Radiation Biology
advertisement
Radiation Biology Robert Metzger, Ph.D. Biologic Effects Many factors determine the biologic response to radiation exposure Radiosensitivity and complexity of the biologic system determine the type of response from a given exposure Usually complex organisms exhibit more sophisticated repair mechanisms Some responses appear instantaneously, others weeks to decades c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed., p. 814. Classification of Bio Effects Biologic effects of radiation exposure can be classified as either stochastic or deterministic (non-stochastic) Stochastic Effect The probability of the effect, rather than its severity, ↑ with dose Radiation-induced cancer and genetic effects Basic assumption: risk ↑ with dose and no threshold Injury to a few cells or even a single cell can theoretically result in manifestation of disease The principal health risk from low-dose radiation Classification of Bio Effects Deterministic or Non-stochastic Effect Predominant biologic effect is cell killing resulting in degenerative changes to the exposed tissue Severity of the effect, rather than its probability, ↑ with dose Require much higher dose to produce an effect Threshold dose below which the effect is not seen Cataracts, erythyma, fibrosis, and hematopoietic damage are some deterministic effects Dx radiology: only observed in some lengthy, fluoroscopically guided interventional procedures Interaction of Radiation with Tissue Ionizing radiation energy deposited randomly and rapidly (< 10-10 sec) via excitation, ionization & thermal heating Interactions producing biologic changes classified as either direct or indirect Direct Critical targets (e.g., DNA, RNA or protein) directly ionized or excited Indirect Radiation interacts within the medium (e.g., cytoplasm) creating reactive chemical species (free radicals) which in turn interact with the a critical target macromolecule Interaction of Radiation with Tissue Vast majority of interactions are indirect (body 70% 85% water) Results in an unstable ion pair, H2O+, H2ODissociate into another ion and a free radical (lifetime is less than 10-5) H2O+ H+ + OH• H2O- H• + OH- Combine w/ other free radicals to form molecules such as hydrogen peroxide (H2O2) → highly toxic to cell Oxygen enhances free radical damage via production of reactive oxygen species (e.g., H• + O2 → HO2•) Interaction of Radiation with Tissue c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed., p. 816. Linear Energy Transfer Biological effect dependent on the dose, dose rate, environmental conditions, radiosensitivity and the spatial distribution of energy deposition Linear Energy Transfer (LET) Amount of energy deposited per unit length (eV/cm) LET q2/KE Describes the energy deposition density which largely determines the biologic consequence of radiation exposure High LET radiation: α2+, p+, and other heavy ions Low LET radiation: Electrons (e-, β- and β+) EM radiation (x-rays or g-rays) High LET >> damaging than low LET radiation Relative Biological Effectiveness (RBE) Although all ionizing radiation capable of producing a specific biological effect, the magnitude/dose differs Compare dose required to produce the same specific biologic response as a reference radiation dose (typically 250 kVp x-rays): Relative Biological Effectiveness (RBE) Essential in establishing radiation weighting factors (wR) X-rays/gamma rays/electrons: LET ≈ 2 keV/μm; wR = 1 Protons (< 2MeV): LET ≈ 20 keV/μm; wR = 5-10 Neutrons (E dep.): LET ≈ 4-20 keV/μm; wR = 5-20 Alpha Particle: LET ≈ 40 keV/μm; wR = 20 H (equivalent dose, Sv) = D (absorbed dose, Gy) ∙ wR LET vs. RBE c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed., p. 817. Cellular Targets Radiation-sensitive targets are located in the nucleus and not the cytoplasm of the cell Cell death may occur if key macromolecules (e.g., DNA) which have no replacement are damaged or destroyed Considerable evidence that damage to DNA is the primary cause of radiation-induced cell death Concept of key or critical targets has led to a model of radiation-induced cellular damage termed target theory in which critical targets may be inactivated by one or more ionization events (hits) Radiation Effects on DNA c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed., p. 819. Cellular Radiosensitivity Studied through radiationinduced cell death (loss of reproductive integrity) Useful in assessing the relative biologic impact of various types of radiation and exposure conditions Cellular inability to form colonies as a function of radiation exposure → cell survival curves Three parameters defining response to radiation: n, Dq and D0 c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 822. Cell Survival Cures: n n: Extrapolation number