A comparison of the effects of radiation on tumour growth delay and cell survival. The effect of radiation quality.

Measurements have been made of the effects of 250 kV X-rays and cyclotron produced neutrons on the delay in growth of a rat fibrosarcoma (RIB5C) and on tumour cell survival assayed in vitro after irradiation in vivo. For doses above 300 rads of neutrons the RBE for cell survival was greaer than that for tumour growth delay. This may be due to X rays causing a greater delay in cell proliferation than neutrons for a given level of cell survival. This would be the opposite effect to that found by BBarendsen and Broerse (1969), irradiating a rat rhabdomyosarcoma. Another possibility is that meassurements of cellular radio-sensitivity which involve removal of cells from their normal environment may lead to incorrect estimates of cell survival in situ. A plot of the RBE for growth delay against the reciprocal of the neutron dose indicated that the dose level at which the RBE became dependent on the fraction of hypoxic tumour cells was larger than that for cell survival, indicating a smaller "effective" hypoxic fraction than that estimated from the cell-survival curves.