  For dose calculations in ion beam therapy, it is vital to accurately
determine the relative stopping power (RSP) distribution within the treated
volume. Currently, RSP values are extrapolated from Hounsfield units (HU),
measured with x-ray computed tomography (CT), which entails RSP inaccuracies
due to conversion errors. A suitable method to improve the treatment plan
accuracy is proton computed tomography (pCT). A typical pCT system consists of
a tracking system and a separate residual energy (or range) detector to measure
the RSP distribution directly. This paper introduces a novel pCT system based
on a single detector technology, namely low gain avalanche detectors (LGADs).
LGADs are fast 4D-tracking detectors, which can be used to simultaneously
measure the particle position and time with precise timing and spatial
resolution. In contrast to standard pCT systems, the residual energy is
determined via a time-of-flight (TOF) measurement between different 4D-tracking
stations. The design parameters for a realistic proton computed tomography
system based on 4D-tracking detectors were studied and optimized using Monte
Carlo simulations. The RSP accuracy and RSP resolution were measured inside the
inserts of the CTP404 phantom to estimate the performance of the pCT system.
After introducing a dedicated calibration procedure for the TOF calorimeter,
RSP accuracies < 0.6 % could be achieved. Furthermore, the design parameters
with the strongest impact on the RSP resolution were identified and a strategy
to improve RSP resolution is proposed.
