Since December 1997 about 30 patients suffering from tumours at the base of the skull and in the pelvis region have been treated successfully with beams of high energy 12C ions at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt. During the irradiation fragmentation reactions between the ions of the beam and atomic nuclei of the tissue produce a characteristic spatial distribution of positron emitters. For the simultaneous monitoring of these ß+-activity distributions we operate a dedicated limited angle positron camera consisting of ECAT block detectors at the therapy beam line. List mode data acquisition has been implemented in order to record PET coincidence data as well as accelerator protocol data in sufficient time resolution. During the irradiation of one field with a dose of approximately 0.5 Gy a ten minute list mode scan of 104 - 105 valid coincidence events is acquired. Due to these sparse PET scans and the limited angle scanner geometry a fully 3D Maximum Likelihood algorithm is used for image reconstruction.
The experience of the patients' treatments shows that the PET in-situ dose localisation is appreciated by the physicians and medical physicists. The PET images from different therapy fractions are very similar and demonstrate the stability and reliability of the heavy ion therapy. In rare therapeutic situations when the therapy beam has to pass very heterogeneous structures the PET images revealed higher ranges of the primary particles than originally planned or a slight misplacement of the delivered dose. PET has become a major part of the quality assurance programme at the heavy ion therapy project.
At the proposed dedicated ion beam facility for cancer therapy designed to treat about 1,000 patients a year, at least one treatment site will be equipped with a positron camera again. There we face the challenge to combine the rotating isocentric ion gantry with a PET scanner. The main demands for this new PET device are
First feasibility studies suggest a dual-head camera design, each detector head of three rectangular segments with 30 cm x 30 cm sensitive area. Due to the higher light ouput LSO seems to be more favourable than BGO as the scintillator material. However, the applicability of LSO to in-beam PET has not been tested yet.