Experience during percutaneous ion beam therapy at Gesellschaft fu¿r Schwerionenforschung showed that in vivo dose monitoring can contribute to the treatment's success. It has been studied whether a similar system, independent of treatment device and based upon the techniques of positron emission tomography can be implemented for high energy photon therapy. Simulation shows a strong correlation between the distribution of the annihilation points of positrons emerging from pair production and the dose. Design parameters of a detector system have been assessed which is capable of registering annihilation radiation during treatment. A test device system has been assembled which can be operated with various scintillator crystals and avalanche photodiodes. A custom digital signal processing system has been implemented. Phantom experiments at a clinical linear accelerator disclosed a severe background that does not solely stem from scattered primary photons but from radiation directly emitted from the treatment head into the detectors. It was not possible to shield the detectors sufficiently and thus there was no success in identifying annihilation radiation.