found by extrapolating the linear portion of the curve back through the y-axis Represents either the number of targets in a cell that must be “hit” once by a radiation event to inactivate the cell or the number of “hits” required on a single critical target to inactivate the cell For mammalian cells: [2,10] c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 822. Cell Survival Curves: D0 D0: Mean lethal dose Radiosensitivity of the cell population under study Dose producing a 63% (1-e-1) reduction in viable cell number: slope = Δy/Δx = .63/D0 (e ≈ 2.72; e-1 = 0.37) reciprocal linear region slope Radioresistant cell D0 > radiosensitive cell D0 ↓ D0 → lesser survival/dose Mammalian cells: [1Gy,2Gy] c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 822. Cell Survival Curves: Dq Dq: Quasithreshold dose (Dq = D0 · logen) Width of the shoulder region and a measure of sublethal damage Irradiated cells remain viable until enough hits received to inactivate the critical target or targets Clear evidence that for lowLET radiation, damage produced by a single radiation interaction with cellular critical target(s) is insufficient to produce reproductive death c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 822. Factors Affecting Cellular Radiosensitivity Conditional factors - physical or chemicals factors that exist previous to and/or at irradiation Dose rate LET Fractionation Presence of oxygen Inherent factors - biologic factors characteristic of the cell Mitotic rate Degree of differentiation Cell cycle phase Conditional Factors-Dose Rate Which has highest D0? Which has highest n? Which has highest Dq? c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed., p. 823. Conditional Factors-LET c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed., p. 824. Conditional Factors-Fractionation c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed., p. 825. Conditional Factors-Presence of Oxygen Increases cell damage by inhibiting Free radical recombination to form harmless chemical species Repair of damage caused by free radicals Oxygen enhancement ratio (OER): ratio of dose producing a given biologic response in the absence of oxygen to that in the presence of oxygen Mammalian cells Low-LET: [2,3] High-LET: [1,2] Conditional Factors - Oxygen Conditional Factors- Oxygen c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed., p. Inherent Factors: Law of Bergonie & Tribondeau Radiosensitivity greatest for cells with High mitotic rate Long mitotic future Undifferentiated c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed., p. 826. Inherent Factors-Cell Cycle Phase Cells are most sensitive to radiation during mitosis (M phase) and RNA synthesis (G2 phase) Less sensitive during the preparatory period for DNA synthesis (G1 phase) Least sensitive during DNA synthesis (S phase) During mitosis (M), the metaphase is the most sensitive c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed., p. 827. Davis Notes-Radiation Biology 4. The quasi-threshold dose (Dq) for cell line C is: A. 500 B. 700 C. 1,000 D. 1,500 E. impossible to determine from this data Huda 2nd Edition-Chapter 10Radiation Biology 1. (A) (B) (C) (D) (E) Radiological LD50 is the radiation dose that kills: 50% of exposed cells 50 exposed cells All exposed cells within 50 days e-50 of exposed cells e/50 of exposed cells Huda 2nd Edition-Chapter 10Radiation Biology 10. Stochastic effects of radiation (A) Can be recognized as caused by radiation (B) Have a dose-dependent severity (C) Have a threshold of 50 mSv/year (D) Include carcinogenesis (E) Involve cell killing Huda 2nd Edition-Chapter 10Radiation Biology 5. (A) (B) (C) (E) The LET of x-rays is: Between 0.3 and 3 keV/μm Cannot be defined for energies greater than 2 MeV Greater than the LET for alpha particles Low energy threshold (D) Independent of relative biological effectiveness (RBE) Huda 2nd Edition-Chapter 10Radiation Biology 4. Which is not true of the interaction of ionizing radiation with tissues? (A) Cellular DNA is a key target (B) Direct action is more frequent than indirect action (D) Ions can dissociate into free radicals (E) It can produce chromosome aberrations (C) Indirect action causes most of the biological damage Huda 2nd Edition-Chapter 10Radiation Biology 3. Which cells are considered to be the least radiosensitive? (A) Bone marrow cells (B) Lymphoid tissues (C) Neuronal cells (D) Skin cells (E) Spermatids Huda 2nd Edition-Chapter 10Radiation Biology 2. (A) (B) (C) (D) (E) The cell division stage most sensitive to radiation is: Anaphase Interphase Metaphase Prophase Telophase Organ Systems Response: Regenerization and Repair c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed., p. 828. Organ Systems Response: Skin c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 