Collision kerma Kc

That part of kerma (i.e. electron kinetic energy) that leads to subsequent collision interactions and hence energy deposition is known as collision kerma (Kc).

Collision part of kerma is NOT ALWAYS equal to absorbed dose. But when charged particle equilbrium exists, for example, at P in the above figure, electron energy escaping from at a point P will be compensated by electron energy entering at P (ENTER DIAGRAM LATER). In this case Kc(P) can be equated to D(P) with a small correction that accounts for the very small attenuation of photons over electron ranges.

Since any dosimetric system, whether it is a Primary Standard or a therapy dosimetry responds to energy absorbed in the sensitive volume, kerma (or exposure) of indirectly ionizing particles can be measured only under electronic equilibrium conditions.