Is cell survival a determinant of the in situ response of 9L tumours?

The influence fo growth rate, location, size and PLD recovery on the cellular radiosensitivity, the tumour response (increase in life-span) and the relationship between these two end points was studied using two sublines of the 9L rat brain tumour, designated 9L/Ro and 9L/SF. The median day of death of rats bearing the intracerebral (i.c.) 9L/Ro tumours was 16-18 days; for i.c. 9L/SF tumours it was 23-25 days. The doubling time of 9L/Ro cells was slightly faster than that for 9L/SF cells both in culture and in the brain. The cellular radiosensitivity of both i.c. tumour cell sublines was identical when measured immediately after irradiation by an in vitro colony formation assay. However, subcutaneous (s.c.) 9L/Ro tumour cells were more resistant (D0 = 332 rad cf 180 rad). There was no evidence of a substantial hypoxic fraction in either site. When i.c. 9L/Ro and 9L/SF tumours of similar size were treated with fractionated doses of BCNU, X-rays or combinations of the two, the responses of the two tumours were essentially identical. The rate of recovery from radiation-induced potentially lethal damage (PLD) was identical in the two sublines and the two sites, although the extent of the PLD recovery was apparently greater in s.c. tumours. Increase in life-span of rats bearing i.c. 9L/Ro tumours appeared to be correlated with the tumour cell kill measured after completion of PLD recovery rather than with the tumour cell kill determined immediately after irradiation.