The effect of radiation on tumour growth delay, cell survival and cure of the animal using a single tumour system

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Abstract
The response of a murine tumor [anaplastic MT mouse tumor] to single doses of X rays was measured using 3 different assays-animal cure, cell survival in vitro after irradiation in vivo, and tumor growth delay. The dose to cure 50% of the animals, TCD50, was 79.0 Gy [grays]. This was not affected by clamping the tumors to render all the cells hypoxic at the time of irradiation, implying that most the cells in the tumor were hypoxic already. The enhancement ratio for the hypoxic cell sensitizer Ro-07-0582 [1-(2-hydroxy-3-methoxypropyl)-nitroimidazole] was 2.1. The cell survival assay gave an enhancement ratio of 1.6 and an hypoxic fraction of 5%. The discrepancy in the estimates of the hypoxic fraction can be explained by the ability of the naturally hypoxic cells, but not the oxic ones, to recover from potentially lethal damage in vivo. Neither the cell survival assay nor the growth delay assay agreed with the TCD50 assay as to the effect of the hypoxic cell sensitizer, even allowing for recovery from potentially lethal damage. It is doubtful whether the measured survival curve would predict the measured TCD50.