Presentation RBE value of therapeutic carbon-ion beam on various biological systems

松藤, 成弘

Escalating relative biological effectiveness (RBE) of carbon-ion beam toward range end is attractive characteristics for treating deep-seated tumors. Here, the RBE value is not unique but dependent on the biological endpoint of concern. At the National Institute of Radiological Sciences (NIRS), depth-dose profile of therapeutic carbon-ion beam has been designed based on one representative biological system, and the treatment planning system (TPS) indicates this reference-RBE-weighted dose distribution irrespective of actual anatomy. In this study the biological effectiveness of the beam for other biological system is estimated in order to evaluate the safety of the C-ion RT. (Methods) As a typical profile, routinely used depth-dose and depth-LET relationship of therapeutic C-290 MeV/n beam with 60mm SOBP was used. This profile was designed to achieve uniform biological effectiveness within the SOBP region with the radiosensitivity of a reference cell line established from human salivary gland tumor cells (HSG) at 10 % survival level, then scaled to the value of 3.0 at the point where the dose-averaged LET was 80 keV/m for clinical usage. On another front, Ando et al investigated the LET dependency of  values on mice skin shrinkage and delay in tumor regrowth derived by fractionated irradiation experiments (JRR 46 51-57 (2005)). The reported radiosensitivity was introduced as a surrogate for yet another tumor and normal-tissue response. Their RBE distribution was estimated with the dose and LET value at corresponding irradiation depth of the therapeutic carbon SOBP beam. (Result) The mice tumor RBE exceeded the HSG RBE while the skin RBE was comparable, which means obviously smaller than the therapeutic RBE in the TPS. (Conclusion) The smaller RBE value on the skin suggests that the current therapeutic RBE system is on the side of prudence in evaluating the skin toxicity. More comparison will be reported at the presentation.

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