BIOLOGICAL EFFECTS OF IONIZING RADIATION

ContentsContents

• Introduction• Radiation Injury Mechanism• Factors That May Modify Radiation

Effects• Biological Effects• Risk Estimates• Summary

IntroductionIntroduction

• The final concern of radiation hazard is its effect on living things.

• The biological effects of ionizing radiation begin with the transfer of radiation energy at the molecular level and culminates with possible damage to the individual.

• At different levels the cells or organisms different types of changes are expressed.

• Damage cells may eventually cause cell death or induce the abnormal cell.

Levels of Radiation EffectsLevels of Radiation Effects

Molecular Level

Cellular Level

Organ Level

Whole Body Level

Radiation Injury MechanismRadiation Injury Mechanism

• Radiation effects begins at the physico-chemical level in cells.

• Interaction of radiation with water is the main basis for biological effects.

• Two type of radiation interactions: Direct and Indirect interactions

• DNA is the primary radiation target.

• Depending on the dose received by the cells, radiation can alter cell’s characteristics and functions, and may cause cell death.

Biological Factors

Types of cells

Activeness in cell division

Stages in the cell cycle

Factors affecting CELLULAR responses

Physical Factors

Type of radiation (LET, RBE), dose, dose rates, dose fractionation

Environmental Factors

Radiation Sensitizers and protectors, OER

Factors That May Modify Radiation Effects

Factors That May Modify Radiation Effects

Factors That May Modify Radiation Effects (cont.)

Factors That May Modify Radiation Effects (cont.)

• Other factors affecting at whole organism levels:

Types of radiation. Total dose and dose rate. Parts of the body of total body exposure. Different organs of the body. Location of the radiation source relative to target

organ. Non-uniform distribution of radionuclide inside the

body.

Biological EffectsBiological Effects

• Radiological sensitivities of cells: Types of cells Divisional activities

• Genetic and somatic damage (define).

• Deterministic and stochastic effects.

Biological Effects (cont.)Biological Effects (cont.)

Deterministic Effect

Stochastic Effect

BIOLOGICALDominant

Recessive

SOMATIC

GENETIC

Biological Effects (cont.)Biological Effects (cont.)

SO

MA

TIC

GENETIC

BIO

LO

GIC

AL

Hereditary effect (Stochastic effect)Hereditary effect (Stochastic effect)

Sto

chas

tic

Eff

ect

Carcinogenic effect (Stochastic effect)

Carcinogenic effect (Stochastic effect)

Det

erm

inis

tic

Eff

ect

Partial exposure - Hematopoietic- Intestinal- Skin- LungWhole body exposure - BM, GI and CNS syndrome

Biological Effects (cont.)Biological Effects (cont.)

BIOLOGICAL

SOMATIC

GENETIC

Dominant

Recessive

Effects on sperms and eggs

(Deterministic effects)

Effects on sperms and eggs

(Deterministic effects)

Deterministic vs Stochastic Effects Deterministic vs Stochastic Effects

• Effects of radiation on biological system is generally divided into TWO, ie somatic and genetic.

• Both types of effects is NOT favored in radiation protection. In radiation protection a quantitative assessment of effects or risks is preferred.

• In 1990, ICRP # 60 proposed Deterministic and Stochastic Effects.

Deterministic vs Stochastic Effects (cont.)

Deterministic vs Stochastic Effects (cont.)

• Deterministic effects: There exist threshold levels for deterministic effects. Severity of its effect is proportional to the dose

received.

• Stochastic effects: Occurrence of effect is probabilistic in nature.

Example: cancer. Probability of its occurrence is proportional to the

dose received. Threshold level of radiation dose does not exist for

stochastic effect.

Stochastic EffectCarcinogenesis

Stochastic EffectCarcinogenesis

• Carcinogenesis is an example of somatic stochastic effect.

• Carcinogenesis may: Occur after low dose exposure; and The probability of malignancy increases with increase in dose.

