Induction of micronuclei in human fibroblasts and keratinocytes by 25 kV x-rays

A relative biological effectiveness (RBE) not much larger than unity is usually assumed for soft x-rays (up to approximately 50 keV) that are applied in diagnostic radiology such as mammography, in conventional radiotherapy and in novel radiotherapy approaches such as x-ray phototherapy. On the other hand, there have been recent claims of an RBE of more than 3 for mammography and respective conventional x-rays. Detailed data on the RBE of soft x-rays, however, are scarce. The aim of the present study was to determine the effect of low-energy x-rays on chromosomal damage in vitro, in terms of micronucleus induction. Experiments were performed with 25 kV x-rays and a 200 kV x-ray reference source. The studies were carried out on primary human epidermal keratinocytes (HEKn), human fibroblasts (HFIB) and NIH/3T3 mouse fibroblasts. Micronucleus (MN) induction was assayed after in vitro irradiation with doses ranging from 1 to 5.2 Gy. Compared to the effect of 200 kV x-rays, 25 kV x-rays resulted in moderately increased chromosomal damage in all cell lines studied. This increase was observed for the percentage of binucleated (BN) cells with micronuclei as well as for the number of micronuclei per BN cell. Moreover, the increased number of micronuclei per micronucleated BN cell in human keratinocytes and 3T3 mouse fibroblasts suggests that soft x-rays induce a different quality of damage. For all cell lines studied the analysis of micronucleus induction by 25 kV soft x-rays compared to 200 kV x-rays resulted in an RBE value of about 1.3. This indicates a somewhat enhanced potential of soft x-rays for induction of genetic effects.