Maria Saager1, Christin Glowa2, Peter Peschke3, Stephan Brons4, Rebecca Grün5, Michael Scholz5, Peter E Huber6, Jürgen Debus7, Christian P Karger8. 1. Dept. of Radiation Oncology, University Hospital of Heidelberg, Germany; Dept. of Medical Physics in Radiation Oncology, German Cancer Research Center (DKFZ), Heidelberg, Germany. Electronic address: m.saager@dkfz.de. 2. Dept. of Radiation Oncology, University Hospital of Heidelberg, Germany; Dept. of Medical Physics in Radiation Oncology, German Cancer Research Center (DKFZ), Heidelberg, Germany. 3. Clinical Cooperation Unit Molecular Radiooncology, German Cancer Research Center (DKFZ), Heidelberg, Germany. 4. Heidelberg Ion Beam Therapy Center (HIT), Heidelberg, Germany. 5. Dept. of Biophysics, Helmholtz Center for Heavy Ion Research (GSI), Darmstadt, Germany. 6. Dept. of Radiation Oncology, University Hospital of Heidelberg, Germany; Clinical Cooperation Unit Molecular Radiooncology, German Cancer Research Center (DKFZ), Heidelberg, Germany. 7. Dept. of Radiation Oncology, University Hospital of Heidelberg, Germany. 8. Dept. of Medical Physics in Radiation Oncology, German Cancer Research Center (DKFZ), Heidelberg, Germany.
Abstract
PURPOSE: 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. METHODS AND MATERIALS: 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). RESULTS: 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. CONCLUSIONS: The measured data improve the knowledge on the accuracy of RBE-calculations for carbon ions.
PURPOSE: 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. METHODS AND MATERIALS: 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). RESULTS: 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. CONCLUSIONS: The measured data improve the knowledge on the accuracy of RBE-calculations for carbon ions.
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