Biological Effects of Ionizing Radiation

Prof. Hamby

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

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

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)

Biological Damage

Damage can occur at various biological levels Sub-cellular Cellular (cell death) Organ (disfunction) 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 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

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

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”)

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

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

Radiation Dosimetry

Radiation dose quantifies energy deposition

Dose categories: local; whole body; extremity shallow; deep internal; external

radiationinteraction

energydeposition

biologicalresponse

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)

Radiation Quality

Not all radiations are created equal What is the “quality” of radiation? Linear Energy Transfer (LET) Energy absorbed per unit length

(keV/m) Essentially a measure of

“ionization density”

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

Radiation Quality

The ionization density is different among radiation types.

X-ray -- not many ionizationsAlpha particle -- very high densityBeta particle -- high density at end

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

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

Dosimetric Quantities

Effective Dose Equivalent, E (rem) Used to “normalize” over different

organ radio-sensitivities Tissue weighting factor, wT,

describes relative cancer risk

E = (H • wT) SI definition: still, 1 Sv = 100 rem Unit of record

Dosimetric Quantities

Internal Dose External Dose Committed Dose Cumulative Dose Population Dose EDE CEDE TEDE

Dose-Response

Dose

Response(CancerFatality)

“Dose-ResponseCurves”

Non-Stochastic (Deterministic) Effects

Occurs above threshold dose Severity increases with dose

Alopecia (hair loss)CataractsErythema (skin reddening)Radiation SicknessTemporary Sterility

Stochastic (Probabilistic) Effects

Occurs by chance Probability increases with dose

CarcinogenesisMutagenesisTeratogenesis