[Bystander effect: critical review and implications for risk assessment in radioprotection]

A growing experimental evidence indicates that ionizing radiation is able to determine relevant biological effects, including reduction in cell survival, cytogenetic alterations, gene mutations, induction of apoptosis and gene expression modification, in cells not directly hit by radiation. Such a phenomenon, known as bystander effect, has been described for radiation of different quality (alpha, gamma, X, beta radiation, heavier ions) and for different types of cells; it is relevant at low doses (even few mGy) and in many cases it seems to be persistent over time. Bystander effect is surely mediated by soluble factors released by irradiated cells into the extracellular environment and/or by the passage of mediators through gap junctions. The search of the effect in vivo, the setting up of models of risk assessment at low doses of ionizing radiation accounting for the bystander effect and the identification and quantification of this effect in relation to chemical agents displaying genotoxic and carcinogenic potential are primary research goals.