Dose distribution of neutron beam and chromosome analysis.

Chromosome analysis using peripheral lymphocytes is a sensitive and reliable indicator of the biological effect of radiation at low radiation doses. In the present study, using this chromosome aberration analysis, we tried to investigate the radiation dose distribution within and around the radiation field of a 6 MeV neutron beam. The efficient induction of dicentrics and rings at the irradiation of neutrons compared to those of gamma or X rays especially in a lower dose range, results in a dominant linear component in the linear quadratic model in the dose response relation of a chromosome aberration formation. A marked increase of RBE (relative biological effectiveness) in a lower dose range of neutrons was demonstrated. The radiation doses of the neutron beam as a function of depth, estimated from the yields of dicentrics and rings in a water phantom revealed a fairly good agreement with doses that were physically obtained. The radiation portal margin of the neutron beam was demonstrated to be not as sharp due to a wide penumbra. This wide penumbra and high RBE value, especially at lower dose range of the neutron beam may contribute to the induction of secondary malignancies in the normal tissue surrounding the tumor mass.