Abstract
High-LET radiations are more effective for cell killing than low-LET ones, thereby having been applied to cancer therapy. Unavoidable, however, is exposure of normal cells which coexist with or surround tumors. Though many lines of evidence have shown that reproductive death occurs in the progeny of cells surviving radiations, its LET dependence remains elusive. We here investigated the cell-killing effectiveness of 60Co gamma-rays (0.2 keV/μm) and six different types of heavy ions (16.2-1610 keV/μm) in normal human fibroblast AG01522 cells, being processed for primary and secondary colony formation. RBE values relative to iso-survival dose of gamma-rays peaked at around 100 keV/μm both in primary and secondary colonies, showing that delayed reproductive death observed in secondary colonies arose in a LET-dependent fashion. On the other hand, very little difference in LET was found in the RBE based on the secondary survival at the primary 10% survival doses, indicating that delayed reproductive death arising only during secondary colony formation depends upon initial damages which would have been fixed during primary colony formation. Further studies to clarify the LET dependence of heterogeneity in colonies derived from irradiated single cells and apoptosis induction are ongoing.
