Estimation of a single property of low LET radiations which correlates with biological effectiveness.

The biological effectiveness of ionising radiation is believed to be related to the initial structure of the radiation tracks. In this paper, an attempt is made to identify a single physical property of the radiation field that correlates with relative biological effectiveness. This property has been sought in terms of a minimum amount of energy deposition in a certain volume. In order to quantify this approach detailed Monte Carlo track structure simulations have been made of x-rays of widely differing energies and the results have been compared with previously reported experiments of cell killing by these radiations. In terms of the above threshold energy concept, we have thus identified a putative critical property of radiations of low ionisation density such as x-rays, namely the rare occurrence of energy deposition of greater than or equal to 100 eV in a volume of about 3 nm diameter.