  Proton beam therapy can potentially offer improved treatment for cancers of
the head and neck and in paediatric patients. There has been a sharp uptake of
proton beam therapy in recent years as improved delivery techniques and patient
benefits are observed. However, treatments are currently planned using
conventional x-ray CT images due to the absence of devices able to perform high
quality proton computed tomography (pCT) under realistic clinical conditions. A
new plastic-scintillator-based range telescope concept, named ASTRA, is
proposed here to measure the proton's energy loss in a pCT system. Simulations
conducted using GEANT4 yield an expected energy resolution of 0.7\%. If
calorimetric information is used the energy resolution could be further
improved to about $0.5\%$. In addition, the ability of ASTRA to track multiple
protons simultaneously is presented. Due to its fast components, ASTRA is
expected to reach unprecedented data collection rates, similar to $10^8$
protons/s. The performance of ASTRA has also been tested by simulating the
imaging of phantoms. The results show excellent image contrast and relative
stopping power reconstruction.
