Comparisonof the Methods of Specifying Doses at NIRS and GSI Carbon Ion

Carbon ions/Carbon-therapy/Biologieal treatment planlRBE. Duc to the RBE variations, the carbon-ion doses (in Gy) are no longer sufficient to moniter adequately the biological effect of these radiations. Therefore, "RBE dose weighting factors" WRBE allowing for the RBE variations with energy, dose and biological system have to be introduced in the treatment plans in order to provide the physician with interpretable inforrnation. This paper compares the methods cmployed for this purpose at NIRS and GSI, which are specific of the beam delivery system of these institutions. NIRS has a "passive" beam delivery system where the dose distribution in the SOBP is determined by a Ridge filter. The dose distribution and thus, the shaping of the filter is chosen according to the clinical situation and determined with respect to WRBE factors in order to yield a biologically iso-effective SOBP. WRBE factors in the SOBP are at first derived from a RBEfLET function for HSG cells, then normalized to 3 at a LET of 80 keVfpm. The latter value of 3 corresponds to the clinical RBE of NIRSneutrons, which were found to exhibit the same radiobiological properties as 80 keVfptm carbon-ions. GSI has a "dynamic" beam delivery system ("spot" or "voxel" scanning) making it possible to irradiate irregular vo]umes and to modulate the radiation intensity according to the radiosensitivity of diffbrent tissues and/or different sub-volumes. Due to the C`power" and the resulting complexity of the system, WRBE factors are determined through an integrated calculation code aHowing iterativc interaction f both physical and radiobiological parameters. The "Local Effect Model" (LEM) was developed in this view with the aim of deriving carbon-ion WRBE factors from the parameters detemining the respense to photons. Advantages and weaknesses of the respective methods will be discussed,