Radiobiological features of the anti-cancer strategies involving synchrotron X-rays.

Synchrotrons are opening new paths in innovative anti-cancer radiotherapy strategies. Indeed, the fluence of X-rays induced by synchrotrons is so high (10(6) times higher than standard medical irradiators) that it enables the production of X-ray beams tunable in energy (monochromatic beams) and in size (micrometric beams). Monochromatic synchrotron X-ray beams theoretically permit photoactivate high-Z elements to be introduced in or close to tumours in order to increase the yield of damage by enhanced energy photoabsorption. This is notably the case of attempts with iodinated contrast agents used in tumour imaging (the computed tomography therapy approach) and with platinated agents used in chemotherapy (the PAT-Plat approach). Micrometric synchrotron X-ray beams theoretically permit very high radiation doses to accumulate in tumours by using arrays of parallel microplanar beams that spare the surrounding tissues (the microbeam radiation therapy approach). These anti-cancer applications of synchrotron radiation have been developed at the European Synchrotron Radiation Facility to be applied to glioma, one of the tumour tissues most refractory to standard treatments. In the present paper the molecular and cellular mechanisms involved in these three approaches are reviewed, in the context of recent advances in radiobiology. Furthermore, by considering the unavoidable biases, an attempt to propose a comparison of the different results obtained in preclinical trials dealing with rats bearing tumours is given.