Radiation tolerance of the rat spinal cord after 6 and 18 fractions of photons and carbon ions: Experimental results and clinical implications

Purpose: The tolerance of the rat spinal cord to photon and carbon ion irradiations was investigated to determine the relative biologic effectiveness (RBE) of carbon ions (C-12) in the plateau region and in a 1 cm spread-out Bragg-peak. Methods and Materials: The cranial part of the cervical and thoracic spinal cord of 336 rats was irradiated with 6 or 18 fractions (Fx) of photons or C-12-ions, respectively. Animals were followed up for 300 days for the onset of paresis grade 11 and dose-response curves were calculated. Results: The D-50-values (dose at 50% complication probability) were 42.9 +/- 0.5 Gy, 62.2 +/- 0.9 Gy (6 and 18 Fx, C-12-plateau) and 19.2 +/- 0.2 Gy, 17.6 +/- 0.2 Gy (6 and 18 Fx C-12-peak), respectively. For photons, the D-50-values were 57.0 +/- 0.7 Gy for 6 and 88.6 +/- 0.7 Gy for 18 Fx. The corresponding RBE-values were 1.33 +/- 0.02, 1.42 +/- 0.02 (6 and 18 Fx, 12 C-plateau) and 2.97 +/- 0.05, 5.04 +/- 0.08 (6 and 18 Fx C-12-peak), respectively. Including data of a previously performed experiment for 1 and 2 Fx (1) the parameter alpha/beta of the LQ-model was found to be 2.8 +/- 0.4 Gy, 2.1 +/- 0.4 Gy and 37.0 +/- 5.3 Gy for photon-, C-12-plateau- and C-12-peak irradiations, respectively. Conclusions: Carbon ion irradiations of the spinal cord are significantly more effective in the peak than in the plateau region. The alpha/beta-values indicate a significant fractionation effect only for the plateau irradiations. In the Bragg-peak, the applied RBE-model correctly describes the main features although it generally underestimates the RBE by 25%. In the plateau region, maximum deviations of up to 20% were found. The acquired data contribute significantly to the validation of the applied RBE-model. (c) 2006 Elsevier Inc.