Apparent absence of a proton beam dose rate effect and possible differences in RBE between Bragg peak and plateau
Schematic figure of the beam line for this experiment.
The flask was placed in a specially molded polyethylene block containing a space to hold it. The thicknesses of the polyethylene block in front of the flask were chosen so that the cells were located at the Bragg peak and plateau, respectively.
(Left) Depth-dose profile of the present experiment. The depth-dose measurement was performed in a water phantom. The dots were the results of a GEANT4 simulation. Cell positions are indicated. (Right) The LET distributions at the Bragg peak (solid line) and plateau (dotted line) were calculated by a GEANT4 simulation. In the figure, the distribution has been normalized so that the integral of the distribution is unity.
The lateral dose profile at the cell position depth. The dose was normalized so that the dose at the center of the cell region represented unity. The measurement was performed at the CDR in an extensive region including point , which was outside of the irradiation field. The dose at was 3% of that delivered to the cell containing area. The DMU produced by the UDR at an arbitrary lateral position was determined by the DMU measurement at for UDR and extrapolation from this profile curve.
Survival curves for HSG cells at the Bragg peak (left) and the plateau (right). All data points were fitted by the LQ model. HLET and LLET indicate high and low LET, respectively, and UDR and CDR indicate ultrahigh and clinical dose rate, respectively.
Estimated SFs and the P-values in unpaired t-test for the difference of SFs according to the dose rate and LET.
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