Dose-effect relationship for 14.5 MeV (d + T) neutron-induced chromosome aberrations in human lymphocytes irradiated in a man phantom

Neutron relative biological effectiveness in Hiroshima and Nagasaki atomic bomb survivors: a critical review

The calculated risk of cancer in humans due to radiation exposure is based primarily on long-term follow-up studies, e.g. the life-span study (LSS) on atomic bomb (A-bomb) survivors in Hiroshima and Nagasaki. Since A-bomb radiation consists of a mixture of γ-rays and neutrons, it is essential that the relative biological effectiveness (RBE) of neutrons is adequately evaluated if a study is to serve as a reference for cancer risk. However, the relatively small neutron component hampered the direct estimation of RBE in LSS data. To circumvent this problem, several strategies have been attempted, including dose-independent constant RBE, dose-dependent variable RBE, and dependence on the degrees of dominance of intermingled γ-rays. By surveying the available literature, we tested the chromosomal RBE of neutrons as the biological endpoint for its equivalence to the microdosimetric quantities obtained using a tissue-equivalent proportional counter (TEPC) in various neutron fields. The radiation weighting factor, or quality factor, Qn, of neutrons as expressed in terms of the energy dependence of the maximum RBE, RBEm, was consistent with that predicted by the TEPC data, indicating that the chromosomally measured RBE was independent of the magnitude of coexisting γ-rays. The obtained neutron RBE, which varied with neutron dose, was confirmed to be the most adequate RBE system in terms of agreement with the cancer incidence in A-bomb survivors, using chromosome aberrations as surrogate markers. With this RBE system, the cancer risk in A-bomb survivors as expressed in unit dose of reference radiation is equally compatible with Hiroshima and Nagasaki cities, and may be potentially applicable in other cases of human radiation exposure.

Neutron RBE values and their relationship to judgements in radiological protection

This review traces the history of radiological protection judgements concerning the relative biological effectiveness (RBE) for neutrons and quality factor. The basis of the different views coming from the International Commission on Radiological Protection (ICRP) and the International Commission on Radiation Units and Measurements (ICRU) is explained. A review of present-day information on neutron RBE relevant to judgements for radiological protection purposes is presented. It is argued that variations in neutron RBE measurements are large so that for practical purposes pragmatic judgements need to be made. It is further suggested that no inconsistency arises if different judgements are made for different purposes. However, this would require some resolution of the contrasting views of ICRP and ICRU.

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.

Chromosome aberrations induced in patients treated with chemotherapeutic drugs and irradiation for acute lymphatic leukemia

Chromosome analyses were carried out in lymphocytes of ten children with ALL, prior to and during combined therapy with antineoplastic drugs and cranial irradiation. Chromosome preparations from group I (four patients) were analysed by conventional staining. For group II (six patients) the FPG technique (fluorescence plus Giemsa) was applied after BrdU treatment of cultures and cells were scored exclusively in first division. A significant clastogenic effect could not be detected in samples collected during or after chemotherapy for either group. After cranial irradiation of those patients in group II. linear dose-effect relationships for dicentric plus ring chromosomes, and for excess acentrics, could be demonstrated after correction for the relative amount of irradiated volume.