Lecture 4-Biological Effects of Ionizing Radiation

1. Biological Effects of
Ionizing Radiation
Prof. Hamby

2. Objectives
Describe how ionizing radiation
interacts with biological material
Discuss the major factors that influence
the severity or type of biological effect
Define terms describing biological
effect
Define radiation dose quantities
Describe meaning of “dose-response”
Define stochastic and non-stochastic
processes

3. Ionizing Radiation
Radiation having adequate
energy to ionize atoms, dissociate
molecules, or alter nuclear
structures
Particles, alpha, beta, electrons,
neutrons, protons
Electromagnetic waves, x-rays,
gamma rays
Direct or indirect ionization of
atoms

4. Energy Deposition
Radiation interacts by either
ionizing or exciting the atoms or
molecules in the body (water)
Energy is deposited and absorbed
as a result of these interactions
Absorbed Dose is defined as the
energy absorbed per unit mass of
material (tissue in this case)

5. Biological Damage
Damage can occur at various
biological levels
Sub-cellular
Cellular (cell death)
Organ (disfunction)
Organism (cancer, death)

6. Cellular Radiosensitivity
Cells that divide more rapidly
are more sensitive to the effects
of radiation ...
… essentially because the resulting
effect is seen more rapidly.

7. Acute Radiation
Syndrome
Sub-clinical
25 - 200 rads; no symptoms, but signs
Hematopoietic
200 - 600 rads; changes in blood
Gastrointestinal
600 - 1000 rads; intestinal lining failure
Cerebral
> 1000 rads; nervous system failure
LD50/30 ~ 400 rads

8. Factors Influencing
Biological Effect
Total absorbed energy (dose)
Dose rate
Acute (seconds, minutes)
Chronic (days, years)
Type of radiation
Source of radiation
External
Internal
Age at exposure

9. Factors Influencing
Biological Effect
Time since exposure
Area or location being irradiated
Localized (cells, organ)
Extremities (hands, forearms, feet,
lower legs)
Entire body (trunk including head)
Superficial dose (skin only shallow)
Deep tissue (“deep dose”)

10. Terms
Acute exposure - dose received in a
short time (seconds, minutes)
Acute effects - symptoms occur
shortly after exposure
Chronic exposure - dose received
over longer time periods (hrs, days)
Delayed effects - symptoms occur
after a latent (dormant) period

11. Terms
Somatic effects - those which
occur in the person exposed
Genetic effects - those which occur
in the offspring of exposed persons
Stochastic effects - likelihood of
effect is random, but increases
with increasing dose
Non-stochastic effects - likelihood
of effect is based solely on dose
exceeding some threshold

12. Radiation Dosimetry
radiation
interaction
energy
deposition
biological
response
Radiation dose quantifies energy
deposition
Dose categories:
local; whole body; extremity
shallow; deep
internal; external

13. Dosimetric Quantities
Erythema; Photographic fog
Exposure (1 R = 1 SC/cm3)
Defined for photons in air
SI definition: 1 X unit = 1 C/kg
Absorbed Dose, D (1 rad = 100
ergs/gm)
Defined for all radiations/all media
SI definition: 1 Gy = 1 J/kg = 100 rads
1 rad (tissue) ~ 1 R (air)

14. Radiation Quality
Not all radiations are created equal
What is the “quality” of radiation?
Linear Energy Transfer (LET)
Energy absorbed per unit length
(keV/µm)
Essentially a measure of
“ionization density”

15. Relative Biological
Effectiveness
RBE is an empirically determined
measure of radiation quality
Expresses the different absorbed
dose required by two radiations in
order to cause the same endpoint
Biological endpoint is undefined
Standard radiations are either 250
kVp x-rays or 60Co gamma rays

16. Radiation Quality
The ionization density is
different among radiation types.
X-ray
-- not many ionizations
Alpha particle -- very high density
Beta particle -- high density at end

17. Dosimetric Quantities
Dose Equivalent, H (rem)
Used to “normalize” over different
radiation types
Quality factor, QF, describes
ionization density (wR)
QF related to both LET and RBE
H = D • QF
SI definition: 1 Sv = 100 rem

18. Dosimetric Quantities
Fatal cancer is the biological
endpoint of importance
Estimates have been made of
organ-specific risks of cancer
fatality
Some cancers can be treated
successfully
Therefore, need to consider
individual organ risks

19. Dosimetric Quantities
Effective Dose Equivalent, E (rem)
Used to “normalize” over different
organ radio-sensitivities
Tissue weighting factor, wT,
describes relative cancer risk
E = Σ (H • wT)
SI definition: still, 1 Sv = 100 rem
Unit of record

20. Dosimetric Quantities
Internal Dose
External Dose
Committed Dose
Cumulative Dose
Population Dose
EDE
CEDE
TEDE

21. Dose-Response
“Dose-Response
Curves”
Response
(Cancer
Fatality)
Dose

22. Non-Stochastic
(Deterministic) Effects
Occurs above threshold dose
Severity increases with dose
Alopecia (hair loss)
Cataracts
Erythema (skin reddening)
Radiation Sickness
Temporary Sterility

23. Stochastic (Probabilistic)
Effects
Occurs by chance
Probability increases with dose
Carcinogenesis
Mutagenesis
Teratogenesis