Biological effect of Radiation : Effective dose

3- Effective dose

We can deal with all these complexi­ties by taking the equivalent dose in each of the major tissues and organs of the body and multiplying it by a weighting factor related to the risk associated with that tissue or organ.

The Sum of these weighted equivalent doses is a quantity called the effec­tive dose: it allows us to represent the various dose equivalents in the body as a single number. The effec­tive dose also takes account of the energy and type of radiation, and therefore gives a broad indication of the detriment to health. Moreover, it applies equally to external and internal exposure and to uniform or non-uniform irradiation.

Tissue or organ	Tissue weighting factor
Gonads	0.20
Bone marrow (red)	0.12
Colon	0.12
Lung	0.12
Stomach	0.12
Bladder	0.05
Breast	0.05
Liver	0.05
Oesophagus	0.05
Thyroid	0.05
Skin	0.01
Bone surface	0.01
Remainder	0.05
Whole body total	1.00

Table (1) : Tissue weighting factor ( UNSCEAR report 2000 )

As an example, consider a circumstance in which a radionuclide causes exposure of the lung, the liver, and the surfaces of the bones.

Suppose that the equivalent doses to the tissues are, respectively,100, 70, and 300 mSv.

The effective dose is calculated as (100 x 0.12) + (70 x 0.05) + (300 x 0.01) = 18,5 mSv

The calculation shows that the risk of harmful effects from this particular pattern of radia­tion exposure will be the same as the risk, from 18.5 mSv received uniformly throughout the whole body.