Tissue weighting factor

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.

Based on the values of tissue weighting factors, tissues are grouped into following to assess the carcinogenic risk:

• high risk (WT = 0.12): stomach, colon, lung, red bone marrow
• moderate risk (WT = 0.05): urinary bladder, esophagus, breast, liver, thyroid
• low risk (WT = 0.01): bone surface, skin

In calculating effective dose, the risk of genetic effects is included, with a weighting factor of 0.20 being assigned to gonads. Remaining tissues (adrenals, brain, kidney, muscle, small intestine, pancreas, spleen, thymus and uterus) for which there is some evidence of cancer induction but insufficient data to provide a specific risk factor; are collectively assigned a low weighting factor of 0.05.

The numerical values given above are valid for legal EU regulations for calculating equivalent dose in an organ or tissue. In 2007, the International Commission on Radiological Protection (ICRP) published a new set of tissue weighting factors as below:

• WT = 0.12: stomach, colon, lung, red bone marrow, breast, remainder tissues
• WT = 0.08: gonads
• WT = 0.04: urinary bladder, esophagus, liver, thyroid
• WT = 0.01: bone surface, skin, brain, salivary glands