I’d like to stress @fogallar 's point about fluence, which as he pointed out is not current, i.e. number of particles. The latter may be meaningfully scored on a surface, but not in a region. The concept of number of particles inside a volume is ill, since a particle traversing the volume cannot be counted as a particle produced in the middle of the volume and immediately absorbed. That’s why USRTRACK provides only fluence, weighing particles by their tracklength.