Microbeams in radiation biology: review and critical comparison.

Microbeams have undergone a renaissance since their introduction and early use in the mid-60s. Recent advances in imaging, software and beam delivery have allowed rapid technological developments in microbeams for use in a range of experimental studies. Microbeams allow the effects of single radiation tracks to be determined in a highly quantified way. They offer a unique tool for following DNA damage and repair in a highly controlled way. More importantly, they allow radiation to be targeted to specific regions within a cell to probe subcellular radiosensitivity. They are also playing an important role in our understanding of bystander responses, where cells not directly irradiated can respond to irradiated neighbours. Although these processes have been studied using a range of experimental approaches, microbeams offer a unique route by which bystander responses can be elucidated. Without exception, all of the microbeams currently active have studied bystander responses in a range of cell and tissue models. Together, these studies have considerably advanced our knowledge of the underpinning mechanisms. Much of this has come from charged particle microbeam studies, but increasingly, X-ray and electron microbeams are starting to contribute quantitative and mechanistic information on bystander effects. A recent development has been the move from studies with 2-D cell culture models to more complex 3-D systems where the possibilities of utilising the unique characteristics of microbeams in terms of their spatial and temporal delivery will make a major impact.