Biomedical Research

The activities of the institute in the field of biomedicine have been devoted to two topics: The continuous operation of positron emission tomography (PET) for quality assurance of heavy ion therapy and the improvement of this method, and as a further application of nuclear physics based technology to a biomedical issue, the preparation of an experimental facility at ELBE for cell radiobiology with quasi-monochromatic X-rays.

During the time period reported here (July 1999 - Dec. 2000) 46 Patients suffering from head and neck tumours received carbon ion therapy at the experimental therapy facility at GSI Darmstadt. All these irradiations have been monitored by means of the Rossendorf in-beam PET scanner BASTEI. It could be confirmed that the modifications of the treatment planning data base, which were initiated by PET observations of carbon ion range deviations especially in highly inhomogeneous target volumes, considerably improved the precision and the reliability of the treatments. This allowed the carbon ion therapy to be extended to more delicate situations. An evaluation of the 45 patients with skull base tumours irradiated with carbon beams between December 1997 and September 1999 was published earlier this year (J. Debus et al., Strahlenther. Onkol. 176 (2000) 211). After a mean follow-up of 9 months it revealed that the irradiations were well tolerated by all patients. Partial tumour remission was seen in 7 patients, one-year local control was 94 %, one patient deceased. No severe unexpected toxicity and no local recurrence within the treated volume were observed. The clinical effectiveness and the technical feasibility of the experimental therapy facility could be demonstrated. To evaluate the clinical relevance, larger patient numbers are necessary and, therefore, the treatments will be continued during three time slots of about 4 weeks a year. Continuing this project a new heavy ion accelerator exclusively for clinical use is planned to be constructed in Heidelberg. The facility will be capable of producing therapeutic beams ranging from protons to oxygen ions, it will be equipped with rotating beam deliveries (gantries) that will be combined with in-beam PET for therapy monitoring. These results and future plans determine the current work on the PET project:

• To increase the flexibility in treatment planning the GSI therapy facility will be equipped with a chair in 2001 for treating patients in a sitting position. This required to built up a completely new PET gantry which allows the detector heads to be rotated around the beam axis.
• It is desirable to quantify deviations between the planned and the applied dose distributions on the basis of PET data. Such deviations are caused by slight mispositioning or by changes of the patient's physical condition during the treatment of more than 3 weeks duration. To prepare for this, improved attenuation and scatter correction methods of the measured data as well as refined models (positron range, photon scattering) for predicting the b⁺-activity from the treatment planning have been introduced.
• Combining an in-beam PET scanner with an ion beam gantry for multi-field irradiations, as it is planned for the Heidelberg clinical facility, requires new technical solutions, namely new scanner configurations. To predict their imaging properties a versatile PET simulation and reconstruction tool has been developed. Gantry-based multi-field therapeutic irradiations may result in PET scans of rather low counting statistics and, thus, an optimization of the signal-to-noise ratio is required. For this we studied the possibility of using the new scintillator material Lutetiumorthosilicate (LSO), which is superior to the currently used Bismutgermanate (BGO) with respect to light output and fluorescence time. We analyzed the influence of the time microstructure of the synchrotron beam to the random coincidence rate registered by the positron camera as the basis for improving the signal-to-noise ratio of the PET images by an effective random coincidence rejection.
• The investigation of an extension of our PET technique to proton therapy monitoring has been initiated.

In preparation of radiobiological investigations with quasi-monochromatic X-rays produced by electron channeling in diamond at ELBE the following activities have to be reported:

• The radiation physics beam line, where the radiobiological experiments will be performed, has been designed with special attention to an electron beam of low divergence for a high channeling radiation yield. On that basis the majority of the beam line components have been purchased.
• By means of Monte Carlo simulations the different radiation components (channeling X-rays, bremsstrahlung, neutrons) contributing to the dose in the cells have been quantified. The beam line configuration has been optimized to reduce the bremsstrahlung and the neutron background. A dedicated channeling radiation production target is under construction. Nevertheless, for energy dependent cell survival studies a further reduction of the bremsstrahlung background is necessary. This will be performed by means of filters of highly oriented pyrolytic graphite.
• The equipment for X-ray dosimetry has been purchased and set into operation. Due to the high attenuation of the rather soft X-rays a reliable dosimetry within the cell monolayers requires special care. As a possible solution the application of thermally stimulated exoelectron emission dosimetry has been studied.
• The experimental techniques for cell survival studies have been trained and successfully applied to the measurements of the relative biological effectiveness of 25 kV X-rays.
• The cell laboratory attached to ELBE has been designed, construction has been started and the main technical equipment (class 2 safety cabinet, incubator, autoclave, centrifuge and microscope) has been purchased.