Doses and Biological Effects

Basis of biological damage - direct and indirect mechanisms

The main cause of damage in cells is ionisation of critical molecules such as DNA. This may be the result of direct interaction of the secondary electrons with the molecular bonds within the DNA strands or it may be via the (indirect) mechanism of damage by free radical formation. More than half of the damage occurs due to free radical interaction rather than direct radiation interaction.

In this latter case the secondary electrons are able to breakdown a molecule into free radicals. For example, action of radiation on water:

H₂0 H₂0⁺   +   e^-    H⁺   +   OH.  +   e^-

e^-  +   H₂0    H₂0^-  H. +   OH^-

H . and the OH . are free radicals.

Free radicals are very reactive and are known to cause damage to biological systems. This damaging effect can be enhanced if there is a surplus of oxygen present. This is because oxygen increases the yield of the very toxic and more damaging ‘hydroperoxy free radical’, (HO₂. ).  This enhancement is called the ‘oxygen effect’.

 

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Classification of Biological Effects

Damage may be:

• to the irradiated individual (somatic) or
• to the offspring (genetic).

Also there are two types of radiation effects:

• stochastic -where there is assumed to be no threshold and the probability of the effect (such as cancer) occurring increases with dose and deterministic (such as erethema, sterility or cataracts)
• non-stochastic - where there is a known threshold and the severity of the effect increases with increasing dose. 

 

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Effects at Cell Level

When a cell is damaged by radiation there are three possible outcomes:

1. Cell repairs itself - no harm done
2. Cell loses ability to replicate (cell death) – no harm if number of cells damaged is not excessive
3. Cell has modified DNA and replication process carries this forward – can be the start of cancer induction if the cell is not recognised and destroyed by the body’s defence mechanism.

The more rapidly dividing the cell is the greater the chance of cell repair not being effective and the more the cell will be affected.

 

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Effects at Tissue Level

• Ionising radiation affects balance between cell death and replacement of tissue cells in body
• Loss of substantial number of healthy cells may lead to failure of organ function / tissue death
• Threshold level exists below which cell replacement implies that probability of harm is low
• Uncontrolled proliferation of mutated tissue cells are termed malignant

 

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