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Investigations into the Relative Biological Effectiveness of high-energy photon radiation


When biological tissue is irradiated with ionizing radiation, the biological effectiveness (i.e. the type and number of the radiation damage caused in the cells) depends, among other things, on the kind and energy of the radiation (i.e. on the radiation quality). To compare the biological effects of radiations of different quality, the Relative Biological Effectiveness (RBE) is indicated which represents the ratio between the biological effects generated at the same dose for the radiation quality of interest and for a reference radiation quality.

In the past few years, the RBE of photon radiation in the energy range from 3 keV to 16 MeV has been systematically investigated in a cooperation between PTB and the Forschungszentrum für Umwelt und Gesundheit (GSF, Research Centre for Environment and Health), the Bundesamt für Strahlenschutz (BfS) and the Ludwig Maximilians University in Munich [1,2,3,4,5]. The biological endpoint of interest was the occurrence of dicentric chromosomes in human lymphocytes (see Figure 1). For the determination of the RBE, different blood samples of the same donor were irradiated with known doses at different radiation qualities; dosimetry was continuously traceable to the primary standards of the Physikalisch-Technische Bundesanstalt (PTB). After that, the frequency of the occurrence of dicentric chromosomes in human lymphocytes was determined1).

Figure 1 : Human lymphocyte in the first cell division cycle in vitro (the arrow points at a dicentric chromosome).

Figure 2 (red points) shows the dependence of the RBE obtained from these experiments on the mean energy of the photon radiation; 60Co gamma radiation was selected as reference radiation. Whereas the development of the RBE for photon radiation with energies smaller than approx. 1 MeV (below 60Co gamma radiation) has already been known and accepted for a longer time, an unexpected increase in the RBE (to approx. the value 2) was observed at higher energies. As photon radiation with energies above 1 MeV is used in radiation therapy for the treatment of cancer diseases, the knowledge gained about the RBE of such radiation is important, for example, for the assessment of possible late damages in radiation therapy and may even influence the therapy scheme.

Figure 2 : Relative Biological Effectiveness of photon radiation as a function of the mean photon energy. In addition to the results obtained by experiment, the spectral fluence of the secondary electrons with an energy of 4 keV calculated by Monte Carlo simulation is shown. The circular symbols indicate the results obtained on the basis of monoenergetic photons, the stars were based on real spectra.

In a first attempt to explain the energy dependence of the RBE obtained by experiment, the radiation transport for the blood irradiations was simulated with the Monte Carlo simulation program EGSnrc [6], and different parameters - which were not experimentally accessible without further ado - were calculated. It turned out that the RBE of photon radiation with energies below the energies of 60Co is very well correlated with the spectral fluence of the generated low-energy secondary electrons. Figure 2 shows - in addition to the RBE obtained by experiment - the calculated relative fluence of secondary electrons with an energy of 4 keV (standardized to the fluence for 60Co). In contrast to the experiments, the Monte Carlo calculations were in most cases performed with monoenergetic photon radiation (circular symbols in Figure 2), some of the calculations were repeated with the actual spectra of the photon radiation used in the experiment (stars in Figure 2) - the obtained results were not, however, affected by this.

The figure shows that the relative spectral fluence of the low-energy secondary electrons very well describes the development of the RBE for photon energies below 1 MeV. However, the increase in the RBE at higher energies has so far not been expressed by the Monte Carlo calculations. The cause for this is not known; here, other mechanisms of effectiveness may be involved. Further investigations will be performed to clarify the cause for the increase in the RBE at energies above 1 MeV.


  1. Bauchinger, M., Schmid, E., Streng, S., Dresp, J.:
    Quantitative analysis of the chromosome damage at first division of human lymphocytes after 60Co γ-irradiation.
    Radiat. Environ. Biophys. 22 (1983), 225-229
  2. Schmid, E., Bauchinger, M., Streng, S., Nahrstedt, U.:
    The effect of 220 kV X-rays with different spectra on the dose response of chromosome abberrations in human lymphocytes.
    Radiat. Environ. Biophys. 23 (1984), 305-309
  3. Schmid, E., Regulla, D., Guldbakke, S., Schlegel, D., Roos, M.:
    Relative biological effectiveness of 144 keV neutrons in producing dicentric chromosomes in human lymphocytes compared with 60Co γ-rays under head-to-head conditions.
    Radiat. Res. 157 (2002), 453-460
  4. Schmid, E., Regulla, D., Kramer, H.M., Harder, D.:
    The effect of 29 kV X rays on the dose response of chromosome aberrations in human lymphocytes.
    Radiat. Res. 158 (2002), 771-777
  5. Schmid, E., Krumrey, M., Ulm, G., Roos, H., Regulla, D.:
    The maximum low-dose RBE of 17.4 and 40 keV monochromatic X rays for the induction of dicentric chromosomes in human peripheral lymphocytes.
    Radiat. Res. 160 (2003), 499-504
  6. Kawrakow, I.:
    Accurate condensed history Monte Carlo simulation of electron transport. I. EGSnrc, the new EGS4 version.
    Med. Phys. 27 (2000), 485-498

1) The biological investigations were performed in the Forschungszentrum für Umwelt und Gesundheit.