3. Effect of ionizing radiation on the living organism, basic dose units.

Non-ionizing radiation

EM spectrum radio wave–visible light range + sound ultrasound mechanical radiations

Ionizing radiations

EM spectrum ultraviolet–gamma radiation range alpha, beta and neutron particle radiations

Direct effect of radiation

the biological molecule is directly hit and inactivated by the radiation • it is the only mechanism taking place when irradiating dried samples • its probability is much smaller than that of hitting a solvent molecule when irradiating solutions

Indirect effect of radiation

In dilute aqueous solutions the probability that the radiation hits a water molecule is much larger than the probability of hitting a target (e.g. enzyme molecule). • Radiation leads to the generation of free radicals from water which reach and inactivate the target

Dose-effect curve

the fraction of surviving individuals (objects) as a function of dose.

target theory

generation of radiation damage is stochastic • the interaction between the radiation and the biological object is not described molecularly • but it appropriately described the dose-effect curves of molecules

molecular theory

• generation of radiation damage is stochastic • radiation damage is described molecularly; the major determinant of radiation damage is DNA double strand break • it is appropriate to describe radiation damage of cells

Factors influencing radiation sensitivity

A. Quality of radiation

1. Ionization density (LET) 2. Penetrability

B. Biological variation

1. Cell cycle 2. Differentiation

C. Time factor

1. Fractionation, the role of DNA repair

D. Metabolism and temperature

Quality and penetrability of radiation

the extent of radiation damage depends on the ionisation density (LET). penetrability: alpha and beta radiation do not penetrate the skin systemic effects can only be induced by penetration into the body

Biological variability

. cells display different radiation sensitivity in different parts of the cell cycle (m phase cells are most sensitive)

the less differentiated the cells are, the higher their radiation sensitivity

Time factor

If a certain dose is given in fractions, a part of the radiation damage can be repaired between fractions  the extent of radiation damage is reduced.

Repair: primarily DNA repair, repair of double strand breaks

Metabolism and temperature

Cells with a higher metabolic rate usually have higher radiation sensitivity.

Since the rate of metabolism increases with temperature, a temperature increase usually leads to higher radiation sensitivity

The effect of oxygen

in the presence of O2 the amount of radiation-generated radicals increases  higher radiation sensitivity