CHROMOSOME-ABERRATIONS IN HUMAN FIBROBLASTS INDUCED BY MONOENERGETIC NEUTRONS .1. RELATIVE BIOLOGICAL EFFECTIVENESS

The relative biological effectiveness (RBE) of neutrons for many biolo gical end points varies with neutron energy. To test the hypothesis th at the RBE of neutrons varies with respect to their energy for chromos ome aberrations in a cell system that does not face interphase death, we studied the yield of chromosome aberrations induced by monoenergeti c neutrons in normal human fibroblasts at the first mitosis postirradi ation. Monoenergetic neutrons at 0.22, 0.34, 0.43, 1, 5.9 and 13.6 MeV were generated at the Accelerator Facility of the Center for Radiolog ical Research, Columbia University, and were used to irradiate plateau -phase fibroblasts at low absorbed doses from 0.3 to 1.2 Gy at a low d ose rate. The reference low-LET, low-dose-rate radiation was Cs-137 ga mma rays (0.66 MeV). A linear dose response (Y = alpha D) for chromoso me aberrations was obtained for all monoenergetic neutrons and for the gamma rays. The yield of chromosome aberrations per unit dose was hig h at low neutron energies (0.22, 0.34 and 0.43 MeV) with a gradual dec line with the increase in neutron energy. Maximum RBE (RBE(M)) values varied for the different types of chromosome aberrations. The highest RBE (24.3) for 0.22 and 0.43 MeV neutrons was observed for intrachromo somal deletions, a category of chromosomal change common in solid tumo rs. Even for the 13.6 MeV neutrons the RBE(M) (11.1) exceeded 10. Thes e results show that the RBE of neutrons varies with neutron energy and that RBEs are dissimilar between different types of asymmetric chromo some aberrations and suggest that the radiation weighting factors appl icable to low-energy neutrons need firmer delineation. This latter may best be attained with neutrons of well-defined energies. This would e nable integrations of appropriate quality factors with measured radiat ion fields, such as those in high-altitude Earth atmosphere. The intro duction of commercial flights at high altitude could result in many mo re individuals being exposed to neutrons than occurs in terrestrial wo rkers, emphasizing the necessity for better-defined estimates of risk. (C) 1996 by Radiation Research Society