Equivalent Dose - Computation

Computation

The equivalent dose is calculated by multiplying the absorbed dose, averaged by mass over an organ or tissue of interest, by a radiation weighting factor appropriate to the type and energy of radiation. To obtain the equivalent dose for a mix of radiation types and energies, a sum is taken over all types of radiation energy dose.

where

H_T is the equivalent dose absorbed by tissue T

D_T,R is the absorbed dose in tissue T by radiation type R

W_R is the radiation weighting factor defined by regulation

Thus for example, an absorbed dose of 1 Gy by alpha particles will lead to an equivalent dose of 20 Sv.

The radiation weighting factor represents the relative biological effectiveness of the radiation. It aims to correct the simple deposited energy of the radiation (a fundamental quantity with clear physical meaning) for different biological effect of different types of radiation. An equivalent dose of radiation is estimated to have the same biological effect as an equal amount of absorbed dose of gamma rays.

If the equivalent dose is uniform throughout the organism, it will be equal to the effective dose. Otherwise, a weighted average of H_T will have to be taken to average out the radiation dose through the body while correcting for the different sensitivities of different tissues. See the article on effective dose for this calculation. In the case of non-uniform radiation, or radiation given to only part of the body, using the local equivalent dose alone would overstate the biological risks to the entire organism.