5: Fundamental Radiation Quantities And Units

Absorbed dose

Absorbed dose is the amount of energy deposited per unit mass.
The SI Unit for energy is the joule (J), and for mass is kilogram (kg). The special unit for absorbed dose is the Gray (Gy).

Integral Dose

The integral dose the the amount of radiation energy, in joules, imparted to a whole tissue or the entire body.


Kerma is an acronym for Kinetic Energy Released in MAterial / MAtter. It is used when describing photon beams to denote how much energy is deposited in a volume - that is, energy transferred to orbital elections through photoelectric, Compton or pair production events. Kerma uses the same units as absorbed dose, but may be different then the absorbed dose in a certain tissue. High energy electrons may travel outside the volume of interest, and are also more likely to undergo radiative collisions with nuclei leading to bremsstrahlung production.

Equivalent Dose

The equivalent dose is the absorbed dose, corrected to account for the different biological damage caused by alternative radiation types. A radiation weighting factor (WR) is used as a correction.

Effective Dose

The effective dose is an additional correction of the equivalent dose, designed to account for the risks of stochastic effects in different organs. The tissue weighting factor (WT) is used for this correction.