The low-energy secondary electrons emerging from the entrance surface of an X-irradiated gold foil increase the dose to cells in contact with or at micrometer distances from this surface (Radiat. Res. 150, 92-100, 1998). We examined the effect of the spectrum of these low-energy electrons on the RBE for cytogenetic effects and showed that this RBE was increased. A monolayer of surface-attached human T lymphocytes was exposed to 60 kV X rays in the absence or presence of a gold foil positioned immediately behind the cell layer or separated from it by a Mylar foil 0.9 or 2 microm thick. The enhancement of dose in the cell nuclei caused by the photoelectrons and Auger electrons emerging from the entrance surface of the gold foil was measured by TSEE dosimetry. Dose enhancement factors of 55.7, 46.6 and 37.5 were obtained with 0, 0.9 and 2 microm of Mylar inserted between the gold surface and the cell layer. This large enhancement results from the photoelectric effect in the gold foil, as shown by the accompanying Monte Carlo calculations of the secondary electron spectra at the gold surface. Auger electrons from the gold foil generally were not able to penetrate into the cell nuclei except for that fraction of the cells that had a very thin (< 0.7 microm) layer of cytoplasm and membranes between gold surface and cell nucleus. The dose-yield curves for dicentric chromosomes plus centric rings and for acentric fragments obtained after exposures without or with the gold foil were linear-quadratic. The coefficient alpha, the slope of the linear yield component, was increased in the presence of the gold foil and showed RBE values ranging from 1.7 to 2.2 compared to exposures in absence of the gold foil. The ratio of the yield of interstitial deletions and dicentrics (H ratio) was significantly increased from about 0.17 in the absence of the gold foil to about 0.22 in the presence of the gold foil. The increases in the RBE and the H ratio are interpreted in microdosimetric terms: The preferred occurrence of electron track ends in the vicinity of the gold surface causes an increase in the dose-mean restricted linear energy transfer in cell nuclei exposed to the photoelectrons and Auger electrons.

The induction of chromosome aberrations in human lymphocytes irradiated in vitro with X rays generated at a tube voltage of 29 kV was examined to assess the maximum low-dose RBE (RBE(M)) relative to higher-energy X rays or 60Co gamma rays. Since blood was taken from the same male donor whose blood had been used for previous irradiation experiments using widely varying photon energies, the greatest possible accuracy was available for such an estimation of the RBE(M), avoiding the interindividual variations in sensitivity or differences in methodology usually associated with interlaboratory comparisons. The magnitude of the linear coefficient alpha of the linear-quadratic dose-effect relationship obtained for the production of dicentric chromosomes by 29 kV X rays (alpha = 0.0655 +/- 0.0097 Gy(-1)) confirms earlier observations of a strong increase in alpha with decreasing photon energy. Relating this value to previously published values of alpha for the dose-effect curves for dicentrics obtained in our own laboratory, RBE(M) values of 1.6 +/- 0.3 in comparison with weakly filtered 220 kV X rays, 3.0 +/- 0.7 compared to heavily filtered 220 kV X rays, and 6.1 +/- 2.5 compared to 60Co gamma rays have been obtained. These data emphasize that the choice of the reference radiation is of fundamental importance for the RBE(M) obtained. A special survey of the RBE(M) values obtained by different investigators in the narrow quality range from about 30 to 350 kV X rays indicates that the present RBE is in fairly good agreement with previously published findings for the induction of chromosome aberrations or micronuclei in human lymphocytes but differs from recently published findings for neoplastic transformation in a human hybrid cell line.

Regulla, D., Panzer, W., Schmid, E., Stephan, G. and Harder, D. Detection of Elevated RBE in Human Lymphocytes Exposed to Secondary Electrons Released from X-Irradiated Metal Surfaces. Radiat. Res. 155, 744-747 (2001). Monolayers of human lymphocytes, attached to a 2-microm Mylar film, were irradiated with 60 kV X rays in the presence and absence of a 150-microm gold film backing the Mylar film. With the gold film present, the absorbed dose imparted to the cells was increased by a factor of 45.4 due to the release of photoelectrons from the gold film. The frequencies of dicentric chromosomes and centric rings as well as of excess acentric fragments were increased in agreement with this dose enhancement, and in addition an RBE of about 1.7 compared to the frequencies observed in the absence of the gold film was found. These radiation effects, which contribute to risk considerations in radiology, are interpreted in terms of the increased dose-mean restricted LET of the photoelectrons backscattered from the metal and slowed down in the Mylar film before they enter the cell layer.

For photon and electron radiations, radiobiological low-dose RBE values, with 250 kV X-rays as the reference radiation, can vary from 0.5 up to 3.5 in dependence upon mean LET. On the other hand epidemiological risk data for the sparsely ionizing radiations are subjected to considerable methodical limitations of accuracy, so that a trend with LET is not detectable. This justifies the strong simplification consisting in the recommendation of Q = 1 and W-R = 1 for all sparsely ionizing radiations, and 250 kV X-rays have to be regarded as representative. In individual cases of radiation risk estimate, radiation quality dependent risk factors should however be employed as far as available even for the photon and electron radiations. The risk data obtained in the Hiroshima and Nagasaki Life Span Study are valid for a very wide photon spectrum whose mean LET value approximates that for Co-60 gamma radiation. For neutron radiation with energies below 1 MeV (fission spectra) the low-dose radiobiological RBE values referred to 250 kV X-rays are found around 20, but vary from 30 to 80 when referred to Co-60 gamma radiation. For reference radiation with strong curvature of the dose effect curves, there are however considerable methodical problems which limit the accuracy of initial slope determination. Since epidemiological risk factors for the neutron radiation are still lacking, experimental RBE values for tumor induction in laboratory animals have served instead as the data base for the recommendations of Q values up to 30 and neutron W-R values up to 20. In comparing these with the constant setting of Q = 1 and W-R = 1 for the manifold of photon and electron radiations, one should consider that this manifold is approximately represented by 250 kV X-rays. Accordingly, the system of Q and WR values corresponds to our present knowledge, but should also be seen in the light of the still existing methodical imperfection.