RBE and weighting of absorbed dose in ion-beam therapy

Absorbed dose is the fundamental quantity used to quantify the exposure of any biological system to ionizing radiation. However, the relationship between dose and biological effect is not unique but varies with fractionation and time factor(s), radiation quality and irradiation conditions. In radiation therapy, weighting factors are used to correlate absorbed dose and clinical effects when altering irradiation conditions, or for combining or comparing different technical modalities. For some well established therapy modalities (e.g. fractionated photon beam therapy), a general agreement on weighting factors has been reached: it is based on the linear-quadratic model. For neutron, proton or ion therapy, the differences in radiation quality are currently accounted for using a diversity of methods (almost hospital specific). This paper reviews the current approaches used for evaluating or selecting the weighting factors and their application in clinical practice. The weighting factors take account of the RBE and other factors, such as fractionation when needed. Harmonization of these approaches will facilitate the exchange of information within the radiation oncology community and between centres using different technical modalities. In any case, when reporting the treatments, absorbed dose and irradiation conditions should always be specified in addition to the weighting factor and the weighted dose.