Radiation Biology

1
Radiation Biology

• Robert Metzger, Ph.D.

2
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, 2nd ed., p. 814.
3
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

4
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

5
Interaction of Radiation with Tissue

• Ionizing radiation energy deposited randomly and
  rapidly (lt 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

6
Interaction of Radiation with Tissue

• Vast majority of interactions are indirect (body
  70 - 85 water)
• Results in an unstable ion pair, H2O, H2O-
• Dissociate 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)

7
Interaction of Radiation with Tissue
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 816.
8
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 a2, p, and other heavy ions
  
• Low LET radiation
• Electrons (e-, ß- and ß)
• EM radiation (x-rays or g-rays)
• High LET gtgt damaging than low LET radiation

9
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 (lt 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

10
LET vs. RBE
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 817.
11
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)

12
Radiation Effects on DNA
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 819.
13
Cellular Radiosensitivity

• Studied through radiation-induced 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.
14
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.
15
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 ?y/?x .63/D0
  (e 2.72 e-1 0.37)
• ? reciprocal linear region slope
• Radioresistant cell D0 gt 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.
16
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 low-LET 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.
17
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

18
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, 2nd ed., p. 823.
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Conditional Factors-LET
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 824.
20
Conditional Factors-Fractionation
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 825.
21
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

22
Conditional Factors- Oxygen
Conditional Factors - Oxygen
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p.
23
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, 2nd ed., p. 826.
24
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, 2nd ed., p. 827.
25
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

26
Huda 2nd Edition-Chapter 10-Radiation Biology

• 1. Radiological LD50 is the radiation dose that
  kills
• (A) 50 of exposed cells
• (B) 50 exposed cells
• (C) All exposed cells within 50 days
• (D) e-50 of exposed cells
• (E) e/50 of exposed cells

27
Huda 2nd Edition-Chapter 10-Radiation 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

28
Huda 2nd Edition-Chapter 10-Radiation Biology

• 5. The LET of x-rays is
• (A) Between 0.3 and 3 keV/µm
• (B) Cannot be defined for energies greater than 2
  MeV
• (C) Greater than the LET for alpha particles
• (E) Low energy threshold

• (D) Independent of relative biological
  effectiveness (RBE)

29
Huda 2nd Edition-Chapter 10-Radiation 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

30
Huda 2nd Edition-Chapter 10-Radiation 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

31
Huda 2nd Edition-Chapter 10-Radiation Biology

• 2. The cell division stage most sensitive to
  radiation is
• (A) Anaphase
• (B) Interphase
• (C) Metaphase
• (D) Prophase
• (E) Telophase

32
Organ Systems Response Regenerization and Repair
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 828.
33
Organ Systems Response Skin
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 830.
34
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 gt 2.5 Gy

35
Organ Systems Response Ocular Effects

• Eye lens contains a population of radiosensitive
  cells
• Cataract magnitude and probability of occurrence
  ? to the dose
• Acute doses
• 2 Gy (200 rad) cataracts in a small percentage
  of people exposed
• gt 7 Gy (700 rad 700 cGy) always produce
  cataracts
• Protracted exposure
• 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

36
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

37
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 lt 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.
38
Acute Radiation Syndrome Interrelationships
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 836.
39
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

40
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.
• 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)

SS c/D2
National Research Council (1995) Radiation Dose
Reconstruction for Epidemiologic Uses. Natl.
Acad. Press
41
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 Hodgkins disease patients
  (x-ray therapy)

42
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 aDose
• Linear-quadratic, non-threshold
• Effect aDose ßDose2
• a/ß 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.
43
Risk Estimation Models-Risk Models

• Multiplicative risk model after a latent period,
  the excess risk is a multiple of the natural
  age-specific 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.
44
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

45
Radiation Standards Organizations

• Independent bodies of experts evaluate
  information on radiation health effects
• 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
• Experts draw upon this collective knowledge to
  develop recommendations for systems of radiation
  protection
• 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

46
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

47
ACRP 60 Risk Estimates
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 847.
48
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 dose-response 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.
49
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)

50
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 gt 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

51
Increased Risk of Induced Breast Cancer Before 65
Years of Age per 25 mSv Breast Organ Dose for Age
at Exposure
52
Comparisons of the Risks of Some Medical Exams
53
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

54
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

55
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-or-nothing 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)

56
Critical Periods for Radiation-Induced Birth
Defects
pre-implantation
major organogenesis
fetal growth
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 855.
57
Relative Incidence of Radiation-Induced Health
Effects at Various Stages in Fetal Development
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 860.
58
Radiation Effects In Utero (2)

• Exposures gt 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)

59
Radiation Effects In Utero (3)

• Recommendations from Wagner are
• If radiation dose received during or prior to the
  first two weeks post conception (lt 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.
60
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)
• Radiation not a sufficient risk to justify
  therapeutic abortion
• Fetal dose between 50-150 mGy (5-15 rad)
• 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.
61
Radiation Effects In Utero (5)

• Fetal dose at 150 mGy
• Up to a 6 probability the child could be
  mentally retarded
• Natural incidence 0.4
• Probability the child will develop cancer lt 3
• Natural incidence 1.4
• Probability of small head size 15 (but does
  not necessarily affect normal mental function)
• 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.
62
Effect of In Utero Risk Factors on Outcome
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 858.
63
In Utero Irradiation Summary
c.f. Bushberg, et al. The Essential Physics of
Medical Imaging, 2nd ed., p. 860.
64
Huda 2nd Edition-Chapter 10-Radiation Biology

• 15. When is gross malformation most likely to
  occur?
• (A) Early fetal period
• (B) Early organogenesis
• (C) Late fetal period
• (D) Late organogenesis
• (E) Preimplantation

65
Huda 2nd Edition-Chapter 10-Radiation 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)

66
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 hospitals 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.