Background: As there is a growing number of long-term cancer survivors, the incidence of carcinogenesis as a late\neffect of radiotherapy is getting more and more into the focus. The risk for the development of secondary\nmalignant neoplasms might be significantly increased due to exposure of healthy tissue outside of the target field\nto secondary neutrons, in particular in proton therapy. Thus far, the radiobiological effects of these neutrons and a\ncomparison with photons on normal breast cells have not been sufficiently characterised.\nMethods: MCF10A cells were irradiated with doses of up to 2 Gy with neutrons of different energy spectra and\nX-rays for comparison. The biological effects of neutrons with a broad energy distribution ( = 5.8 MeV),\nmonoenergetic neutrons (1.2 MeV, 0.56 MeV) and of the mixed field of gamma�s and secondary neutrons ( =\n70.5 MeV) produced by 190 MeV protons impinging on a water phantom, were analysed. The clonogenic survival\nand the DNA repair capacity were determined and values of relative biological effectiveness were compared.\nFurthermore, the influence of radiation on the sphere formation was observed to examine the radiation response of\nthe potential fraction of stem like cells within the MCF10A cell population.\nResults: X-rays and neutrons caused dose-dependent decreases of survival fractions after irradiations with up to\n2 Gy. Monoenergetic neutrons with an energy of 0.56 MeV had a higher effectiveness on the survival fraction with\nrespect to neutrons with higher energies and to the mixed gamma - secondary neutron field induced by proton\ninteractions in water. Similar effects were observed for the DNA repair capacity after exposure to ionising radiation\n(IR). Both experimental endpoints provided comparable values of the relative biological effectiveness. Significant\nchanges in the sphere formation were notable following the various radiation qualities.\nConclusion: The present study compared the radiation response of MCF10A cells after IR with neutrons and photons.\nFor the first time it was shown that monoenergetic neutrons with energies around 1 MeV have stronger radiobiological\neffects on normal human breast cells with respect to X rays, to neutrons with a broad energy distribution ( = 5.\n8 MeV), and to the mixed gamma - secondary neutron field given by interactions of 190 MeV protons in water. The\nresults of the present study are highly relevant for further investigations of radiation-induced carcinogenesis and are very\nimportant in perspective for a better risk assessment after secondary neutron exposure in the field of conventional and\nproton radiotherapy.
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