830. Organ Systems Response: Reproductive System Gonads are very radiosensitive Females Temporary sterility: 1.5 Gy (150 rad) acute dose Permanent sterility: 6.0 Gy (600 rad) acute dose* *reported for doses as low as 3.2 Gy Males Temporary sterility: 2.5 Gy (250 rad) acute dose* *reported for doses as low as 1.5 Gy Permanent sterility: 5.0 Gy (500 rad) acute dose Reduced fertility 20-50 mGy/wk (2-5 rad/wk) when total dose > 2.5 Gy Organ Systems Response: Ocular Effects Eye lens contains a population of radiosensitive cells Cataract magnitude and probability of occurrence to the dose Acute doses Protracted exposure = 2 Gy (200 rad) cataracts in a small percentage of people exposed > 7 Gy (700 rad = 700 cGy) always produce cataracts 2 months: 4 Gy threshold 4 months: 5.5 Gy threshold Unlike senile cataracts that typically develop in the anterior pole of the lens radiation-induced cataracts begin with a small opacity in the posterior pole and migrate anteriorly Acute Radiation Syndrome Characteristic clinical response when whole body (or large part thereof) is subjected to a large acute external radiation exposure Organism response quite distinct from isolated local radiation injuries such as epilation or skin ulcerations Combination of subsyndromes occurring in stages over hours to weeks as the injury to various tissues and organs is expressed In order of their occurrence with increasing radiation dose: Hematopoietic syndrome Gastrointestinal syndrome Neurovascular syndrome ARS Sequence of Events Prodromal symptoms typically begin within 6 hours of exposure No symptoms during the latent period, which may last up to 6 weeks for dose < 1 Gy Manifest illness stage: onset of organ system damage clinical expression which can last 2-3 wks Most difficult to manage from a therapeutic standpoint Treatment during the first 6-8 wks essential to optimize recovery Higher risk of cancer and genetic abnormalities in future progeny if patient survives c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., pp. 832-3. Acute Radiation Syndrome Interrelationships c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 836. Epidemiologic Investigations of Radiation Induced Cancer Dose-response relationships for cancer induction at high dose and dose rate have been well established Not so for low dose exposures like those resulting from diagnostic and occupational exposures Very difficult to detect a small increase in the cancer rate due to radiation Natural incidence of many forms of cancer is high Latent period for most cancers is long To rule out simple statistical fluctuations, a very large irradiated population is required Difficulties in Quantifying Low Dose Risk If excess risk proportional to dose, then large studies are required for low absorbed dose to maintain statistical precision and power; the necessary sample power increases approximately as the inverse square of dose This relationship reflects a decline in the signal (radiation risk) to noise (natural background risk) ratio as dose decreases. SS = c/D2 500 persons needed to quantify the effect of a 1,000 mSv dose 50,000 for a 100 mSv dose 5 million for a 10 mSv dose (a single body CT = 7.5 mSv) National Research Council (1995) Radiation Dose Reconstruction for Epidemiologic Uses. Natl. Acad. Press What is the Evidence? Major epidemiological investigations that form the basis of current cancer dose-response estimates in human populations: Atomic-bomb survivors (Japan) life span study (LSS) Anklyosing spondylitis (UK) Postpartum mastitis study (New York) Radium dial painters (Tritium) Thorotrast (radioactive Thorium x-ray contrast agent) Massachusetts tuberculosis patients (multiple chest fluoroscopy) Stanford University Hodgkin’s disease patients (x-ray therapy) Risk Estimation Models Dose-Response Curves Dose-response models predict cancer risk from exposure to low levels of ionizing radiation → dose-response curves Linear, non-threshold (LNT) Effect = α∙Dose Linear-quadratic, nonthreshold Effect = α∙Dose + β∙Dose2 α/β: [1Gy-10Gy] appears linear for low dose appears quadratic (non-linear) for higher dose c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 844. Risk Estimation Models-Risk Models Multiplicative risk model: after a latent period, the excess risk is a multiple of the natural agespecific cancer risk for the population in question Additive risk model: fixed or constant increase in risk unrelated to the spontaneous age-specific cancer risk at the time of exposure Latency periods: Leukemia 10 yrs average Solid tumors 25 yrs average c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 845. Risk Estimation Models-Risk Expression Relative Risk Ratio of the cancer incidence in the exposed population to that in the general (unexposed) population RR of 1.2 would