The Gray Cancer institute X-ray microprobe and its radiobiological applications

Abstract
Radiation microbeams represent a unique and powerful tool to study and quantify the effects of precise doses of radiation delivered with micron precision to selected biological samples. The Gray Cancer Institute has developed two independent but complementary micro-irradiation facilities, specifically developed for the targeted irradiation of cells and structured tissues; a charged-particle microbeam that uses collimated protons or helium ions and an ultra-soft X-ray microprobe. The ultra-soft X-ray facility employs a focused electron bombardments source to produce a near monochromatic C X-ray beam. Highly efficient zone plates optimised for the appropriate wavelength are used to focus the characteristic X-rays into a sub-micron spot. The facility is also equipped with a three-axis micro-positioning stage, an epi-fluorescent UV microscope with intensified CCD camera coupled to a fast PC for a automatic, fast and accurate samples recognition and alignment with the probe. Recent experiments have been directed to investigate the bystander effect by irradiating only one cell within a population of V79 cells that are subsequently individually revisited for colony formation. A clear bystander effect has been detected (~10% reduction in survival) when a single cell has been irradiated. The effect is triggered by very low doses (~100 mGy) and it is largely dose independent.