Maria Saager1, Christin Glowa2, Peter Peschke3, Stephan Brons4, Michael Scholz5, Peter E Huber6, Jürgen Debus7, Christian P Karger8. 1. Department of Radiation Oncology, University Hospital of Heidelberg, Heidelberg, Germany; Department of Medical Physics in Radiation Oncology, German Cancer Research Center, Heidelberg, Germany. Electronic address: m.saager@dkfz.de. 2. Department of Radiation Oncology, University Hospital of Heidelberg, Heidelberg, Germany; Department of Medical Physics in Radiation Oncology, German Cancer Research Center, Heidelberg, Germany. 3. Clinical Cooperation Unit Molecular Radiooncology, German Cancer Research Center, Heidelberg, Germany. 4. Heidelberg Ion Beam Therapy Center (HIT), Heidelberg, Germany. 5. Department of Biophysics, Helmholtz Center for Heavy Ion Research (GSI), Darmstadt, Germany. 6. Department of Radiation Oncology, University Hospital of Heidelberg, Heidelberg, Germany; Clinical Cooperation Unit Molecular Radiooncology, German Cancer Research Center, Heidelberg, Germany. 7. Department of Radiation Oncology, University Hospital of Heidelberg, Heidelberg, Germany. 8. Department of Medical Physics in Radiation Oncology, German Cancer Research Center, Heidelberg, Germany.
Abstract
PURPOSE: To measure the relative biological effectiveness (RBE) of carbon ions in the rat spinal cord as a function of linear energy transfer (LET). METHODS AND MATERIALS: As an extension of a previous study, the cervical spinal cord of rats was irradiated with single doses of carbon ions at 6 positions of a 6-cm spread-out Bragg peak (16-99 keV/μm). The TD50 values (dose at 50% complication probability) were determined according to dose-response curves for the development of paresis grade 2 within an observation time of 300 days. The RBEs were calculated using TD50 for photons of our previous study. RESULTS: Minimum latency time was found to be dose-dependent, but not significantly LET-dependent. The TD50 values for the onset of paresis grade 2 within 300 days were 19.5 ± 0.4 Gy (16 keV/μm), 18.4 ± 0.4 Gy (21 keV/μm), 17.7 ± 0.3 Gy (36 keV/μm), 16.1 ± 1.2 Gy (45 keV/μm), 14.6 ± 0.5 Gy (66 keV/μm), and 14.8 ± 0.5 Gy (99 keV/μm). The corresponding RBEs increased from 1.26 ± 0.05 (16 keV/μm) up to 1.68 ± 0.08 at 66 keV/μm. Unexpectedly, the RBE at 99 keV/μm was comparable to that at 66 keV/μm. CONCLUSIONS: The data suggest a linear relation between RBE and LET at high doses for late effects in the spinal cord. Together with additional data from ongoing fractionated irradiation experiments, these data will provide an extended database to systematically benchmark RBE models for further improvements of carbon ion treatment planning.
PURPOSE: To measure the relative biological effectiveness (RBE) of carbon ions in the rat spinal cord as a function of linear energy transfer (LET). METHODS AND MATERIALS: As an extension of a previous study, the cervical spinal cord of rats was irradiated with single doses of carbon ions at 6 positions of a 6-cm spread-out Bragg peak (16-99 keV/μm). The TD50 values (dose at 50% complication probability) were determined according to dose-response curves for the development of paresis grade 2 within an observation time of 300 days. The RBEs were calculated using TD50 for photons of our previous study. RESULTS: Minimum latency time was found to be dose-dependent, but not significantly LET-dependent. The TD50 values for the onset of paresis grade 2 within 300 days were 19.5 ± 0.4 Gy (16 keV/μm), 18.4 ± 0.4 Gy (21 keV/μm), 17.7 ± 0.3 Gy (36 keV/μm), 16.1 ± 1.2 Gy (45 keV/μm), 14.6 ± 0.5 Gy (66 keV/μm), and 14.8 ± 0.5 Gy (99 keV/μm). The corresponding RBEs increased from 1.26 ± 0.05 (16 keV/μm) up to 1.68 ± 0.08 at 66 keV/μm. Unexpectedly, the RBE at 99 keV/μm was comparable to that at 66 keV/μm. CONCLUSIONS: The data suggest a linear relation between RBE and LET at high doses for late effects in the spinal cord. Together with additional data from ongoing fractionated irradiation experiments, these data will provide an extended database to systematically benchmark RBE models for further improvements of carbon ion treatment planning.
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