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

The relative biological effectiveness of carbon ion irradiations of the rat spinal cord increases linearly with LET up to 99 keV/μm

“…Briefly, the spinal cord was located at 127 mm (doseaveraged LET 99 keV/lm) of a 6 cm SOBP ranging from 70 to 130 mm water-equivalent depth (beam energy 187-260 MeV/ u), which was calculated with the treatment planning system TRiP [12]. The field size was 10 Â 15 mm 2 including the cervical segments C1-C6 [8][9][10][11]. The position of the spinal cord within the SOBP was adjusted using an appropriate polymethyl-methacrylate (PMMA)-bolus.…”

“…These data showed a linear increase of RBE with LET, where the slope of the curve increased with decreasing fractional dose [11]. For the single fraction experiment at 99 keV/lm, however, an inconsistently low RBE combined with a shallow dose-response curve was found.…”

“…For comparison with measurements, the RBE was calculated at the position of the spinal cord within the SOBP and for the measured TD 50 -value using LEM I and IV together with the standard parameters of the two versions [11].…”

“…This was the starting point of a second series of experiments, where the rat spinal cord was irradiated with single and split doses at six different positions of a 6 cm SOBP covering a LET range from 16 to 99 keV/lm [10,11]. These data showed a linear increase of RBE with LET, where the slope of the curve increased with decreasing fractional dose [11].…”

“…To validate the LEM in vivo, the RBE for radiation-induced myelopathy in the rat spinal cord has been determined in several studies [8][9][10][11] and comparisons with LEM predictions were made. In these studies RBEs were calculated for the tolerance doses at 50% complication probability (TD 50 ) determined from dose-response experiments.…”