Today tumour diseases are the second most cause of death in Western countries. But only 45 percent of the patients can be cured by the established treatment methods. The further improvement of the these forms of therapy and the development of new therapeutical approaches is urgent. A substantial proportion of the patients could benefit from particle therapy with heavy ions.

Beams of accelerated heavy ions (e.g. carbon, nitrogen or oxygen) with an energy between 70 and 500 AMeV are characterised by physical and biological properties superior to the radiation used in conventional radiotherapy (photons, electrons, neutrons). They form a sharp dose maximum (Bragg peak) shortly before coming to rest and are scarcely scattered while penetrating tissue. Because of the increased relative biological efficiency of these ions in the Bragg peak region they are suitable for precision therapy of deeply seated, compact, radioresistant tumours near to organs at risk.

For a safe application of heavy ions close to radiosensitive structures (brain stem, optical nerves, eyes) an in situ monitoring of the therapy is desirable. This can be accomplished by positron emission tomography (PET), since fragmentation reactions between the stable ions of the therapy beam and the atomic nuclei of the tissue generate a dynamic spatial distribution of positron emitters (ß⁺-emitters) that can be observed by a positron camera.

At the Gesellschaft für Schwerionenforschung in Darmstadt a medical treatment site for heavy ion therapy has been established in co-operation with the Radiologische Universitätsklinik Heidelberg, the Deutsches Krebsforschungszentrum Heidelberg and the Forschungszentrum Rossendorf. The fast variation of the beam energy in conjunction with the vertical and horizontal beam deflection by dipole magnets (raster scanning) allows the three-dimensional, strictly tumour shape conformed irradiations. The dual head positron camera BASTEI has been installed at the treatment place in order to measure the decay of the ß⁺-emitters during the irradiation and a few minutes after. Two ways to verify the treatment plan by PET are possible.

• In critical situations when the beam has to pass very heterogeneous structures and radiosensitive organs are situated in the direction of the beam behind the Bragg peak, a monoenergetic low dose beam pulse can be applied to the patient. The range of the particles can be derived from the simultaneous PET scan, so that the correct range calculation of the treatment plan is ensured before the therapeutical irradiations are started.
• During each fraction of the heavy ion therapy the ß⁺-activity distributions are measured routinely. Based on the time course of every individual therapy fraction the expected ß⁺-emitter distribution is computed. By comparing the simulated with the measured data the precision of the dose deposition of this single therapy fraction is assessed. If a considerable disagreement between these two distributions is revealed by this comparison the treatment plan has to be modified before proceeding with the following therapy fraction.

The PET data are recorded in list mode, together with a protocol of important accelerator parameters of the irradiation. Because of the half-lives of the most abundant ß⁺-emitters ¹¹C and ¹⁵O it is on principle impossible to obtain the precise position of the ¹²C therapy beam by PET during the irradiation.

Physiological processes within the patientĘs body influence the ß⁺-activity distribution produced by heavy ion irradiation. The washout of ß⁺-emitters from soft tissue by perfusion is the dominant effect. A measurement of the blood flow in the target region could help to improve the accuracy of the predicted ß⁺-emitter distribution.

The results from the previous treatments show that PET has become a main part of quality assurance at heavy ion therapy. It is the only method that is capable of confirming the dose deposition of every single therapy fraction at the right place inside the patient. Therfore it is intended to equip the proposed clinical therapy facility for ion beam cancer treatment with a positron camera again.