Cure, regression and cell survival: a comparison of common radiobiological endpoints using an in vitro tumour model.

Multicell spheroids of Chinese hamster V79-171 cells grown in suspension culture display many of the characteristics of solid tumours in vivo, and can be used as an in vitro tumour model. Two populations of spheroids differing in age and radiosensitivity were exposed to single doses of gamma-radiation and their response assayed by several techniques: (1) spheroids were reduced to single cells by trypsinization at various times post-irradiation, and viability of the single cells determined by colony formation; (2) entire spheroids were placed in petri dishes and observed for cellular outgrowth; (3)spheroid volume and cell content were monitored as a function of time after irradiation. It was found that spheroid volume changes could not be correlated with either the amount of radiation given or with the relative radiosensitivity. In contrast, the number of cells per spheroid, or cellularity, decreased exponentially with exposure dose at sufficiently long times after irradiation. Radiosensitivity was then quantified by calculating the per cent decrease of cellularity per rad. "Cure" of spheroids as defined by lack of outgrowth when entire spheroids were placed in petri dishes, correlated well with single cell survival, and was achieved 50 per cent of the time after 2,630 rads for the smaller spheroids and 3,750 rads for the larger ones. Since these spheroids contained an average of similar 7,600 and similar 30,700 cells respectively, comparison of cure data with single cell survival data showed that cures were achieved only when every cell was killed. This result may have significant therapeutic implications, since cells of the most radioresistant population of the spheroid, the chronically hypoxic internal cells, were capable of proliferation even when the spheroid was not reduced to single cells after irradiation.