Chromosome aberration frequencies in human lymphocytes irradiated in a phantom by a mixed beam of fission neutrons and gamma -rays

Abstract

Purpose: To provide further information on the existence of a significant quadratic component of the dose-response relationship for the production of dicentrics in human lymphocytes by mixed fission neutron and gamma -ray irradiation, which has been observed previously employing the same beam under free-in-air conditions. Materials and methods: Irradiation of blood samples and dosimetry was performed at 2 cm depth in a polymethyl methacrylate (PMMA) phantom of 16 cm side-length where the PMMA material fully surrounded the blood specimen. Chromosome analysis was carried out exclusively in complete first division metaphases. Results: Dicentric yields induced by absorbed doses between 0.043 and 2.68 Gy fit a linear-quadratic model with a quadratic coefficient significantly different from zero. The distribution of dicentrics is overdispersed compared to Poisson at lower doses, but is poissonian at higher doses. Conclusion: The presence of a significant quadratic dose-response coefficient for dicentrics, both for free-in-air and phantom irradiation, is caused by the various degraded fission spectra that produce neutrons or recoil protons over a broad energy range, rather than by the gamma -ray component of the beam.