Image based in-vivo dosimetry: from PET to in-beam SPECT

Radiation therapy is an important treatment modality in cancer therapy. New radiation species, like protons and light ions have the potential of increasing tumor conformality of irradiation. Because of the way these particles deposit their energy on their path through tissue they allow for an increased dose deposition in the tumor volume and reduce the damage of the surrounding healthy tissue. Such high precision radiotherapy treatment requires efficient quality assurance techniques. Small changes in the irradiated volume will lead to a mismatch of the deposited dose maximum and the tumor. This causes missing dose in the tumor volume and potential damage to healthy tissue. 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 für Schwerionenforschung, Darmstadt, Germany, for monitoring the dose delivered by 12C beams. Since the positive clinical impact of the method has been shown at the Dresden Proton Therapy facility an in-room PET/CT will be installed for the same purpose.
Due to inherent limitations 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. For this purpose 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. Now the construction of a first prototype is under way.