Debus, J., Scholz, M., Haberer, T., Peschke, Jäkel, O., Karger, C. P. and Wannenmacher, M. Radiation Tolerance of the Rat Spinal Cord after Single and Split Doses of Photons and Carbon Ions. Radiat. Res. 160, 536–542 (2003).

The sensitivity of the rat spinal cord to single and split doses of radiation and the resulting relative biological effectiveness (RBE) were determined for carbon-ion irradiations (¹²C) in the plateau and Bragg-peak regions. The cranial part of the cervical and thoracic spinal cords of 180 rats were irradiated with one or two fractions of ¹²C ions or photons, respectively. Dose–response curves for the end point symptomatic myelopathy were established, and the resulting values for the ED₅₀ (dose for 50% complication probability) were used to determine the RBEs. A median latency for myelopathy of 167 days (range, 121–288 days) was found. The ED₅₀ values were 17.1 ± 0.8 Gy, 24.9 ± 0.7 Gy (one and two fractions, ¹²C plateau) and 13.9 ± 0.8, 15.8 ± 0.7 Gy (one and two fractions, ¹²C Bragg peak), respectively. For photons we obtained ED₅₀ values of 24.5 ± 0.8 Gy for single doses and 34.2 ± 0.7 Gy when two fractions were applied. The corresponding RBEs were 1.43 ± 0.08, 1.37 ± 0.12 (one and two fractions, ¹²C plateau) and 1.76 ± 0.05, 2.16 ± 0.11 (one and two fractions, ¹²C Bragg peak), respectively. Hematoxylin and eosin staining revealed necrosis of the white matter in the spinal cord in all symptomatic animals. In summary, from one- and two-fraction photon, ¹²C plateau and Bragg-peak irradiation of the rat spinal cord, we have established RBEs as well as the individual ED₅₀'s. From the latter there is a clear indication of repair processes for fractionated photons and ¹²C plateau ions which are significantly reduced by using Bragg-peak ions. Additional studies are being carried with 6 and 18 fractions to further refine and define the RBE and ED₅₀ values and estimate the α/β ratios.