The tissue weighting factor (WT) is a relative measure of the risk of stochastic effects that might result from irradiation of that specific tissue. It accounts for the variable radiosensitivities of organs and tissues in the body to ionizing radiation.
To calculate the effective dose, the individual organ equivalent dose values are multiplied by the respective tissue weighting factor and the products added. The sum of the weighting factors is 1.
In 2007, the International Commission on Radiological Protection (ICRP) published a set of tissue weighting factors 2 as below:
WT = 0.12 (for each of 6): stomach, colon, lung, bone marrow (red), breast & remainder tissues*
WT = 0.08: gonads
WT = 0.04: urinary bladder, esophagus, liver, thyroid
WT = 0.01: bone surface, skin, brain, salivary glands
*Remaining tissues, collectively (13 organs): adrenals, extrathoracic region, gallbladder, heart, kidney, lymph nodes, muscle, oral mucosa, pancreas, small intestine, spleen, thymus, and uterus/cervix (♀), prostate(♂).
Calculating sum of all weighted tissue factors: (0.12 x 6) + 0.08 + (0.04 x 4) + (0.01 x 4) = 1.0
The numerical values given above are valid for legal EU regulations for calculating equivalent dose in an organ or tissue.
Note: Changes in respective WT from last ICRP publication-60 of 19913 to ⟶ ICRP publication-103 of 2007 are:
gonads: 0.20 ⟶ 0.08
breast and reminder tissue: 0.05 ⟶ 0.12
bladder, esophagus, liver, thyroid: 0.05 ⟶ 0.04
addition of brain and salivary glands to 0.01 category
The tissue weighting factors in ICRP-60 were themselves an update on the values given in ICRP-264 of 1977.
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