Answer to Question #11413 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
If two of the organs (kidney and spleen) in the remainder list are irradiated, what tissue weighting factor should be used for each of these organs?
The answer to your question depends partly on which specific recommendations you are following. The most recent, overall recommendations of International Commission on Radiological Protection (ICRP) 103 (2007) include a tissue weighting factor of 0.12 for the entire remainder, and the remainder consists of 14 tissues: the adrenals, extrathoracic (ET) region, gall bladder, heart, kidneys, lymphatic nodes, muscle, oral mucosa, pancreas, prostate (males), small intestine, spleen, thymus, and uterus/cervix (females).
The recommended procedure for establishing the weighted dose to the remainder is to calculate the average equivalent dose among all remainder tissues and to multiply this average dose by the weighting factor for the remainder. If only two of the tissues of the remainder received dose, the dose would still be averaged among the 13 tissues (for males the uterus/cervix is eliminated, and for females the prostate is eliminated) that constitute the remainder and the factor of 0.12 then applied. This is mathematically equivalent to assuming a tissue weighting factor of 0.12/13 for each tissue of the remainder and applying this individually to each of the two irradiated tissues.
For example, if the equivalent doses to the kidneys and spleen were 5 millisieverts (mSv) and 8 mSv, respectively, and no other remainder tissues received dose, the average equivalent dose among the 13 tissues would be (5 mSv + 8 mSv)/13 = 1 mSv, and the weighted equivalent dose to the remainder would then be (1 mSv)(0.12) = 0.12 mSv. This is the same result that we would obtain by applying an individual weighting factor of 0.12/13 to the kidneys and to the spleen; i.e., equivalent dose to remainder = (5 mSv)(0.12/13) + (8 mSv)(0.12/13) = 0.12 mSv.
If you are following earlier ICRP 60 (1990) recommendations, the remainder consists of 10 tissues: the adrenals, brain, small intestine, upper large intestine, kidney, muscle, pancreas, spleen, thymus, and uterus, and the combined weighting factor for all of these tissues is 0.05. The dose to the remainder is calculated using the same mechanics as discussed above for the ICRP 103 recommendations, simply adjusting for the different number of tissues in the remainder and the different weighting factor. The implied tissue weighting factor for the kidneys and the spleen would be 0.05/10 for each (including uterus for a female); for the numerical values of dose used in the above example, the equivalent dose to the remainder would be (5 mSv)(0.05/10) + (8 mSv)(0.05/10) = 0.07 mSv.
Finally, if you are using even earlier recommendations embodied in ICRP 26 (1977), on which some current requirements for U.S. licensees of the U.S. Nuclear Regulatory Commission and agreement states are based, the remainder consists of five tissues, each with an implied tissue weighting factor of 0.06. In this case, for the situation you describe and the tissue doses used in the above examples, the equivalent dose to the remainder would be (5 mSv)(0.06) + (8 mSv)(0.06) = 0.8 mSv.
I hope this is helpful to you.
George Chabot, PhD
International Commission on Radiological Protection. Recommendations of the International Commission on Radiological Protection. Oxford: Pergamon Press; ICRP Publication 26; 1977.
International Commission on Radiological Protection. The 1990 recommendations of the International Commission on Radiological Protection. Oxford: Pergamon Press; ICRP Publication 60; 1990.
International Commission on Radiological Protection. The 2007 recommendations of the International Commission on Radiological Protection. Amsterdam: Elsevier; ICRP Publication 103; 2007.