Maria Saager1, Peter Peschke2, Stephan Brons3, Jürgen Debus4, Christian P Karger2. 1. Dept. of Medical Physics in Radiation Oncology, German Cancer Research Center (DKFZ), Heidelberg, Germany; National Center for Radiation Research in Oncology (NCRO), Heidelberg Institute for Radiation Oncology (HIRO), Heidelberg, Germany. Electronic address: m.saager@dkfz.de. 2. Dept. of Medical Physics in Radiation Oncology, German Cancer Research Center (DKFZ), Heidelberg, Germany; National Center for Radiation Research in Oncology (NCRO), Heidelberg Institute for Radiation Oncology (HIRO), Heidelberg, Germany. 3. Heidelberg Ion Beam Therapy Center (HIT), Heidelberg, Germany; National Center for Radiation Research in Oncology (NCRO), Heidelberg Institute for Radiation Oncology (HIRO), Heidelberg, Germany. 4. Dept. of Radiation Oncology, University Hospital of Heidelberg, Heidelberg, Germany; National Center for Radiation Research in Oncology (NCRO), Heidelberg Institute for Radiation Oncology (HIRO), Heidelberg, Germany.
Abstract
BACKGROUND AND PURPOSE: To determine the relative biological effectiveness (RBE) of protons in the rat spinal cord as a function of linear energy transfer (LET) and dose. MATERIALS AND METHODS: The rat cervical spinal cord was irradiated with single or two equal fractions (split doses) of protons at four positions (LET 1.4-5.5 keV/µm) along a 6 cm spread-out Bragg peak (SOBP). From dose-response analysis, TD50- (dose at 50% effect probability) and RBE-values were derived using the endpoint of radiation-induced myelopathy. RESULTS: Along the SOBP, the TD50-values decreased from 21.7 ± 0.3 Gy to 19.5 ± 0.5 Gy for single and from 32.3 ± 0.3 Gy to 27.9 ± 0.5 Gy for split doses. The corresponding RBE-values increased from 1.13 ± 0.04 to 1.26 ± 0.05 (single doses) and from 1.06 ± 0.02 to 1.23 ± 0.03 (split doses). CONCLUSIONS: For the relative high fractional doses, the experimental RBE at the distal edge of the proton SOBP is moderately increased. The conventionally applied RBE of 1.1 appears to be valid for the mid-SOBP region, but the higher values occurring more distally could be of clinical significance, especially if critical structures are located in this area. Further in vivo studies at lower fractional doses are urgently required.
BACKGROUND AND PURPOSE: To determine the relative biological effectiveness (RBE) of protons in the rat spinal cord as a function of linear energy transfer (LET) and dose. MATERIALS AND METHODS: The rat cervical spinal cord was irradiated with single or two equal fractions (split doses) of protons at four positions (LET 1.4-5.5 keV/µm) along a 6 cm spread-out Bragg peak (SOBP). From dose-response analysis, TD50- (dose at 50% effect probability) and RBE-values were derived using the endpoint of radiation-induced myelopathy. RESULTS: Along the SOBP, the TD50-values decreased from 21.7 ± 0.3 Gy to 19.5 ± 0.5 Gy for single and from 32.3 ± 0.3 Gy to 27.9 ± 0.5 Gy for split doses. The corresponding RBE-values increased from 1.13 ± 0.04 to 1.26 ± 0.05 (single doses) and from 1.06 ± 0.02 to 1.23 ± 0.03 (split doses). CONCLUSIONS: For the relative high fractional doses, the experimental RBE at the distal edge of the proton SOBP is moderately increased. The conventionally applied RBE of 1.1 appears to be valid for the mid-SOBP region, but the higher values occurring more distally could be of clinical significance, especially if critical structures are located in this area. Further in vivo studies at lower fractional doses are urgently required.
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