Kerma (kinetic energy released per unit mass) is defined as the sum of the initial kinetic energies of all charged ionising particles liberated by uncharged ionising particles (such as X-rays, gamma rays, fast neutrons) per unit mass of interacting medium.
Kerma therefore is a quantity of transferred energy and it is measured in joules per kilogram (J/kg) or grays (Gy) in SI units. Although kerma and absorbed dose are both measured in the same units, they are different quantities.
Depending on the energies involved kerma dose is different from absorbed dose partially because ionisation energy is not accounted for. At low energies kerma is roughly equal to absorbed dose however at higher energies kerma is much higher than absorbed dose because some energy escapes from the absorbing volume in the form of bremsstrahlung or highly energetic secondary electrons.