Biological Effects of Ionizing Radiation Michael Hajek Radiation Safety and Monitoring Section Division of Radiation, Transport and Waste Safety Department of Nuclear Safety and Security

Outline Introduction and historical background Targets for biological radiation damage Deterministic and stochastic effects ICRP system of radiological protection Biological Radiation Effects

Ionization energy of soft tissue Ionizing Radiation Ionizing radiation Composed of particles that individually carry enough kinetic energy to liberate an electron from an atom or molecule Kinetic energy > 12.4 eV Non-ionizing radiation Ionizing radiation Ionization energy of soft tissue 12.4 eV or 100 nm Biological Radiation Effects

Discovery of Ionizing Radiation X-rays (1895) Natural radioactivity (1896) Wilhelm Conrad Roentgen Antoine Henri Becquerel Nobel Prize in Physics 1901 Nobel Prize in Physics 1903 Biological Radiation Effects

First Medical Observations Skin-burn attributed to radiation ─ 1901 Radiation-induced leukaemia ─ 1911 Clinical syndrome following exposure to atomic bomb explosions ─ 1946 P. D. Keller, J. Am. Med. Assoc. 131, 504 (1946). Holzknecht’s chromoradiometer related to skin erythema ─ 1902 Biological Radiation Effects

Targets for Biological Radiation Damage Human tissues are formed from cells that are grouped into organs and systems of the body to perform the many specialized functions Each cell is defined by a membrane enclosing Cytoplasm containing up to 85% water Structures such a nucleus Biological Radiation Effects

Chromosomes and DNA Chromosomes are organized structures of supercoiled deoxyribonucleic acid (DNA) and proteins found in cells DNA macromolecules encode genetic information used in development and functioning of all known living organisms Biological Radiation Effects

Structure of DNA Double-stranded helices, with nucleobases (G, A, T, C) attached to sugar-phosphate backbones Each type of nucleobase on one strand bonds with just one type of nucleobase on the other strand (complementary base pairing) Hydrogen bond 1 nm 3.4 nm Biological Radiation Effects

Direct and Indirect Radiation Effects Indirect action predominant with low-LET radiation (X- and gamma rays) Direct action predominant with high-LET radiation (alpha particles) Biological Radiation Effects

Consequences of DNA Damage Repair Cell death Mutation Viable cell Deterministic effect Stochastic effect Biological Radiation Effects

Mechanism of DNA Repair DNA damage occurs at a rate of ~ 100,000 per cell per day Genetic mutations drive evolution Biological Radiation Effects

Deterministic Radiation Effects Occur at high doses when enough cells in an organ or tissue are killed or prevented from functioning normally Threshold dose, above which effects are clinically observable Severity increases with dose Acute effects, non-malignant late effects Examples: Cataracts, erythema, acute radiation syndromes (ARS) DOSE SEVERITY 100% Threshold dose Biological Radiation Effects

Deterministic Radiation Effects Data on deterministic radiation effects come from Survivors of atomic bombs on Hiroshima and Nagasaki Effects on early radiologists Consequences of severe accidents with industrial radiation sources Studies of side effects of radiotherapy Biological Radiation Effects

Deterministic Radiation Effects Organ or tissue Acute dose (Gy) Type of effect Time of occurrence Bone marrow 1 ARS 1 to 2 months Skin 3 Erythema 1 to 3 weeks Thyroid 5 Hypothyroidism ≥ 1 year Lens of the eye 2 Cataract ≥ 6 months Gonads Permanent sterility Several weeks Foetus 0.1 Teratogenesis ─ Biological Radiation Effects

Deterministic Effects after Chernobyl Chernobyl experience ARS and radiation burns Biological Radiation Effects

Stochastic Radiation Effects Occur at all dose levels as a result of damage to the DNA Random or non-threshold effects Probability of occurrence increases with dose Late effects, often decades after exposure Examples: Radiation-induced cancers, hereditary effects DOSE RISK Linear-no-threshold hypothesis Quadratic response Biological Radiation Effects

Stochastic Radiation Effects Principal sources of information on stochastic effects are Epidemiological studies on atomic-bomb survivors Patients exposed to radiation for medical treatment or diagnosis Some groups of occupationally exposed workers (uranium miners, nuclear industry workers, radium-dial painters) Biological Radiation Effects

Stochastic Radiation Risks DOSE RISK Risk factor Relationship is irrelevant Background incidence Background dose Average 2.4 mSv Typical 10. mSv High 100. mSv Increment of dose Increment of probability Biological Radiation Effects

ICRP Nominal Risk Coefficients ICRP detriment-adjusted nominal risk coefficients (10−2 Sv−1) for stochastic effects after exposure to radiation at low dose rate Combined detriment due to excess cancer and hereditary effects ~ 5% per Sv Exposed population Cancer Hereditary effects Total Publ.103 Publ. 60 Whole 5.5 6.0 0.2 1.3 5.7 7.3 Adult 4.1 4.8 0.1 0.8 4.2 5.6 Biological Radiation Effects

ICRP System of Radiological Protection … to contribute to an appropriate level of protection for people and the environment against the detrimental effects of radiation exposure ... Justification Any decision that alters the radiation exposure situation should do more good than harm. Optimization (ALARA) The likelihood of incurring exposure, the number of people exposed, and the magnitude of their individual doses should all be kept as low as reasonably achievable, taking into account economic and societal factors. Limitation The total dose to any individual from regulated sources in planned exposure situations other than medical exposure of patients should not exceed the appropriate limits specified by the Commission. Biological Radiation Effects

Philosophy of Individual Dose Limitation Prevention of deterministic effects Dose limits lower than threshold Reduction of stochastic effects to acceptable level Comparison with risks for other occupations Ethical judgment Annual dose limits Occupational exposure → 20 mSv (whole-body exposure) → 20 mSv (lens of the eye), 500 mSv (extremities) General public → 1 mSv (whole-body exposure) → 15 mSv (lens of the eye), 50 mSv (skin) Biological Radiation Effects

Further Information IAEA Safety Standards IAEA Safety Standards Series No. GSR Part 3 (Interim) “Radiation Protection and Safety of Radiation Sources: International Basic Safety Standards” (2011) IAEA Safety Standards Series No. SF-1 “Fundamental Safety Principles” (2006) No. RS-G-1.1 “Occupational Radiation Protection” (1999) Practical Radiation Technical Manuals “Health Effects and Medical Surveillance” (2004) “Personal Protective Equipment” (2004) Biological Radiation Effects

Thank you for your kind attention! Biological Radiation Effects