Kerma and energy deposition

Kerma is only defined for photons which interact with matter and convert to electron energy.

Not all of kerma ends up as energy deposition in the medium. Part of kerma (electron kinetic energy produced) is converted into bremmstrahlung which escapes from the region of interest, so that only that part of kerma called the collision part of kerma is energy that is dissipated in the medium by ionization and excitation events.