IV deficiency. Mutations of the A-T gene havebeen associated with breast and prostate cancer, head and neck cancer, lymphoma, andleukemia.Potential mechanisms of carcinogenesisSeveral mechanisms by which ionizing radiation could cause cancer have been proposed.Ionizing radiation may induce DNA damage directly, resulting in single-strand breaks,double-strand breaks, modifications of deoxyribose rings and bases, intra- and interstrandDNA-DNA cross-links, and DNA-protein cross-links. Epigenetic mechanisms that resultin alteration in the expression of genomic information also have been proposed. Theseproposed mechanisms include radiation-induced genomic instability, induction ofmutations by cytoplasmic irradiation, and “bystander effects,” which are based onmutational events occurring in cells that do not directly receive exposure to ionizingradiation.xii

06/18/03RoC Background Document for X & Gamma Radiation and NeutronsAbbreviationsBqBecquerelC kg-1Coulomb per kilogram of airCiCurieCLLChronic Lymphatic LeukemiaENUEthylnitrosoureaEPDElectronic Pocket dosimetersEVElectron-voltGMGeiger-Mueller counterGyGrayHIDAN-substituted-2,6-dimiethyl phenyl carbamoylethyl iminodiacetic acid(hepatic iminodiacetic acid);HMPAOHexamethyl propyleneamine oximeHPGeHigh purity germaniumICIonization chamberJJouleKermaKinetic energy released in matterLETLinear Energy transferMAAMacroaggregated albuminMDPMethylene diphosphonateMNUMethylnitrosoureaPCProportional CounterRRoentgenRBERelative biological effectivenessRSDRadiation Sensitive disordersxiii

06/18/03RoC Background Document for X & Gamma Radiation and NeutronsSIStandard International unitsSRPDSelf-Reading Pocket dosimetersSvSievertTEPCTissue-Equivalent Proportional CounterTLDThermoluminescent Detectorxiv

06/18/03RoC Background Document for X & Gamma Radiation and NeutronsTable of Contents1Introduction. 11.1Basic information on ionizing radiation . 11.1.1Photon radiation.

of gamma rays in the environment is naturally occurring potassium-40. Artificial sources include plutonium-239 and cesium-137. Gamma rays can easily pass through the human body or be absorbed by tissue, thus constituting a radiation hazard for the entire body. Gamma rays resulting from radioactive decay consist of monoenergetic photons with