The success of clinical boron neutron capture therapy (BNCT) lies in the ability to manage the radiobiological effect on the tumour and healthy tissue, and thus, accurate dosimetry measurements is pertinent for each individual patient. In the present work we investigate the possibility of performing online prompt gamma tomography (PGT) during BNCT. A prototype detector system was constructed, which is in principle a pin-hole collimator with a HPGe crystal to be mounted on a C-bow device, with shielding of lithium-plastic and lead. The detector system was used to measure on a phantom placed in an epithermal neutron beam and on a 137 Cs-source. The possibility of tomographic reconstruction using the detector system was tested on a phantom filled with a 131 I-solution with a smaller sphere inserted containing a higher specific activity (ratio 10:1). The detector system was possible to operate up to about 6 × 10 8 cm -2 s -1 thermal neutron fluence at the peak in the phantom, at which time it was saturated. A 478 keV boron-peak was visible in the measured spectra but the signal-to-noiseratio was rather low. No post-irradiation damage or neutron activation was detectable. A tomographic reconstruction of the phantom filled with 131 I-solutions was performed using an algorithm developed in house and based on the MLEM method. The image quality is fairly good and the results provide a clear indication that the detector system can be used to obtain data that enables tomographic reconstruction. A spatial resolution of the detector system of about 2 cm was obtained from both the measurement on the 137 Cs-source and the 131 I-phantom. In conclusion, the presented feasibility study on a prototype PGT system is encouraging further studies specifically directed at improving the signal-to-noise-ratio in measurements in epithermal neutron beams. KEYWORDS: Detector modelling and simulations I (interaction of radiation with matter, interaction of photons with matter, interaction of hadrons with matter, etc); Neutron sources; Dosimetry concepts and apparatus.