indicate 20% increase over the spontaneous rate Excess relative risk is simply RR - 1 Absolute Risk Expressed as the number of excess radiation-induced cancers per 104 people/Sv-yr For a cancer with a radiation-induced risk of 4 per 10,000 person/Sv-yr and a latency period of 20 years, the risk of developing cancer from a dose of 0.1 Sv (~13x body CT dose) within the next 40 years would be: (40-20) or 20 years x 0.1 Sv x 4 per 10,000 person/Sv-yr = 8 per 10,000 or 0.08% Radiation Standards Organizations Independent bodies of experts evaluate information on radiation health effects Experts draw upon this collective knowledge to develop recommendations for systems of radiation protection BIER - National Academy of Sciences/National Research Council Committee on the Biological Effects of Ionizing Radiation UNSCEAR - United Nations Scientific Committee on the Effects of Radiation RERF - Radiation Effects Research Foundation NCRP – National Council on Radiation Protection and Measurements ICRP – International Commission on Radiological Protection Radiation protection regulatory framework NRC – Nuclear Regulatory Commission EPA - Environmental Protection Agency BEIR V Risk Estimates BEIR published a report in 1990 entitled, “The Health Effects of Exposure to Low Levels of Ionizing Radiation” or the BEIR V report Single best estimate of radiation-induced mortality at low exposure levels is 4% per Sv (0.04% per rem) for a working population (ICRP - 5% per Sv for the whole population - takes children into account) The single best estimate of radiation-induced mortality at high doses applied at high dose rate is 8% per Sv (0.08% per rem) The BEIR V Committee believed that the LNT dose-response model was best for all cancers except leukemia and bone cancer; for those malignancies, a linear-quadratic model was recommended According to the LNT model, an exposure of 10,000 people to 10 mSv would result in approximately 4 cancer deaths in addition to the 2,200 (22%) normally expected in the general population ACRP 60 Risk Estimates c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed., p. 847. Specific Cancer Risk Estimates: Leukemia Natural incidence in US population: 1 in 104 (0.01%) 17% of total mortality from radiocarcinogenesis The incidence of leukemia greatly influenced by age at the time of exposure BEIR V: nonlinear doseresponse model predicting excess life-time risk of 10 in 104 (0.1%) after exposure to 0.1 Gy (10 rad) Average latent period = 10 yrs c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 849. Specific Cancer Risk Estimates: Thyroid Cancer 6-12% of total mortality from radiocarcinogenesis Females 3-5x greater risk than males Latency period Benign nodules: 5-35 yrs Thyroid malignancies: 10-35 yrs Dose-response curve: LNT Associated mortality rate: 5% However, other responses such as hypothyroidism and thyroiditis with thresholds: 2 Gy for external irradiation 50 Gy for internal radiation (radioactive materials like 131I) Specific Cancer Risk Estimates: Breast Cancer One of 8 US women at risk of developing breast cancer 180,000 new cases/yr 1 in 30 women die of breast cancer Low LET radiation risk age dependent, ≈ 50 times greater for the 15 yo age group (≈ 0.3% per year) after exposure of 0.1 Gy than those > 55 yo The risk estimates for women in the 25, 35 and 45 yo age groups are 0.05%, 0.04% to 0.02% respectively (BEIR V) Dose-response curve: LNT w/ dose of ≈ 0.8 Gy doubling the natural incidence Latent period [10yrs,40yrs]; longer latencies with younger women Increased Risk of Induced Breast Cancer Before 65 Years of Age per 25 mSv Breast Organ Dose for Age at Exposure Increased Risk/25 mSv 0.160% 0.140% 0.120% 0.100% 0.080% 0.060% 0.040% 0.020% 0.000% 10 20 30 40 50 Years of Age at Exposure 60 Comparisons of the Risks of Some Medical Exams Davis Notes- Radiation Biology 9. The overall fatal cancer risk per rad of whole body low LET radiation of a population selected at random would be on the order of: A. 104 B. 102 C. 10-4 D. 10-6 E. 106 Risk ≈ 1 cSv (1 rad) · 0.04/Sv = 0.0004 = 4x10-4 Genetic Effects in Humans Genetic effects the result of radiation exposure to the gonads Epidemiological investigations have failed to demonstrate radiation-induced genetic effects Current risk estimates are based on animal experiments For workers, the risk of severe hereditary effects is 0.8% per Sv of gonadal radiation according to the ICRP For a whole population, the risk of severe hereditary effects is 1.3% per Sv which is higher because of the inclusion of children Radiation Effects In Utero Gestational period divided into 3 stages: Relatively short preimplantation stage (day 0-9) Extended period of major organogenesis (day 9-56) Fetal growth stage (day 45 to term) Preimplantation: conceptus extremely sensitive