  Particle therapy is an established method to treat deep-seated tumours using
accelerator-produced ion beams. For treatment planning, the precise knowledge
of the relative stopping power (RSP) within the patient is vital. Conversion
errors from x-ray computed tomography (CT) measurements to RSP introduce
uncertainties in the applied dose distribution. Using a proton computed
tomography (pCT) system to measure the SP directly could potentially increase
the accuracy of treatment planning. A pCT demonstrator, consisting of
double-sided silicon strip detectors (DSSD) as tracker and plastic scintillator
slabs coupled to silicon photomultipliers (SiPM) as a range telescope, was
developed. After a significant hardware upgrade of the range telescope, a 3D
tomogram of an aluminium stair phantom was recorded at the MedAustron facility
in Wiener Neustadt, Austria. In total, 80 projections with 6.5x10^5 primary
events were acquired and used for the reconstruction of the RSP distribution in
the phantom. After applying a straight-line approximation for the particle path
inside the phantom, the most probable value (MPV) of the RSP distribution could
be measured with an accuracy of 0.59%. The RSP resolution inside the phantom
was only 9.3% due to a limited amount of projections and measured events per
projection.
