Health Risks from Exposure to Low Levels of Ionizing Radiation BEIR VII Phase 2 (2006) / Chapter Skim
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1 Background Information
1 Background Information
Pages 19-42
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From page 19... ...
Different Types of Ionizing Radiation Ionizing radiation, by definition, contains enough energy to displace electrons and break chemical bonds. Charged 1Restricted linear energy transfer, L, results when, within the charged particles, such as high-energy electrons, protons, -particles, particle tracks, secondary electrons (-rays)
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From page 20... ...
A photon interacts with an atomic nucleus, and the the X-rays. Discrete energy "spikes" also occur in the X-ray photon energy is converted into a positron and an electron.
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From page 21... ...
of photon radiation with that of electron radiation, the mean X- and -rays produce electrons with relatively low linear range of electrons of specified energy is given in the same energy transfer, (i.e., energy loss per unit track length) and diagram.
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From page 22... ...
A similar deseparate tracks. This accounts in an approximate way for the pendence on photon energy is seen in the related microincreased local energies due to -rays and therefore provides dosimetric parameter dose lineal energy, y, which has been larger values that are more meaningful than those of unre used as reference parameter by the liaison committee of the stricted LET.
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From page 23... ...
10 8 0.24 mm m) 6 m 0.96 mm /(keV/ 4 y D d = 3.9 mm 2 0 0.01 0.1 1 photon energy / MeV FIGURE 1-5 Measured dose average lineal energy, yD, for monoenergetic photons and for different simulated site diameters, d.
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From page 24... ...
ear-quadratic fits to the dose dependencies. The closed Since the dose dependence for solid tumors among A-bomb circles relate to -rays and to broad X-ray spectra; the survivors indicates little curvature, the dependence of risk squares, to characteristic X-rays and monoenergetic photons on photon energy may be similarly weak for tumor induction in man.
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From page 25... ...
Squares are for monoenergetic photons; circles are X-ray spectra or -rays. The two data points in the lower panel labeled 220 kV both had 220 kV generating voltage, but the filtration was different.
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From page 26... ...
The confidence limit for the study of cervical carcinoma patients is recalculated. Cohorts: LSS: Life Span Study of atomic bomb survivors; MasTb: Massachusetts tuberculosis patients; PPMast: New York postpartum mastits patients; SwBBD: Swedish benign breast disease patients; CervCa: cervical cancer patients (case-control study)
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From page 27... ...
A dose of 1 Gy corresponds to about 1000 tracks Relative Biological Effectiveness of Neutrons This report assesses the biological effects of low-LET radiation, that is, photons and electrons. It does not deal with densely ionizing radiation, such as heavy ions (including particles)
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From page 28... ...
, for photons can in This large maximal value might be seen as an indication of many radiobiological investigations be described as a linear an exceptionally high effectiveness of neutrons at low doses. quadratic function of absorbed dose: In fact the dose-effect relationship for neutrons is simply linear, and the high maximal RBE of neutrons is merely a E(D)
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From page 29... ...
Their calculations also indicate that the DNA damage tends to be along short lengths of DNA: 134 base H2O+ + H2O OH· + H3O+. pairs (bp)
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tive base products of ionizing radiation are chemically iden
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From page 31... ...
In contrast, the D37 DNA damage produced by background radiation and the low dose for low-LET ionizing radiation produces only 1000 doses of radiation to which some workers are exposed does SSBs and 40 DSBs -- damage that is not characteristic of lenot add appreciably to the extensive spontaneous and meta- thal doses of H2O2. Such data suggest that DSBs and other bolic damage.
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From page 32... ...
endonuMOLECULAR MECHANISMS OF DNA REPAIR clease, APE1, occupies a pivotal position in BER of anoma Ionizing radiation can cause a wide array of damage to lous residues, recognizing and cleaving at the 5 side of individual DNA bases and SSBs and DSBs resulting from abasic sites generated by spontaneous hydrolysis, reactive deoxyribose destruction (for basic biological and genetic oxygen species, and DNA glycosylases. Abasic sites generconcepts, see Appendix A)
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From page 33... ...
BACKGROUND INFORMATION 33 FIGURE 1-9 Base-excision repair. This pathway repairs single-base damage (from X-rays, reactive oxygen species, methylation, or deamination)
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From page 34... ...
Repair of Double-Strand Breaks Nucleotide-Excision Repair of Cyclodeoxynucleosides Exposure of DNA to ionizing radiation produces about The great majority of endogenous DNA lesions produced 57% as many DSBs as SSBs (e.g., see earlier discussion of by reactive oxygen species are corrected by the BER path- Nikjoo and others 1997, 2000)
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From page 35... ...
In some cases, endonuclease, and DNA polymerase . Attempted repair by nonhomologous end joining appears to be responsible for large these BER enzymes can enhance DSB formation and loss of DNA deletions and chromosome aberrations.
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From page 36... ...
. Bacteria live in a highly variable environment and have evolved efficient inducible DNA repair processes to deal Homologous Recombination with sudden challenges of DNA damage from oxygen free Repair of a DSB by HR involves matching the two broken radicals, ionizing radiation, chemicals, and ultraviolet radiaends of a DNA strand with identical sequences of intact DNA tion.
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From page 37... ...
The specific activity of ATM is increased after introduction of DSBs in DNA through ionizing radiation or other means; this then activates other proteins by phosphorylation (denoted by amino acid symbol and number) and in a cell cycle-specific manner.
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From page 38... ...
HR requires an identical sequence spanning the part of the DNA molecule containing the break and extensive remodeling of the broken DNA termini. Mre11/Rad50/Nbs1 resects individual strands by its 5- to 3-exonuclease activity and binds homologous doublestranded DNA by the Rad50 moiety.
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From page 39... ...
. and beta particles, which are important contributors to direct Most of the members of the signal transduction pathways DNA damage after ionizing radiation exposure, and introincluding ATM, p53, Chk1, Chk2, Brca1, and hMre11/ duces a special subject -- the effect that neutron RBEs have hRad50/Nbs1 are protein products of tumor-suppressor on low-LET radiation risk estimates.
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The relevance of these com Large numbers of genes not obviously involved in DNA parisons to this report lies in the attempts that have been repair fall within the list of sensitive mutants to ionizing ra- made to explain low-dose ionizing radiation as no more than diation and oxidants. Several genes whose deletion produced a special case of oxidative damage (Pollycove and Feinensensitivity to radiation and oxidants were involved in DNA degen 2003)
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(2002) H 2O2-sensitive H 2O2-sensitive 260 total 525 total FIGURE 1A-2 Venn diagram representing the overlap among genes involved in resistance against ionizing radiation and various oxidizing agents as indicated in the reports cited.
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From page 42... ...
rarely found among those involved in resistance to ionizing These results in S cerevisiae, however, provide no supradiation or oxidative damage in yeast, where they play a port for the attempts to equate low-dose ionizing radiation very minor role.
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