• Christian P Karger, Peter Peschke, Michael Scholz, Peter E Huber, and Jürgen Debus.
• Department of Medical Physics in Radiation Oncology, German Cancer Research Center (DKFZ), Heidelberg, Germany. c.karger@dkfz.de
• Int. J. Radiat. Oncol. Biol. Phys. 2013 Jul 1; 86 (3): 450-5.

PurposeTo determine the relative biological effectiveness (RBE) and the effective α/β ratio for local tumor control of a radioresistant rat prostate tumor (Dunning subline R3327-AT1) after 6 fractions of carbon ions and photons.Methods And MaterialsA total of 82 animals with tumors in the distal thigh were treated with 6 fractions of either photons or carbon ions, by use of increasing dose levels and a 2-cm spread-out Bragg peak. Endpoints of the study were local control (no tumor recurrence within 300 days) and volumetric changes after irradiation. The resulting values for dose at 50% tumor control probability were used to determine RBE values. Including data for 1 and 2 fractions from a previous study, we estimated α/β ratios.ResultsFor 6 fractions, the values for dose at 50% tumor control probability were 116.6 ± 3.0 Gy for photons and 43.7 ± 2.3 Gy for carbon ions and the resulting RBE was 2.67 ± 0.15. The α/β ratio was 84.7 ± 13.8 Gy for photons and 66.0 ± 21.0 Gy for carbon ions. Using these data together with the linear-quadratic model, we estimated the maximum RBE to be 2.88 ± 0.27.ConclusionsThe study confirmed the increased effectiveness of carbon ions relative to photons over the whole dose range for a highly radioresistant tumor. The maximum RBE below 3 is in line with other published in vivo data. The RBE values may be used to benchmark RBE models. Hypoxia seems to have a major impact on the radiation response, although this still has to be confirmed by dedicated experiments.Copyright © 2013 Elsevier Inc. All rights reserved.

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