Biological factors influencing the RBE of neutrons: implications for their past, present and future use in radiotherapy.

The recent resurgence of interest in fast-neutron therapy, particularly for the treatment of prostate cancer, warrants a review of the original radiobiological basis for this modality and the evolution of these concepts that resulted from subsequent experimentation with the fast-neutron beams used for randomized clinical trials. It is clear from current radiobiological knowledge that some of the postulates that formed the mechanistic basis for past clinical trials were incorrect. Such discrepancies, along with the inherent physical disadvantages of neutron beams in terms of collimation and intensity modulation, may partially account for the lack of therapeutic benefit observed in many randomized clinical trials. Moreover, it is equally apparent that indiscriminate prescription of fast-neutron therapy is likely to lead to an adverse clinical outcome in a proportion of patients. Hence any renewed efforts to establish a niche for this modality in clinical radiation oncology will necessitate the development of a triage system that can discriminate those patients who might benefit from fast-neutron therapy from those who might be harmed by it. In the future, fast-neutron therapy might be prescribed based upon the relative status of appropriate molecular parameters that have a differential impact upon radiosensitivity to photons compared to fast neutrons.