|Table 1. Tissue Weighting Factors (WT) for Individual Tissues and Organs|
|Tissue or Organ||Tissue Weighting Factor (WT|
|Gonads (testes or ovaries)||0.20|
|Red bone marrow||0.12|
|** The remainder is composed of the following additional tissues and organs: adrenal, brain, upper large intestine, small intestine, kidney, muscle, pancreas, spleen, thymus and uterus.|
The tissue weighting factor (WT) is an important unit of radiation measurement that weights the equivalent dose in a particular tissue or organ in terms of its relative contribution to the total deleterious effects resulting from uniform irradiation of the whole body. This weighting factor takes account of the probability of fatal cancer, the probability of nonfatal cancer, weighted for severity, and the average length of life lost due to an induced cancer.
The values shown in Table 1 were developed from a reference population of equal numbers of both sexes and a wide range of ages. In the definition of effective dose they apply to workers, to the whole population, and to either sex. When an equivalent dose to an organ is multiplied by the (wT) for that organ, the result is the effective dose to that organ:
- Effective dose = sum of [organ doses x tissue weighting factor]
The unit of effective dose is the sievert (Sv).
According to the International Agency for Research on Cancer (IARC) "For purposes of calculation, the 'remainder' category in Table 1 is composed of the following additional tissues and organs: adrenals, brain, upper large intestine, small intestine, kidney, muscle, pancreas, spleen, thymus and uterus. The list includes organs which are likely to be selectively irradiated. Some organs in the list are known to be susceptible to cancer induction. If other tissues and organs subsequently become identified as having a significant risk of induced cancer they will then be included either with a specific WT or in this additional list constituting the remainder. The latter may also include other tissues or organs selectively irradiated."
In those exceptional cases in which a single one of the remainder tissues or organs receives an equivalent dose in excess of the highest dose in any of the twelve organs for which a weighting factor is specified, a weighting factor of 0.025 should be applied to that tissue or organ and a weighting factor of 0.025 to the average dose in the rest of the remainder as defined above.
- International Agency for Research on Cancer, Monographs on the Evaluation of Carcinogenic Risks to Humans, 2001
- 1990 Recommendations of the International Commission on Radiological Protection. ICRP Publication 60, Annals of the ICRP 21(1-3), Pergamon Press, Oxford.