Definition of Sievert (Sv) unit - is it whole body mass? I'm wondering about the definition of the Sievert  (Sv) unit. It is J/Kg but is that Kg the mass of the whole body or just of the exposed body part? For instance, when a table says that an x-ray of a limb gives a dose of 0.1 mSv, does that mean that the body has been committed to 0.1 * N J of energy where N is the weight of the limb (in kg)? Or is N the mass of the whole body (so that the dose received by the limb in question is actually greater)?
It's a bit strange that this is J/Kg because people readily translate the radiation dose into a cancer risk (saying a dose of X mSv increases cancer risk by Y%), since I would assume the risk must depend not only on the radiation dose per unit of mass, but also on the amount of mass exposed to this level of radiation, i.e. related more to the actual J than to J/Kg?
 A: The Sievert is a derived measure of stochastic health risk.  It's used only in cases of low dosage ionizing radiation.  High dosages that produce deterministic health effects are measured in the Gray (Gy), a purely physical term which represents the actual deposit of one joule of energy in one kilogram of matter.
Unlike the Gray, the Sievert does not measure an actual deposit of energy into tissue.  The Sievert is an equivalent dose which represents the likelihood of the effect of depositing one joule of energy in one kilogram of matter.  The Sievert is used to calculate an equivalent dose, which is computed using the actual deposited dose multiplied by a weighting factor which depends on the type of ionizing radiation to which one has been exposed.
The Sievert also is used to measure effective dose, which takes into consideration the biological susceptibility of the particular targeted tissue.  If you go to this link: http://www.researchgate.net/post/What_is_the_difference_between_Sievert_and_Gray_A_practical_question_concerning_the_SI_units_for_ionizing_radiation, and scroll down to Jean Louis M. Genicot's comment, you will find he discusses the ways in which the Sievert is used, and how it is differentiated from the Gray.  This chart summarizes the relationship between equivalent and effective dose: https://upload.wikimedia.org/wikipedia/commons/0/03/SI_Radiation_dose_units.png.
In making these calculations, the intensity of radiation (microSieverts per hour, for example), rather than the mass of the tissue irradiated, is the basis for determining risk.  This link is  a comparative chart of exposures and risks: http://www.radiologyinfo.org/en/info.cfm?pg=safety-xray.  However, note at the bottom of the chart that "actual dose can vary substantially, depending on a person's size".  In other words, volume is important because it may determine how much ambient radiation is absorbed.
If your whole body is exposed to ionizing radiation, then your weight is a proxy for your volume.  But if the radiation is directed only at one area of tissue, with no leakage to the surrounding area, only the targeted area tissue should be used in the calculation.  The Wikipedia article on Equivalent Dose has a section that discusses this (scroll down to "Related Quantities"): https://en.wikipedia.org/wiki/Equivalent_dose.
