Comparison of the Methods of Specifying Carbon Ion Doses at NIRS and GSI.

Due to the RBE variations, the carbon-ion doses (in Gy) are no longer sufficient to monitor adequately the biological effect of these radiations. Therefore, "RBE dose weighting factors" - W(RBE) - 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 information. This paper compares the methods employed 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 W(RBE) factors in order to yield a biologically iso-effective SOBP. W(RBE )factors in the SOBP are at first derived from a RBE/LET function for HSG cells, then normalized to 3 at a LET of 80 keV/mum. The latter value of 3 corresponds to the clinical RBE of NIRS-neutrons, which were found to exhibit the same radiobiological properties as 80 keV/mum carbon-ions. GSI has a "dynamic" beam delivery system ("spot" or "voxel" scanning) making it possible to irradiate irregular volumes and to modulate the radiation intensity according to the radiosensitivity of different tissues and/or different sub-volumes. Due to the "power" and the resulting complexity of the system, W(RBE )factors are determined through an integrated calculation code allowing iterative interaction of both physical and radiobiological parameters. The "Local Effect Model" (LEM) was developed in this view with the aim of deriving carbon-ion W(RBE )factors from the parameters determining the response to photons. Advantages and weaknesses of the respective methods will be discussed.