The effect of single and fractionated doses of X rays on the effective proportion of hypoxic cells in C3H mouse mammary tumours

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Abstract
C3H mice bearing first-generation transplanted mammary tumours were given priming doses consisting of a single dose of 300 rads or of a fractionated dose (5 × 300 rads in nine days). They were then tested with large single doses of 250 kV X rays given under conditions of either hypoxia, air, or high-pressure oxygen breathing to investigate the number and proportion of hypoxic cells in the tumours at the time of giving the test dose. The response was expressed in terms of the dose required to produce local control in 50 per cent of a population of treated animals (i.e. TCD 50 at 150 days). The results indicate that, after single or fractionated doses, re-oxygenation of originally hypoxic cells took place to a limited extent depending upon the size of the priming dose. The effectiveness of breathing high-pressure oxygen was greater when combined with the multidose priming treatment than with the single priming dose of 300 rads. Estimates of the “effective proportion” of hypoxic cells are compared with results previously obtained for a single priming dose of 1,500 rads. It is concluded that, although cell killing is relatively inefficient with this fractionated pretreatment, improvements in oxygenation can be equally great at a certain time after the pre-treatment.