Relative biological effectiveness of ionizing radiations determined in tissue (RBE) fails in assessing comparative relative effectiveness in the tissue cells.

The value of the RBE of a test radiation is conventionally determined against a known standard radiation for a chosen response of a selected biological tissue and is expressed as the ratio of tissue absorbed doses at equal effect, or as ratio of magnitudes of the effect at equal absorbed dose. If such an effect is observable as a consequence of responses of individual elements of this tissue, namely the cells, such as induction of cancer that arises from a single cell, the relative biological effectiveness should be expressed as the ratio of the incidences of the effects at equal mean absorbed dose to the cells rather than at equal absorbed dose to tissue. This cell based relative biological effectiveness is here termed the relative local efficiency. Since tissue absorbed dose is a product of the number of energy deposition events in cells of that tissue (N(H)) and the mean absorbed dose to these cells in the exposed tissue (z(1)), per tissue mass equal tissue absorbed doses from different radiation qualities have different values of N(H) and z(1) As a result, for pink mutations in Tradescantia cells, the relative biological effectiveness of 0.43 MeV neutrons is 48 but the relative local efficiency in fact is 2.8.