Dose fractionation studies with a murine sarcoma under conditions of air or carbogen (95% OI 2 + 5% CO 2) breathing

Groups of C 3H mice bearing transplantable KHT Sarcomas were given multiple daily doses of either 200, 300, 500 or 1000 rad. The mice were breathing either air during the irradiation or carbogen (95% O 2:5% CO 2) for 15 min before and during the irradiation. Tumor cell survival curves were determined for each fractionation schedule using either a lung colony assay or an in vitro agar colony assay. The results with mice breathing air at the time of irradiation indicate that, in this tumor, decreasing the fraction size gives a small increase in the slope of the resultant survival curve. Carbogen breathing leads to a reduction in the slope of the survival curve relative to the equivalent curve in air, for large dose fractions, however, this effect declines with fraction size so that for 200 rad fractions very little, if any, difference is observed. Both of these results can be explained on the basis of reoxygenation which is known to occur in this tumor. Isoeffect curves calculated at a survival level of 3 × 10 −3 (equivalent to a single dose of 2000 rad given under air breathing conditions) are initially convex upwards but between 2 and 14 fractions are straight lines with slopes of 0.09 (air) or 0.19 (carbogen).