CHROMOSOME ABERRATION FREQUENCIES IN HUMAN-LYMPHOCYTES IRRADIATED IN A PHANTOM BY A MIXED BEAM OF FISSION NEUTRONS AND GAMMA-RAYS

Purpose: To provide further information on the existence of a signific ant quadratic component of the dose-response relationship for the prod uction of dicentrics in human lymphocytes by mixed fission neutron and gamma-ray irradiation, which has been observed previously employing t he same beam under free-in-air conditions. Materials and methods: Irra diation of blood samples and dosimetry was performed at 2cm depth in a polymethyl methacrylate (PMMA) phantom of 16 cm side-length where the PMMA material fully surrounded the blood specimen. Chromosome analysi s was carried out exclusively in complete first division metaphases. R esults: Dicentric yields induced by absorbed doses between 0.043 and 2 .68 Gy fit a linear-quadratic model with a quadratic coefficient signi ficantly different from zero. The distribution of dicentrics is overdi spersed compared to Poisson at lower doses, but is poissonian at highe r doses. Conclusion: The presence of a significant quadratic dose-resp onse coefficient for dicentrics, both for free-in-air and phantom irra diation, is caused by the various degraded fission spectra that produc e neutrons or recoil protons over a broad energy range, rather than by the gamma-ray component of the beam.