and radiation damage can result in prenatal death: “All-ornothing response” Animal experiments have demonstrated an increase in the spontaneous abortion rate after doses as low as 50 to 100 mGy (5 to 10 rad) Critical Periods for RadiationInduced Birth Defects p r e i m p l a n t a t i o n major organogenesis c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2nd ed., p. 855. fetal growth Relative Incidence of Radiation-Induced Health Effects at Various Stages in Fetal Development c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed., p. 860. Radiation Effects In Utero (2) Exposures > 1 Gy associated with a high incidence of CNS abnormalities Growth retardation after in utero exposure ≥ 100 mGy demonstrated Fetal doses generally are much less than 100 mGy in most diagnostic and nuclear medicine procedures and thought to carry negligible risk compared with the spontaneous incidence of congenital abnormalities (4%6%) A conservative estimate of the excess risk of childhood cancer from in utero irradiation is ≈ 6% per Gy (0.06% per rad) Radiation Effects In Utero (3) Recommendations from Wagner* are: If radiation dose received during or prior to the first two weeks post conception (< 14 days) Exposure to diagnostic radiation is not an indication for therapeutic abortion For patients exposed to radiation between the 2nd and 8th weeks post-conception (days 14-56): Therapeutic abortion based solely on radiation exposure is not advised for dose less than 150 mGy (15 rad) Dose exceeding 150 mGy (15 rad) may be an indication for therapeutic abortion in the presence of less severely compromising factors. However, diagnostic studies rarely result in such dose levels. * Wagner, et al. Exposure of the Pregnant Patient to Diagnostic Radiation, pp. 166-7. Radiation Effects In Utero (4) For a conceptus exposed between the 8th and 15th week post-conception (days 56-105): Fetal dose below 50 mGy (5 rad) Fetal dose between 50-150 mGy (5-15 rad) Radiation not a sufficient risk to justify therapeutic abortion therapeutic abortion is not advisable on the basis of the radiation risk alone Fetal dose above 150 mGy (15 rad) In this time interval there is scientific evidence that may support a recommendation for therapeutic abortion based on the radiation exposure. However, this does not mean an abortion is necessarily recommended. Diagnostic studies rarely result in such dose levels. * Wagner, et al. Exposure of the Pregnant Patient to Diagnostic Radiation, pp. 166-7. Radiation Effects In Utero (5) Fetal dose at 150 mGy: Up to a 6% probability the child could be mentally retarded Probability the child will develop cancer < 3% Natural incidence = 1.4% Probability of small head size ≈ 15% (but does not necessarily affect normal mental function) Natural incidence = 0.4% Natural incidence = 4% IQ may fall a few points short of its full potential Except for possible effects to individual organs from radionuclide studies, no other risks have been demonstrated. However, always practice ALARA! * Wagner, et al. Exposure of the Pregnant Patient to Diagnostic Radiation, pp. 166-7. Effect of In Utero Risk Factors on Outcome c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed., p. 858. In Utero Irradiation Summary c.f. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed., p. 860. Huda 2nd Edition-Chapter 10Radiation Biology 15. (A) (B) (C) (D) (E) When is gross malformation most likely to occur? Early fetal period Early organogenesis Late fetal period Late organogenesis Preimplantation Huda 2nd Edition-Chapter 10Radiation Biology 16. What “threshold” embryo/fetal dose corresponds to a radiation risk smaller than those normally encountered during pregnancy? (A) Less than 10 mGy (1 rad) (B) 10 mGy (1 rad) (C) 30 mGy (3 rad) (D) 100 mGy (10 rad) (E) More than 100 mGy (10 rad) Davis Notes-Radiation Biology 6. A barium enema was performed on a 25 year-old female who was determined to be three weeks pregnant at the time of examination. As the consulting radiologist, you should: A. Recommend a therapeutic abortion. B. Counsel the patient that the embryo is at a significantly high risk for gross malformations as a result of the radiation exposure; however, an abortion is not necessarily warranted. C. Discuss the implications of the radiation exposure with the hospital’s legal department. D. Do not discuss any potential effects of the radiation exposure on the embryo because very little is known about in utero radiation exposure and your comment would be totally speculative and unsubstantiated. E. Explain to the referring physician and patient that the radiation received by the embryo by this diagnostic procedure is relatively small and that the increase in risk is negligible compared to the spontaneous incidence of congenital abnormalities.
