[From the carbon track to therapeutic efficiency of hadrontherapy]

Carbon ions, thanks to their relative biological effectiveness much higher than that of photons and protons and their ballistic characteristics similar to those of protons, can effectively treat radioresistant tumours. The reasons for this increased efficiency are found in the microdosimetric and radiobiological features of ions. The energy deposit or linear energy transfer increases along the range and reaches a very high level at the end producing the Bragg peak, where the linear energy transfer is about hundred times higher than that of photons. These massive energy deposits create multiple DNA lesions that are difficult to repair. DNA repair is associated with longer blockage of the cell cycle and more frequent chromosomal aberrations that are lethal to cells. The types of cell death are identical to those triggered in response to photon irradiation, but the response is earlier and more important at equivalent physical dose. Radiobiological differences between carbon ions and photons have been studied for some years and many aspects remain to be explored. In general, these phenomena tend to reduce the differences of radiosensitivity among different tissues. It is therefore in situation where tumours are relatively radioresistant compared to healthy tissue, that carbon ions must be used and not in the opposite situations where the fractionation of low linear energy transfer radiation is sufficient to provide the necessary differential effect to cure the tumour.