• In man there is a latent period between initiation and expression of effects. Latent period is about 8 years for induced leukemia and 2-3 X longer for solid tumor (eg lung and breast cancers).

• Stochastic effect is influenced by LET and dose rate.

• Occurs in germinal cells of the gonads.

• Radiation induces changes in genetic information (through mutation or chromosomal aberrations) and subsequently expressed in the next generation.

• Stochastic hereditary effect may be divided into different categories based on severity of genetic effects. Dominant mutation: fatality may be expressed in the first

generation. Recessive mutation produces less effect to the first generation

but more towards increasing the content of genetic damage in the general genetic pool.

Stochastic EffectHereditary

Stochastic EffectHereditary

Acute Radiation Syndrome (ARS)Acute Radiation Syndrome (ARS)

• Caused by acute high dose exposure (> 0.5 Sv) to the whole body.

• Usually due to external exposure.

• ARS is divided into three types:Bone Marrow Syndrome;Gastrointestinal Syndrome; and Central Nervous System Syndrome.

Four stages in ARSFour stages in ARS

• Prodromal Syndrome: Classic symptoms include nausea, vomiting,

possibly diarrhoea occurring within minutes - days following exposure. Symptoms last from minutes - several days.

• Latent Stage: Where patient looks and feels healthy for few

hours - weeks.

Four stages in ARS (cont.)Four stages in ARS (cont.)

• Manifest Illness Stage: Depends on the specific radiation syndrome (BMS,

GIS, CNSS). Illness last between hours – months.

• Recovery or Death Stage: Patient either recovers or dies. In case of CNSS there is no recovery and death

within days of exposure. Recovery may occur for BMS although the LD30/60 is about 2.5 - 5 Gy.

Acute Radiation SyndromeAcute Radiation Syndromefollowing acute exposure to low LET uniform whole body radiation of following acute exposure to low LET uniform whole body radiation of

human being (ICRP #60)human being (ICRP #60)

Whole body absorbed dose

(Gy)

Radiation syndrome Time of death after

exposure (days)

3-5

5-15

>15

Bone Marrow Syndrome

Gastrointestinal Syndrome and lungs

Central Nervous System Syndrome

30-60

10 – 20

1-5

Bone Marrow Syndrome (BMS)Bone Marrow Syndrome (BMS)

• Following acute exposure to dose 3 – 5 Gy.

• Mild symptom may occur as low as 0.3 Gy.

• Syndrome is caused by damages to the blood system and stem cell.

• Symptoms: nausea, vomiting and paleness.

• Changes blood picture.

Gastrointestinal Syndrome (GIS)Gastrointestinal Syndrome (GIS)

• Full syndrome usually occurs with a dose of between 5 -15 Gy.

• Syndrome causes damages to the gastro-intestinal system.

• Prognosis 100%.

Central Nervous System Syndrome (CNSS)

Central Nervous System Syndrome (CNSS)

• Syndrome occur with dose > 15 Gy.

• Death due to damages to central nervous system.

• Victims suffer motor incoordination, convulsion, coma and death.

• Prognosis is 100% death within 1 - 5 days.

Risk EstimatesRisk Estimates

Cancer Type Probability of Fatal Cancer, 10-4 Sv-1

Bone Marrow 50 Bone surface 5

Lung 85Thyroid 8Breast 20Colon 85

TOTAL cont. …

Estimates of probabilities of occurrence for various types of cancer due to exposure to radiation

ICRP gave the value of 0.6 x 10-2 Sv-1 year-1 for the risk of genetic effects due to exposure to radiation

Cancer Type Probability of Fatal Cancer, 10-4 Sv-1

Stomach 110Bladder 30Oesophagus 30Liver 15Skin 2Gonad 10Others 50

TOTAL 500 x 10-4 Sv-1

Risk Estimates (cont.)Risk Estimates (cont.)

Thank YouThank Youfor your attention