1. Module 4 Biological Effects of Ionizing Radiation

2. Terminology Acute vs. Chronic Exposure Direct vs. Indirect Action
Prompt vs. Delayed Effects
Stochastic vs. Non-stochastic Effects
An understanding of what Health Physicists mean when we use a particular term will help in discussing radiation and its effects in this class and as you read about and discuss these issues in the future. A.First lets look at the differences between acute and chronic exposure.1.An acute exposure means an exposure that is received over a short time, perhaps as long as a few days.2.A chronic exposure is one that is received continuously (or repeatedly) over a long period of time. Most occupational exposures fall into this category.B.Next consider prompt vs. delayed effects.1.Prompt effects are those that appear within a few days of exposure.2.Delayed effects are those that appear several years after the exposure.3.We’ll look at some examples of both cases later.C.We’ll discuss Direct vs. Indirect action and Stochastic and Non-stochastic effects in detail.
Module #4

3. Chain of Events for Direct Action
Incident particle or photon
Excitation or Ionization
Dissociation of a molecule due to the excitation or ionization on one of the molecules atoms
Biological Effects possible depending on the molecule dissociated
Module #4

4. Direct Action
A break in the DNA molecule due to direct action of the radiation or an electron that is freed from an atom
Module #4

5. Chain of Events for Indirect Action
1. Incident Particle or Photon
Ionization of Water Molecule
Dissociation of Water Molecule
Free Radicals Produced
Chemical Changes
Possible Biological Effects
Module #4

6. Indirect Action H2O  H2O+ + e- H2O+  H+ + OH H2O + e -  H2O-
Radiation ionizes a water molecule
The positive ion dissociates
The electron attaches to a neutral water molecule
The negative ion then dissociates
Module #4

7. Indirect Action (cont.)
H + H H2
OH + OH  H2O2
H + O2 HO2
HO2 + H  H2O2
The H radicals combine to form hydrogen gas
Hydrogen peroxide is produce through two mechanisms
Module #4

8. Stochastic (Random) Effects
Occur by chance
Occur in both exposed and unexposed individuals
Are not unequivocally related to the radiation exposure
Become more likely as dose increases
Severity is independent of the dose
Module #4

9. Linear No Threshold Model
Assumes that any amount of radiation has a detrimental effect
Is not a predictive model
Is used to establish regulatory dose limits (NRC) 1
Probability of Occurrence
0.5
Dose
Module #4

10. Examples
Cancer
Mental Retardation
Genetic Effects
Module #4

11. Cancer
Radiation induced tumors most frequent in the hemopoietic system, thyroid, and skin.
Cancer induction is well documented at doses of 100 rad or more
Induction at lower doses is inconclusive (possible exceptions are leukemia and thyroid cancer)
Tumor induction has a latent time of 5-20 years
Module #4

12. Cancer (cont.)
Radiation induced leukemia in Atomic bomb survivors has been documented at doses above 40 rad
Bone Cancer induction has been documented in laboratory animals for large injection of “bone seeking” radionuclide
Radiation induced lung cancer is seen mainly in underground miners exposed to high Radon concentrations
Module #4

13. Mental Retardation
Most pronounced in those exposed between the 8th and 17th week of pregnancy
Brain cells divide rapidly during this period
Has been observed in children exposed in-utero to radiation from the atomic bombs in Japan
Module #4

14. Genetic Effects
No radiation induced genetic effects have been observed in humans
Genetic effects have been observed in animal studies
Module #4

15. Non-stochastic (Deterministic) Effects
A certain minimum dose must be exceeded before the effect occurs
The severity of the effect increases as dose increase
There is a clear causal relationship between exposure and occurrence
Module #4

16. Nonlinear Threshold Response
No response is seen until the threshold dose is exceeded
At some dose all individual experience the effect
Applies to non-stochastic effects
100
Present Response 50
Dose
Module #4

17. Non-stochastic Effects
Sterility
Cataracts
Skin Erythema
Hemopoietic Syndrome
Gastrointestinal (GI) Syndrome
Central Nervous System Syndrome
Module #4

18. Sterility Temporary sterility had been observed
In men at doses as low as 30 rads
In women at doses as low as 300 rads
The higher the dose the longer the period of sterility
Module #4

19. Cataracts
Cataracts
Threshold eye dose of about 200 rads of beta or gamma radiation
Threshold may be as low as 60 rads for neutron radiation
Long latent period
Module #4

20. Erythema & Other Skin Effects
Reddening of the skin (erythema) occurs at photon or beta doses of about 300 rads
Higher does may cause epilation, blistering, necrosis, and ulceration
Module #4

21. Hemopoietic Syndrome
Blood changes may be seen at doses as low as 14 rads
Blood changes almost certain at doses above 50 rads
Module #4

22. Hemopoietic Syndrome Hemopoietic Syndrome appears at about 200 rads
Characterized by depression or ablation of the bone marrow
May be accompanied by nausea, vomiting, fatigue, and increased temperature
Death occurs within 1-2 months unless medical treatment is successful
Module #4

23. Gastrointestinal Syndrome
Occurs at a whole body dose of 1000 rads or greater
Characterized by the destruction of the intestinal epithelium and complete destruction of the bone marrow
Accompanied by severe nausea, vomiting, and diarrhea soon after exposure
Death occurs within a few weeks
Module #4

24. Central Nervous System Syndrome
Occurs at whole body doses of 2000 rads or more
Damages the central nervous system as well as all other organs and systems
Unconsciousness occurs within minutes
Death follows in a matter of a few hours to a few days
Module #4

25. Summary
Biological effects of concern in the occupational setting do not appear until several years after radiation exposure if they appear at all
The probability of these effects increases with dose
In any individual case it can never be determine with 100% confidence that radiation was the cause
Module #4