The tissue weighting factor (WT) is a measure of relative contribution of an organ or tissue to the total health detriment due to stochastic effects resulting from a uniform irradiation of the entire body 1. It accounts for the variable sensitivity to ionising radiation and size of a given organ or tissue.
In 2007, the International Commission on Radiological Protection (ICRP) published a set of tissue weighting factors 1 as below:
WT = 0.12 (each of following six): stomach, colon, lung, bone marrow (red), breast, and remainder tissues*
WT = 0.08: gonads
WT = 0.04 (each of following four): urinary bladder, oesophagus, liver, thyroid
WT = 0.01 (each of following four): bone surface, skin, brain, salivary glands
*Remainder tissues (13 organs collectively): adrenals, extrathoracic region, gallbladder, heart, kidney, lymph nodes, muscle, oral mucosa, pancreas, small intestine, spleen, thymus, and uterus/cervix (♀) or prostate (♂).
Per definition, the sum of tissue weighting factors for all organs and tissues in the human body equals one: (0.12 x 6) + 0.08 + (0.04 x 4) + (0.01 x 4) = 1.
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