  A proton pencil beam is associated with a surrounding low-dose envelope,
originating from nuclear interactions. It is important for treatment planning
systems to accurately model this envelope when performing dose calculations for
pencil beam scanning treatments, and Monte Carlo (MC) codes are commonly used
for this purpose. This work aims to validate the nuclear models employed by the
Geant4 MC code, by comparing the simulated absolute dose distribution to a
recent experiment of a 177 MeV proton pencil beam stopping in water.
  Striking agreement is observed over five orders of magnitude, with both the
shape and normalisation well modelled. The normalisations of two depth dose
curves are lower than experiment, though this could be explained by an
experimental positioning error. The Geant4 neutron production model is also
verified in the distal region. The entrance dose is poorly modelled, suggesting
an unaccounted upstream source of low-energy protons. Recommendations are given
for a follow-up experiment which could resolve these issues.
