The superconducting electron linear accelerator ELBE, at present under construction at Forschungszentrum Rossendorf, is going to be used to produce various types of secondary radiation, among which are X-rays in the energy range 100 eV - 100 keV. At the first stage, monochromatic X-rays in the energy range 10 keV - 50 keV will be obtained by means of planar channeling of high-energy electrons in a diamond crystal. Such an unconventional X-ray source is rather compact, tuneable and allows the possibility of delivering a beam in picosecond pulses [1]. The relative biological effectiveness (RBE) of the X-rays in this energy range will be determined by studies of cell survival and cytogenetic damage. Precise RBE values are required for risk assessment in diagnostic radiology, such as mammography, and radiotherapy with soft X-rays (brachytherapy). It has been previously shown that soft X-rays are more effective in cell killing [2] and chromosome abberations induction [3]. In preparation of radiobiological studies at ELBE, experiments were performed with a 25 kV soft X-ray tube and a 200 kV reference X-ray tube at TU Dresden. The cell line NIH/3T3 mouse fibroblasts was chosen because it is widely used, easy to handle, and contact-inhibited. The cells were cultured in DMEM supplemented with 10 % bovine serum, 10 mM HEPES buffer, 1 mM Sodium Pyruvate, 1 % non-essential aminoacids, 100 U/ml penicillin and 100 mg/ml streptomycin (all from Biochrom Seromed) at 37^oC in air containing 5 % CO₂. Exponentially growing cells were irradiated in 25 cm² polystyrene flasks at dose rates 1.22 Gy/min (200 kV, 20 mA, 0.3 mm Al filter) and 1.67 Gy/min (25 kV, 20 mA, 0.5 mm Cu filter). After irradiation the cells were harvested, counted and seeded at appropriate densities (6-8 replicates per dose point). After 12 days, colonies having at least 50 cells were scored as offspring from 1 surviving cell.
The RBE value at 10 % survival, determined by a linear-quadratic model fit was found to be 1.28 ± 0.5. As the photon energy is decreasing, the secondary electron spectrum becomes dominated by low-energy electrons and this leads to higher localisation of damage. The comparison of the energy spectrum of the 25 kV X-ray tube (measured with a Si PIN photodiode) and the 200 kV X-ray tube (modelled) revealed that in the first case the main contribution to the dose is coming by the photons of energy 10 - 20 keV, whereas in the second case, photons of this energy range contribute to the absorbed dose only by 4 %. Although the results obtained are not in contradiction with published data, there is a demand for accurate determination of RBE photon energy dependence.

Fig. 1 Clonogenic survival of NIH/3T3 cells after 25 kV and 200 kV X-ray tube irradiation.

¹ Dresden University of Technology, Germany
² Centre of Oncology, Krakow, Poland

References

[1] W. Enghardt et. al., Acta Physica Polonica, Vol. 30 (1999), No. 5, pp. 1639-1645
[2] M. Bistrovic et. al., Radiother. Oncol., Vol. 7 (1986), No. 2, pp. 175-180
[3] R.P. Virsik et. al., Radiat. Environ. Biophys., Vol. 14 (1977), No. 2, pp. 109-121  IKH 06/27/01 © A. Panteleeva