Exploration of "over kill effect" of high-LET Ar- and Fe-ions by evaluating the fraction of non-hit cell and interphase death

The reason why RBE for cell killing fell to less than unity (1.0) with very high-LET heavy-ions ((40)Ar: 1,640 keV/microm; (56)Fe: 780, 1,200, 2,000 keV/microm) was explored by evaluating the fraction of non-hit cell (time-lapse observation) and cells undergoing interphase death (calculation based on our previous data). CHO cells were exposed to 4 Gy (30% survival dose) of Ar (1,640 keV/microm) or Fe-ions (2,000 keV/microm). About 20% of all cells were judged to be non-hit, and about 10% cells survived radiation damage. About 70% cells died after dividing at least once (reproductive death) or without dividing (interphase death). RBE for reproductive (RBER]) and interphase (RBEI]) death showed a similar LET dependence with maximum around 200 keV/microm. In this LET region, at 30% survival level, about 10% non-survivors underwent interphase death. The corresponding value for very high-LET Fe-ions (2,000 keV/microm) was not particularly high (approximately 15%), whereas that for X-rays was less than 3%. However, reproductive death (67%) predominated over interphase death (33%) even in regard to rather severely damaged cells (1% survival level) after exposure to Fe-ions (2,000 keV/microm). These indicate that interphase death is a type of cell death characteristic for the cells exposed to high-LET radiation and is not caused by "cellular over kill effect". Both NHF37 (non-hit fraction at 37% survival) and inactivation cross-section for reproductive death (sigmaR]) began to increase when LET exceeded 100 keV/microm. The exclusion of non-hit fraction in the calculation of surviving fraction partially prevented the fall of RBER] when LET exceeded 200 keV/microm. On the other hand, the mean number of lethal damage per unit dose (NLD/Gy) showed the same LET-dependent pattern as RBER]. These suggest that the increase in non-hit fraction and sigmaR] with an increasing LET is caused by enhanced clustering of ionization and DNA damage which lowers the energy efficiency for producing damage and RBE.