Calculation of clinical r.b.e. values for neutrons

A method is described for calculating r.b.e. values for normal tissues at risk in clinical neutron beam therapy. This is based on the assumption that with high l.e.t. radiations the slope of the cell survival curve is steeper, mainly in the initial or low-dose region. This effect is quantified by using two coefficients, one (epsilon) to produce a proportionate increase in the initial slope, and a second (eta) determining the change in the terminal slope (D0) of the survival curve. Analysis of published experimental data shows epsilon to be a variable quantity, different for different tissues; epsilon is larger when the survival curve has a large shoulder or slope ratio (rho). By contrast, eta is relatively constant (for a given beam) and less dependent on the tissue or end-point studied. For low doses, the r.b.e. approaches epsilon, which can be calculated given eta (characteristic of the beam) and rho (characteristic of the relevant tissue) [epsilon = eta + rho(eta - 1)]. This provides a useful approximation to the clinical r.b.e. for specific tissues relative to conventionally fractionated low-l.e.t. photons.