Protons are excellent particles for tumour treatment due to the increased ionization density close to their stopping point. In practice, the uncertainty on the particle range compromises the achievable accuracy. Compton cameras imaging prompt gamma rays, a by-product of the irradiation, have been proposed for indirect range verification years since. At Universitats Protonen Therapie Dresden, two BGO block detectors (from PET scanners) arranged as Compton camera are deployed for imaging tests with high energy prompt gamma rays produced in PMMA by a proton pencil beam. Target shifts, thickness increase and beam energy variation experiments are conducted. Each measurement lasts about 15 minutes at a low proton beam current. The effect of one centimetre proton range deviations on the backprojected images is analysed. In conclusion, the experimental results highlight the potential application of Compton cameras for high energy prompt gamma ray imaging of pencil beams, as a real-time and in vivo range verification method in proton therapy.