  Treatment planning in ion beam therapy requires accurate knowledge of the
relative stopping power (RSP) distribution within the patient. Currently, RSP
maps are obtained via conventional x-ray computed tomography (CT) by converting
the measured attenuation coefficients of photons into RSP values for ions.
Alternatively, to avoid conversion errors that are inherent to this method, ion
computed tomography (iCT) can be used since it allows determining the RSP
directly. In typical iCT systems, which usually consist of a tracking system
and a separate residual energy detector, the RSP is obtained by measuring the
particle trajectory and the corresponding water equivalent path length (WEPL)
of single ions travelling through the patient. Within this work, we explore a
novel iCT approach which does not require a residual energy detector. Instead,
the WEPL is estimated indirectly by determining the change in time of flight
(TOF) due to the energy loss along the ion's path. For this purpose, we have
created a Geant4 model of a TOF-iCT system based on low gain avalanche
detectors (LGADs), which are fast 4D-tracking detectors that can measure the
time of arrival and position of individual particles with high spatial and time
precision. To assess the performance of this TOF-iCT concept, we determined the
RSP resolution and accuracy for different system settings using the Catphan
CTP404 sensitometry phantom. Within the set of investigated system parameters,
the lower limit of the RSP accuracy was found at 0.91%, demonstrating the
proof-of-principle of this novel TOF-iCT concept. The main advantage of using
this approach is that it could potentially facilitate clinical integration due
to its compact design, which, however, requires experimental verification and
an improvement of the current WEPL calibration procedure.
