Influence ofin vitroassay conditions on the assessment of radiobiological parameters of the MT tumour

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Abstract
The survival parameters of [mouse subcutaneous] anaplastic MT tumor cells following irradiation with 60Co .gamma. rays in vitro and in vivo, comparing colony formation in soft agar and monolayer. Following in vitro irradiation under oxic or hypoxic conditions, or in vivo hypoxic irradiation, the Do [mean lethal dose] values were about 30% greater when measured in soft agar compared with monolayer (soft agar, oxic Do = 1.6 Gy [gray], hypoxic Do = 3.7 Gy; monolayer, oxic Do = 1.2 Gy, hypoxic Do = 2.8 Gy). Other parameters were similar for each assay (oxic and hypoxic n .apprx. 11; hypoxic fraction in vivo .apprx. 0.07; PLD [potentially lethal damage] repair in naturally hypoxic cells increased the Do by a factor of .apprx. 1.3). Recently, McNally and Sheldon (1977) measured cell-survival parameters for this tumor using only a monolayer colony assay, and from these estimated TCD50 [mean tissue culture dose] for the tumor. Their estimate was lower than the directly measured value of 79 Gy. TCD50 was estimated from cell-survival data after correction for RBE [relative biological effectiveness] (.gamma. rays were used while McNally and Sheldon used 250 kV X-rays). The measured TCD50 from monolayer data could not be predicted, even when PLD repair was taken into account. The soft agar data with PLD repair could predict the observed TCD50.