The term "first collision dose" has been used in several senses which physically differ from one another in subtle ways: First collision dose = absorbed dose in an isolated small mass; First collision dose =that part of the absorbed dose contributed by "primary" radiation only, in a small mass located anywhere; First collision dose = kerma. Users of this term have been not always aware of the fact that it conveys different pictures to different people. Ther term "kerma" cannot be indiscriminately substituted wherever first collision dose is now employed.