Mutagenicity of low-filtered 30 kVp X-rays, mammography X-rays and conventional X-rays in cultured mammalian cells.

PURPOSE

To measure the mutagenic effectiveness of low-filtered 30 kVp X-rays, mammography X-rays and conventional (200 kVp) X-rays in mammalian cells.

MATERIALS AND METHODS

Two different cell lines and mutation assays were used. Exponentially growing SV40-transformed human fibroblasts were exposed to graded doses of mammography (29 kVp, tungsten anode, 50 microm Rh filter) or conventional X-rays and the frequency of 6-thioguanine-resistent HPRT-deficient mutants was determined. Exponentially growing hamster A(L) cells, which contain a single human chromosome 11 conferring the expression of the human surface protein CD59, were subjected to magnetic cell separation (MACS) in order to remove spontaneous mutants before irradiation with low-filtered 30 kVp (tungsten anode, 0.5 mm Al filter) or conventional X-rays. Fractions of radiation-induced CD59- mutants were quantified by flow-cytometry after immunofluorescence labelling of CD59 proteins.

RESULTS

Mammography X-rays were more effective than conventional X-rays at inducing killing of human fibroblasts, whereas 30 kVp X-rays and conventional X-rays were about equally effective at killing Al. cells. Mutant frequencies were linearly related to dose in both mutation assays. An RBE = 2.7 was calculated for the yield of HPRT mutants in human fibroblasts exposed to mammography relative to conventional X-rays and an RBE = 2.4 was obtained for the CD59 mutant frequency in A(L) cells irradiated with low-filtered 30 kVp relative to conventional X-rays.

CONCLUSIONS

Both low-filtered 30 kVp and mammography X-rays are mutagenic in mammalian cells in vitro. It is unknown if and how the enhanced mutagenicity of mammography X-rays measured in human cells in vitro translates into breast cancer risk for predisposed women with an enhanced inherited risk for breast cancer. Although the ICRP guidelines attribute the same relative biological effectiveness to all radiations of low LET, including X- and gamma-radiations of all energies for radiobiological protection purposes including the assessment of risks in general terms, they also state that 'for the estimation of the likely consequences of an exposure of a known population, it will sometimes be better to use absorbed dose and specific data relating to the relative biological effectiveness of the radiations concerned and the probability coefficients relating to the exposed population' (ICRP 1991: 32). This latter statement may apply for the population of familial predisposed women. We hope that the presented data on the enhanced mutagenicity of mammography X-rays may stimulate a re-evaluation of the risk assessment of mammography for familial predisposed women. In the meantime, one should be cautious and avoid early and frequent mammography exposure of predisposed women. Alternative examination methods should be applied for these women with an inherited increased risk for breast cancer.