The heavy ion therapy facility at GSI is only capable of delivering a fixed horizontal beam, where the patients have been treated in supine position up to now. This reduces the flexibility in treatment planning for the most frequently treated skull base tumours [1] as a recent analysis of the treatment geometry of such tumours revealed: The heavy ion beam portals giving an optimum dose conformation to the target are found to have an inclination of about 10 to 45 deg with respect to the frontal plane of the patient in supine treatment position [2]. A maximum flexibility for choosing the beam portals as it is usual at medical electron linear accelerators for photon and electron therapy would be achieved with a rotating beam delivery (gantry) [3]. Since such a solution is not feasible at the GSI experimental heavy ion therapy unit, a chair for irradiating patients in a sitting position will be mounted at the therapy site in 2001.
The original design of the positron camera BASTEI [4] was based on the assumption that patients are exclusively irradiated in supine position and thus the detector heads were mounted below and above the table. Evidently they would interfere with a sitting patient. In this case the detector heads have to be rotated into a horizontal or oblique position. Since the positron camera cannot be moved from its park position being 110 cm upbeam to the treatment site before immobilizing the patient, and considering the size of the beam delivery (see Fig. 1), the gap between the heads has to be increased from 83.2 to 115 cm before rotating. After this the camera can be moved downbeam to the patient position and the heads are radially removed to their standard imaging distance of 83.2 cm. To meet these requirements the PET gantry had to be completely rebuilt (Fig. 1).

Fig. 1 The new PET gantry in treatment position.
(Photo: A. Zschau, GSI Darmstadt)

The gantry reconstruction has been coordinated by Dr.-Ing. Haderthauer, nukmed konstruktion, Tülau-Fahrenhorst, mechanical engineering was carried out by Bernd Flach, Maschinenbau & Kunststoffverarbeitung, Schönheide under the technical leadership of K. Spitzner, and the control system based on the SPS process control units of SIEMENS including the position readout via network by the PET workstation has been developed by Automatisierungstechnik Egbert Neuschulz GmbH, Salzwedel. The reliability of the new gantry was demonstrated during the therapy of Nov./Dec. 2000, when 535 PET scans have been performed.
¹ Gesellschaft für Schwerionenforschung Darmstadt

References
[1] J. Debus et al., Strahlenther. Onkol. 176 (2000) 211
[2] O. Jäkel, J. Debus, Phys. Med. Biol. 45 (2000) 1229
[3] K.D. Gross, M. Pavlovic (eds.), Proposal for a dedicated ion beam facility for cancer therapy,i
GSI Darmstadt, 1998
[4] W. Enghardt et al., Strahlenther. Onkol. 175/II (1999) 33  IKH 06/25/01 © W. Enghardt