Understanding 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 interaction energy deposition biological response Radiation Dosimetry • 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/mm) • 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 = S (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