Influence of in vitro assay conditions on the assessment of radiobiological parameters of the MT tumour.

We have examined the survival parameters of anaplastic "MT" tumour cells following irradiation with 60Co gamma rays in vitro acd in vivo, comparing colony formation in soft agar and monolayer. Following in vitro irradiation under oxic or hypoxic conditions, or in vivo hypoxic irradiation, we found that the D0 values were about 30% greater when measured in soft agar compared with monolayer (soft agar, oxic D0 = 1.6 Gy, hypoxic D0 = 3.7 Gy; monolayer, oxic D0 = 1.2 Gy, hypoxic D0 = 2.8 Gy). However, other parameters were similar for each assay (oxic and hypoxic n approximately 11; hypoxic fraction in vivo approximately 0.07; PLD repair in naturally hypoxic cells increased the D0 by a factor of approximately 1.3). Recently, McNally and Sheldon (1977) measured cell-survival parameters for this tumour using only a monolayer colony assay, and from these estimated TCD50 for the tumour. However, their estimate was lower than the directly measured value of 79 Gy. We have also estimated TCD50 from our cell-survival data after correction for RBE (we used gamma rays while McNally and Sheldon used 250 kV X rays). Like McNally and Sheldon we were unable to predict the measured TCD50 from our monolayer data, even when PLD repair was taken into account. However, the soft agar data with PLD repair could predict the observed TCD50.