Answer to Question #8966 Submitted to "Ask the Experts"
Category: Radiation Basics
The following question was answered by an expert in the appropriate field:
I have a question regarding the dose-response curve presented in the LSS (Life Span Study) versus the dose-response curve in International Commission on Radiological Protection (ICRP) Recommendation 103. In the LSS (Preston 2007), the incidence of solid cancer versus dose curve is presented in terms of weighted colon dose. However, in ICRP 103, the incidence of solid cancer versus dose is presented in terms of Sv (total effective dose). How does ICRP convert the weighted colon dose to the effective dose?
ICRP lists recommended tissue weighting factors to be used in consideration of stochastic effects and determination of effective dose. The tissue weighting factor is a relative measure of the risk of certain serious stochastic effects that might result from irradiation of that specific tissue. The sum of all the tissue weighting factors used in effective dose determinations is 1.0. The effective dose, as you likely know, is determined by multiplying the equivalent dose (for beta radiation, gamma radiation, and x rays, the equivalent dose is numerically equal to the absorbed dose) to each significantly irradiated tissue by its respective tissue weighting factor and then summing up all such products to obtain effective dose. If only a single organ is irradiated, the effective dose would be the equivalent dose to that organ multiplied by its tissue weighting factor.
ICRP lists tissue weighting factors in Table 3, page 65, of Publication 103, which you cite. Individual tissue weighting factor values range from 0.01 to 0.12. For the colon, the ICRP-recommended tissue weighting factor is 0.12. Thus, the equivalent dose to the colon would be multiplied by this factor to obtain the contribution of the colon dose to effective dose. Thus, under current recommendations, if the colon received an absorbed dose of 1.0 Gy from gamma rays, the equivalent dose to the colon would be 1.0 Sv, and the contribution to the effective dose would be 0.12 Sv. Under earlier recommendations of the ICRP (ICRP Publication 60), the colon was among remainder tissues and the default weighting factor was 0.06 compared to the current value of 0.12. Use of the older factor would have reduced the effective dose component from the colon by a factor of two compared to the value calculated using the current weighting factor.
Keep in mind that the quantity effective dose is used in radiation protection when attempting to assess the overall potential stochastic risk associated with radiation exposure. It is not intended for use in judging the impact of dose on individual organs or tissues. Since Dale Preston and colleagues were concerned with individual organs for purposes of assessing risk of radiation-induced cancer in those organs, such doses would presumably have been reported as absorbed doses or kerma. When weighted organ doses are used in investigations of cancer incidence, the weighting usually refers to multiplying the absorbed dose, generally expressed in grays (Gy), from a given radiation type by an appropriate radiation weighting factor to convert to equivalent dose, usually expressed in sieverts (Sv). The population of exposed survivors from the World War II bombings of Japan was exposed to gamma radiation and, in part, neutron radiation, and the radiation weighting factor has often been taken as 1.0 for the gamma radiation and 10 for the neutron radiation.
I looked at the figures in the cited Preston publication and noticed that the cancer response plots are in fact shown with excess relative risk plotted against weighted organ dose in Gy, the unit of absorbed dose or kerma. If the radiation weighting factors had been applied, I would have expected the dose unit to be Sv, representative of equivalent dose. I do not currently have access to the text of the publication, so I can not say for certain what the dose weighting refers to, but the weighting would not have anything to do with the tissue weighting factors used to estimate stochastic impact, as when effective dose is determined. If you require more specific information about the dose weighting process used by Preston and colleagues, I suggest that you attempt to contact one of the authors. Good luck.
George Chabot, PhD, CHP