  Range uncertainties in proton therapy hamper treatment precision. Prompt
gamma-rays were suggested 16 years ago for real-time range verification, and
have already shown promising results in clinical studies with collimated
cameras. Simultaneously, alternative imaging concepts without collimation are
investigated to reduce the footprint and price of current prototypes. In this
manuscript, a compact range verification method is presented. It monitors
prompt gamma-rays with a single scintillation detector positioned coaxially to
the beam and behind the patient. Thanks to the solid angle effect, proton range
deviations can be derived from changes in the number of gamma-rays detected per
proton, provided that the number of incident protons is well known. A
theoretical background is formulated and the requirements for a future
proof-of-principle experiment are identified. The potential benefits and
disadvantages of the method are discussed, and the prospects and potential
obstacles for its use during patient treatments are assessed. The final
milestone is to monitor proton range differences in clinical cases with a
statistical precision of 1 mm, a material cost of 25000 USD and a weight below
10 kg. This technique could facilitate the widespread application of in vivo
range verification in proton therapy and eventually the improvement of
treatment quality.
