Biological Effects of Ionizing Radiation Prof Hamby Objectives

Biological Effects of Ionizing Radiation Prof. Hamby

Objectives n n n 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

Ionizing Radiation n n 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

Energy Deposition n 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)

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

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

Acute Radiation Syndrome n Sub-clinical u 25 n - 200 rads; no symptoms, but signs Hematopoietic u 200 n Gastrointestinal u 600 n - 600 rads; changes in blood - 1000 rads; intestinal lining failure Cerebral u> 1000 rads; nervous system failure LD 50/30 ~ 400 rads

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

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

Terms n n 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

Terms n n 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

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

Dosimetric Quantities n n Erythema; Photographic fog Exposure (1 R = 1 SC/cm 3) u Defined for photons in air u SI definition: 1 X unit = 1 C/kg n Absorbed Dose, D (1 rad = 100 ergs/gm) u Defined for all radiations/all media u SI definition: 1 Gy = 1 J/kg = 100 rads u 1 rad (tissue) ~ 1 R (air)

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

Relative Biological Effectiveness n n 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 k. Vp x-rays or 60 Co gamma rays

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

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

Dosimetric Quantities n n 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

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

Dosimetric Quantities n n n n Internal Dose External Dose Committed Dose Cumulative Dose Population Dose EDE CEDE TEDE

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

Non-Stochastic (Deterministic) Effects Occurs above threshold dose n Severity increases with dose n FAlopecia (hair loss) FCataracts FErythema (skin reddening) FRadiation Sickness FTemporary Sterility

Stochastic (Probabilistic) Effects Occurs by chance n Probability increases with dose n FCarcinogenesi s FMutagenesis FTeratogenesis