Split dose carbon ion irradiation of the rat spinal cord: Dependence of the relative biological effectiveness on dose and linear energy transfer.

Abstract

To measure the relative biological effectiveness (RBE) of carbon ions relative to 15 MeV photons in the rat spinal cord for different linear energy transfers (LET) to validate model calculations. The cervical spinal cord of rats was irradiated with 2 fractions of carbon ions at six positions of a 6 cm spread-out Bragg-peak (SOBP, 16-99 keV/μm). TD50-values (dose at 50% complication probability) were determined from dose-response curves for the endpoint radiation induced myelopathy (paresis grade II) within 300 days after irradiation. Using previously published TD50-values for photons (Karger et al., 2006; Debus et al., 2003), RBE-values were determined and compared with predictions of two versions of the local effect model (LEM I and IV). TD50-values for paresis grade II were 26.7 ± 0.4 Gy (16 keV/μm), 24.0 ± 0.3 Gy (21 keV/μm), 22.5 ± 0.3 Gy (36 keV/μm), 20.1 ± 1.2 Gy (45 keV/μm), 17.7 ± 0.3 Gy (66 keV/μm), and 14.9 ± 0.3 Gy (99 keV/μm). RBE-values increased from 1.28 ± 0.03 (16 keV/μm) up to 2.30 ± 0.06 at 99 keV/μm. At the applied high fractional doses, LEM I fits best at 16 keV/μm and deviates progressively toward higher LETs while LEM IV agrees best at 99 keV/μm and shows increasing deviations, especially below 66 keV/μm. The measured data improve the knowledge on the accuracy of RBE-calculations for carbon ions.