The role of repair in the survival of mammalian cells from heavy ion irradiation: Approximation to the ideal case of target theory

Theories of cellular radiation sensitivity that preclude a significant role for cellular repair processes in the final biological expression of cellular damage induced by ionizing radiation are unsound. Experiments are discussed here in which the cell-cycle dependency of the repair deficiency of the S/S variant of the L5178Y murine leukemic lymphoblast was examined by treatment with the heavy ions, ²⁰Ne, ²⁸Si, ⁴⁰Ar, ⁵⁶Fe and ⁹³Nb. Evidence from those studies, which will be described in detail elsewhere, provide support for the notion that as the linear energy transfer (LET_∞) of the incident radiation increases the ability of the S/S cell to repair radiation damage decreases until effectively it is eliminated around 500 keV/μm. In the region of the latter LET_∞ value, the behavior of the S/S cell approximates the ideal case of target theory where post-irradiation metabolism (repair) does not influence cell survival. The expression of this phenomenon among different cell types and tissues will depend upon the actual repair systems involved and other considerations.