Encapsulated iodine-125 in radiation oncology. II. Study of the dose rate effect on potentially lethal damage repair (PLDR) using mammalian cell cultures in plateau phase.

Potentially lethal damage repair (PLDR) is considered to be an important mechanism of cellular radioresistance. Studies have suggested that radioresistant human tumor cells may have increased PLDR capability. Studies have also shown that PLDR may be decreased with smaller dose fractions. PLDR occurring with low dose rate continuous irradiation (LDCI) has not been assessed. The C3H/10T1/2 murine cell line in plateau phase (15% cycling fraction) was used to study the effect of dose fraction size and LDCI on PLDR. The cells were found to have substantial PLDR capability. Uncorrected data showed increased PLDR with higher single fractions. However, when the single fraction results are extrapolated to multiple fractions to give equivalent cell survival for the large and small fractions, no difference in PLDR was observed. The data from the LDCI experiments demonstrated some survival enhancement with delayed plating after 5 and 8 hours of irradiation, but not after 16 or more hours. The results indicate that PLDR occurs during the LDCI and that the magnitude of PLDR is similar for LDCI relative to acute irradiation.