Proceedings of the British Institute of Radiology

The effect of single and divided doses of 15 MeV electrons on spontaneous mammary tumours arising in C3H mice has been studied by measuring the change in tumour volume. Both in unirradiated tumours of control mice and in the subsequent regrowth of tumours following irradiation it has been found that growth occurs exponentially with a doubling time depending to a certain extent on the dose of radiation. The number of doublings of the viable cell population occurring during the time taken for the tumour to regrow to the size at irradiation was calculated using the doubling time of the tumour regrowth. This number of doublings is proportional to loge  S where S is the surviving fraction. LogeS may also be considered equal to —D/D  0.