Answer to Question #1974 Submitted to "Ask the Experts"

Q

I am trying to understand the differences between absorbed dose and effective dose. One is measured in rads or grays and the other in rems or sieverts. Is it possible to calculate one from the other by taking into account other factors such as the patient's mass? Or is it possible to calculate the effective dose from the exposure factors used? If either of these are possible, what is the formula used or where could I find this information?

A

There is a nice self-paced tutorial on the University of Michigan Radiation and Health Physics Web page that will guide you through a lot of basics of radiation and radioactivity. Click on "Introduction to Radiation," then on "Basics of Radiation and Radioactivity." In a brief answer to your question, "absorbed dose" is energy absorbed per unit mass and is measured in basic units of erg/g or joules/kg, or in special units of rad (100 erg/g) or gray (Gy) (1 joule/kg = 100 rad). "Dose equivalent" is a derived quantity, obtained by multiplying values of absorbed dose by dimensionless "radiation weighting factors" which account for the ability of particular types of radiation to cause biological damage. An absorbed dose of 1 rad multiplied by the radiation weighting factor gives the dose equivalent in rem; an absorbed dose of 1 gray (Gy) multiplied by the radiation weighting factor gives the dose equivalent in sievert (Sv). You actually asked about "effective dose," which is a special calculation with the dose equivalent, looking at individual values of dose equivalent to specific organs of the body and weighting each one by risk factors related to their likelihood of expressing biological effects. Michael G. Stabin, PhD, CHP