Techniques for Image Based in-vivo Dosimetry: from Particle Therapy PET to in-beam Prompt Gamma Imaging

Radiation therapy is an important treatment modality in cancer therapy. New radiation species, like protons and light ions have the potential to increase tumor conformality of irradiation and to decrease normal tissue dose. Such high precision radiotherapy treatment requires efficient quality assurance techniques. Therefore, a dose monitoring system is highly desirable. Between 1997 and 2008, the in-beam Positron Emission Tomography (PET) method was used at the GSI Helmholtzzentrum fr Schwerionenforschung, Darmstadt, Germany, for monitoring the dose delivered by 12C beams. The spatial distribution of positron emitters generated via nuclear interactions is measured during and shortly after the irradiation. By means of a comparison between measured and simulated activity distribution conclusions on the accuracy of the dose deposition can be drawn. Different modalities of PET, i.e. measuring during the irradiation versus taking data after the treatment have been compared. Recent investigation and limits of the PET method used for in-vivo dose monitoring at ion beams will be presented and discussed. Due to inherent physical restrictions of this method, a direct quantification of the delivered dose is not feasible. Therefore, another approach based on dose monitoring by detection of prompt gamma rays is currently under investigation. In contrast to PET this method relies on the detection of prompt gamma rays between 0 and 10 MeV emitted almost instantaneously during the therapeutic irradiation. To measure these photons a Compton camera design was evaluated with respect to the special requirements and conditions that arise from this application. Different concepts were compared by means of simulation. The complete chain from simulation based on the treatment plan to the iterative reconstruction of the data was developed and is now under optimization. First measurements have been successfully performed with radioactive sources and ion beams.